View source for Astronomy:Solar System

{{Short description|The Sun and objects orbiting it}}
{{Pp-semi-protected|small=yes}}
{{Infobox planetary system
| title = Solar System
| image = Solar System true color.jpg
| image_alt = A true-color image of the Solar System with sizes, but not distances, to scale. The order of the planets are from right to left.
| image_size = 300px
| caption = {{Longitem|The [[Astronomy:Sun|Sun]], [[Astronomy:Planets of the solar system|planets, moons and dwarf planets]]{{Efn|The [[Astronomy:Asteroid belt|asteroid belt]] and [[Astronomy:Kuiper belt|Kuiper belt]] are not added because the individual asteroids are too small to be shown on the diagram.}}<br/>(true color, size to scale, distances not to scale)|style=padding:2px 0 4px 0;}}
| age = 4.568&nbsp;billion years
| location = {{Longitem|Local Interstellar Cloud, [[Astronomy:Local Bubble|Local Bubble]], Orion–Cygnus Arm, [[Astronomy:Milky Way|Milky Way]]}}
| neareststar = {{Longitem|{{Ublist|[[Astronomy:Proxima Centauri|Proxima Centauri]] (4.2465&nbsp;[[Astronomy:Light-year|ly]]) |[[Astronomy:Alpha Centauri|Alpha Centauri]] (4.344&nbsp;ly)}}}}
| nearestplanetary = {{Longitem|[[Astronomy:Proxima Centauri|Proxima Centauri]] system (4.2465&nbsp;ly)}}
| semimajoraxis = {{Longitem|30.11 [[Astronomy:Astronomical unit|AU]]<br/>(4.5&nbsp;bill.&nbsp;km; 2.8&nbsp;bill.&nbsp;mi)}}
| Kuiper_cliff = ~50 AU from the Sun
| noknown_stars = yes
| stars = 1{{Nbsp}}([[Astronomy:Sun|Sun]])
| planets = {{Longitem|{{Plainlist|*[[Astronomy:Mercury (planet)|Mercury]]
*[[Astronomy:Venus|Venus]]
*[[Astronomy:Earth|Earth]]
*[[Astronomy:Mars|Mars]]
*[[Astronomy:Jupiter|Jupiter]]
*[[Astronomy:Saturn|Saturn]]
*[[Astronomy:Uranus|Uranus]]
*[[Astronomy:Neptune|Neptune]]}}}}
| outerplanetname = [[Astronomy:Neptune|Neptune]]
| dwarfplanets = {{Longitem|{{Plainlist|
*{{Dp|Ceres}}

*{{Dp|Orcus}}

*[[Astronomy:Pluto|Pluto]]

*[[Astronomy:Haumea|Haumea]]

*{{Dp|Quaoar}}

*[[Astronomy:Makemake|Makemake]]
*{{Dp|Gonggong}}

*[[Astronomy:Eris (dwarf planet)|Eris]]

*{{Dp|Sedna}}}}}}
| satellites = {{Longitem|{{Hlist|758{{Nbsp}}(285&nbsp;planetary|473&nbsp;[[Astronomy:Minor planet|minor planet]]ary)<ref name="JPLbodies">{{Cite web |title=Solar System Objects |url=https://ssd.jpl.nasa.gov |url-status=live |archive-url=https://web.archive.org/web/20210707142304/https://ssd.jpl.nasa.gov |archive-date=7 July 2021 |access-date=14 August 2023 |publisher=NASA/JPL Solar System Dynamics}}</ref>}}}}
| minorplanets = 1,298,410{{Efn|name=AsOf|As of 14 August 2023.}}<ref name=MPCSummary/>
| comets = 4,586{{Efn|name=AsOf}}<ref name="MPCSummary">{{Cite web |title=Latest Published Data |url=https://minorplanetcenter.net/mpc/summary |url-status=live |archive-url=https://web.archive.org/web/20190305034947/https://minorplanetcenter.net/mpc/summary |archive-date=5 March 2019 |access-date=14 August 2023 |website=The International Astronomical Union Minor Planet Center}}</ref>
| roundsat = 19
| roundsatlink = Planetary-mass moon
| inclination = 60.19°{{Nbsp}}(ecliptic)<!-- If anyone can find a cited value for the inclination of the Solar System's invariable plane to the galactic plane, please replace this value -->
| galacticcenter = 27,000&thinsp;±&thinsp;1,000 ly
| orbitalspeed = 220&nbsp;km/s; 136&nbsp;mi/s
| orbitalperiod = 225–250 [[Astronomy:Myr|myr]]
| spectral = [[Astronomy:G-type main-sequence star|G2V]]
| frostline = ≈5 AU<ref name="Mumma">{{Cite journal |last1=Mumma |first1=M. J. |last2=Disanti |first2=M. A. |last3=Dello Russo |first3=N. |last4=Magee-Sauer |first4=K. |last5=Gibb |first5=E. |last6=Novak |first6=R. |year=2003 |title=Remote infrared observations of parent volatiles in comets: A window on the early solar system |journal=Advances in Space Research |volume=31 |issue=12 |pages=2563–2575 |bibcode=2003AdSpR..31.2563M |citeseerx=10.1.1.575.5091 |doi=10.1016/S0273-1177(03)00578-7}}</ref>
| heliopause = ≈120 AU
| hillsphere = ≈1–3 ly
|background=#efefef}}
The '''Solar System'''<ref group="lower-alpha">Capitalization of the name varies. The [[Organization:International Astronomical Union|International Astronomical Union]], the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar System" and "solar system" structures in their [http://www.iau.org/public/themes/naming/ naming guidelines document] {{Webarchive|url=https://web.archive.org/web/20210725053113/https://www.iau.org/public/themes/naming |date=25 July 2021 }}. The name is commonly rendered in lower case ('solar system'), as, for example, in the ''Oxford English Dictionary'' and [http://www.m-w.com/dictionary/solar%20system ''Merriam-Webster's 11th Collegiate Dictionary''] {{Webarchive|url=https://web.archive.org/web/20080127201148/http://www.m-w.com/dictionary/solar%20system |date=27 January 2008 }}.</ref> is the [[Physics:Gravity|gravitationally]] bound system of the [[Astronomy:Sun|Sun]] and the objects that orbit it.<ref name="IAU Office of Astronomy for Education y607">{{cite web | title=IAU Office of Astronomy for Education | website=IAU Office of Astronomy for Education | url=https://astro4edu.org/resources/glossary/term/314/ | access-date=December 11, 2023}}</ref> The largest of these objects are the eight [[Astronomy:Planet|planets]], which in order from the Sun are four [[Astronomy:Terrestrial planet|terrestrial planets]] ([[Astronomy:Mercury (planet)|Mercury]], [[Astronomy:Venus|Venus]], [[Astronomy:Earth|Earth]] and [[Astronomy:Mars|Mars]]); two [[Astronomy:Gas giant|gas giants]] ([[Astronomy:Jupiter|Jupiter]] and [[Astronomy:Saturn|Saturn]]); and two [[Astronomy:Ice giant|ice giants]] ([[Astronomy:Uranus|Uranus]] and [[Astronomy:Neptune|Neptune]]). The Solar System [[Astronomy:Formation and evolution of the Solar System|developed 4.6 billion years ago]] when a dense region of a [[Astronomy:Molecular cloud|molecular cloud]] collapsed, forming the Sun and a protoplanetary disc. 

All four terrestrial planets belong to the inner Solar System and have solid surfaces. Inversely, all four giant planets belong to the outer Solar System and do not have a definite surface, as they are mainly composed of [[Physics:Gas|gases]] and [[Physics:Liquid|liquids]]. 99.86% of the Solar System's mass is in the Sun and nearly 90% of the remaining mass are in Jupiter and Saturn. There is a strong consensus among astronomers that the Solar System also has nine [[Astronomy:Dwarf planet|dwarf planets]], which consist of one [[Astronomy:Asteroid belt|asteroid-belt]] object – {{Dp|Ceres}}; five [[Astronomy:Kuiper belt|Kuiper-belt]] objects – [[Astronomy:Pluto|Pluto]], {{Dp|Orcus}}, {{Dp|Haumea}}, {{Dp|Quaoar}}, and {{Dp|Makemake}}; and three [[Astronomy:Scattered disc|scattered-disc]] objects – {{Dp|Gonggong}}, {{Dp|Eris}}, and {{Dp|Sedna}}.

There are a vast number of smaller objects orbiting the Sun, called [[Astronomy:Small Solar System body|small Solar System bodies]]. This category includes [[Astronomy:Asteroid|asteroids]], [[Astronomy:Comet|comets]], [[Astronomy:Centaur (minor planet)|centaurs]], [[Astronomy:Meteoroid|meteoroids]] and [[Astronomy:Interplanetary dust cloud|interplanetary dust clouds]]. Many of these objects are in the [[Astronomy:Asteroid belt|asteroid belt]] between the orbits of Mars and Jupiter (1.5–4.5 AU), and the [[Astronomy:Kuiper belt|Kuiper belt]] just outside Neptune's orbit (30–50 AU).{{efn|For more classifications of Solar System objects, see [[Astronomy:List of minor-planet groups|List of minor-planet groups]] and {{Section link|Comet|Classification}}.}} Six of the major planets, the six largest possible dwarf planets, and many of the smaller bodies are orbited by [[Astronomy:Natural satellite|natural satellites]], commonly called "moons" after Earth's [[Astronomy:Moon|Moon]]. Two natural satellites, Jupiter's moon [[Astronomy:Ganymede (moon)|Ganymede]] and Saturn's moon [[Astronomy:Titan (moon)|Titan]], are larger than Mercury, the smallest terrestrial planet, though they are less massive.

The Solar System is constantly flooded by the Sun's [[Physics:Charged particle|charged particle]]s, the [[Astronomy:Solar wind|Solar wind]], forming the [[Astronomy:Heliosphere|heliosphere]]. Pushed against by the surrounding [[Astronomy:Interstellar medium|interstellar medium]] of the Local Cloud, the Solar wind starts slowing at 75 to 90 AU (the termination shock), before being halted, resulting in the heliopause, the boundary of the Solar System to interstellar space. The outermost region of the Solar System is the [[Astronomy:Oort cloud|Oort cloud]], the source for long-period comets, extending from 2,000 AU to the edge of the Solar System's [[Astronomy:Sphere of influence (astrodynamics)|sphere of gravitational influence]] at up to {{cvt|200000|AU|ly|lk=out}}. The [[Astronomy:List of nearest stars and brown dwarfs|closest star]] to the Solar System, [[Astronomy:Proxima Centauri|Proxima Centauri]], is 4.25 ly away. The Solar System orbits the [[Galactic Center]] of the [[Astronomy:Milky Way|Milky Way]] galaxy, as part of its Orion Spur, at a distance of 26,000 ly.

== Formation and evolution ==
{{Main|Astronomy:Formation and evolution of the Solar System}}
The Solar System formed 4.568&nbsp;billion years ago from the gravitational collapse of a region within a large [[Astronomy:Molecular cloud|molecular cloud]].{{Refn |group=lower-alpha |The date is based on the oldest [[Earth:Inclusion (mineral)|inclusions]] found to date in [[Astronomy:Meteorite|meteorite]]s, {{Val|4568.2|+0.2|-0.4}} million years, and is thought to be the date of the formation of the first solid material in the collapsing nebula.<ref>{{Cite journal |last1=Bouvier |first1=A. |last2=Wadhwa |first2=M. |year=2010 |title=The age of the Solar System redefined by the oldest Pb–Pb age of a meteoritic inclusion |journal=Nature Geoscience |volume=3 |issue=9 |pages=637–641 |bibcode=2010NatGe...3..637B |doi=10.1038/NGEO941 |s2cid=56092512}}</ref>}} This initial cloud was likely several light-years across and probably birthed several stars.<ref name="Arizona">{{Cite web |last=Zabludoff |first=Ann |title=Lecture 13: The Nebular Theory of the origin of the Solar System |url=http://atropos.as.arizona.edu/aiz/teaching/nats102/mario/solar_system.html |url-status=live |archive-url=https://www.webcitation.org/617GeDn2a?url=http://atropos.as.arizona.edu/aiz/teaching/nats102/mario/solar_system.html |archive-date=22 August 2011 |access-date=27 December 2006 |website=NATS 102: The Physical Universe |publisher=University of Arizona}}</ref> As is typical of molecular clouds, this one consisted mostly of hydrogen, with some helium, and small amounts of heavier elements fused by previous generations of stars.<ref name=":3" />

As the pre-solar nebula<ref name=":3">{{Cite conference |last=Irvine |first=W. M. |date=1983 |title=The chemical composition of the pre-solar nebula |volume=1 |pages=3 |bibcode=1983coex....1....3I |book-title=Cometary exploration; Proceedings of the International Conference}}</ref> collapsed, conservation of angular momentum caused it to rotate faster. The center, where most of the mass collected, became increasingly hotter than the surrounding disc.<ref name="Arizona" /> As the contracting nebula rotated faster, it began to flatten into a protoplanetary disc with a diameter of roughly {{Convert|200|AU|e9km e9mi|abbr=unit|lk=off}}<ref name="Arizona" /> and a hot, dense [[Astronomy:Protostar|protostar]] at the center.<ref>{{Cite journal |last=Greaves |first=Jane S. |date=7 January 2005 |title=Disks Around Stars and the Growth of Planetary Systems |journal=Science |volume=307 |issue=5706 |pages=68–71 |bibcode=2005Sci...307...68G |doi=10.1126/science.1101979 |pmid=15637266 |s2cid=27720602}}</ref><ref>{{Cite book |last1=National Research Council, Division on Engineering and Physical Sciences |title=Strategy for the Detection and Study of Other Planetary Systems and Extrasolar Planetary Materials: 1990–2000 |last2=Space Studies Board, Committee on Planetary and Lunar Exploration |date=1990 |publisher=National Academies Press |isbn=978-0309041935 |publication-place=Washington D.C. |pages=21–33 |chapter=3. Present Understanding of the Origin of Planetary Systems |access-date=9 April 2022 |chapter-url=https://books.google.com/books?id=y56pS7SJs_8C&pg=PT29 |archive-url=https://web.archive.org/web/20220409211803/https://books.google.com/books?id=y56pS7SJs_8C&pg=PT29&lpg=PT29 |archive-date=9 April 2022 |url-status=live}}</ref> The planets formed by [[Physics:Accretion (astrophysics)|accretion]] from this disc,<ref>{{Cite journal |last1=Boss |first1=A. P. |last2=Durisen |first2=R. H. |date=2005 |title=Chondrule-forming Shock Fronts in the Solar Nebula: A Possible Unified Scenario for Planet and Chondrite Formation |journal=[[Astronomy:The Astrophysical Journal|The Astrophysical Journal]] |volume=621 |issue=2 |page=L137 |arxiv=astro-ph/0501592 |bibcode=2005ApJ...621L.137B |doi=10.1086/429160 |s2cid=15244154}}</ref> in which dust and gas gravitationally attracted each other, coalescing to form ever larger bodies. Hundreds of protoplanets may have existed in the early Solar System, but they either merged or were destroyed or ejected, leaving the planets, dwarf planets, and leftover [[Astronomy:Small Solar System body|minor bodies]].<ref name="bennett_8.2" /><ref>{{Cite book |last1=Nagasawa |first1=M. |title=Protostars and Planets V |last2=Thommes |first2=E. W. |last3=Kenyon |first3=S. J. |last4=Bromley |first4=B. C. |last5=Lin |first5=D. N. C. |date=2007 |publisher=University of Arizona Press |editor-last=Reipurth |editor-first=B. |publication-place=Tucson |pages=639–654 |chapter=The Diverse Origins of Terrestrial-Planet Systems |bibcode=2007prpl.conf..639N |access-date=10 April 2022 |editor-last2=Jewitt |editor-first2=D. |editor-last3=Keil |editor-first3=K. |chapter-url=https://jila.colorado.edu/~pja/astr5820/nagasawa.pdf |archive-url=https://web.archive.org/web/20220412010025/https://jila.colorado.edu/~pja/astr5820/nagasawa.pdf |archive-date=12 April 2022 |url-status=live}}</ref>
[[File:Soot-line1.jpg|center|thumb|400x400px|Diagram of the early Solar System's protoplanetary disk, out of which Earth and other Solar System bodies formed]]
Due to their higher boiling points, only metals and silicates could exist in solid form in the warm inner Solar System close to the Sun (within the [[Physics:Frost line (astrophysics)|frost line]]). They would eventually form the rocky planets of Mercury, Venus, Earth, and Mars. Because metallic elements only comprised a very small fraction of the solar nebula, the terrestrial planets could not grow very large.<ref name="bennett_8.2" />

The giant planets (Jupiter, Saturn, Uranus, and Neptune) formed further out, beyond the frost line, the point between the orbits of Mars and Jupiter where material is cool enough for volatile icy compounds to remain solid. The ices that formed these planets were more plentiful than the metals and silicates that formed the terrestrial inner planets, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium, the lightest and most abundant elements.<ref name="bennett_8.2" />

Leftover debris that never became planets congregated in regions such as the asteroid belt, Kuiper belt, and Oort cloud.<ref name="bennett_8.2">{{Cite book |last=Bennett |first=Jeffrey O. |title=The cosmic perspective |date=2020 |publisher=Pearson |isbn=978-0-134-87436-4 |edition=9th |location=Hoboken, NJ |chapter=Chapter 8.2}}</ref> 

Within 50 million years, the pressure and density of hydrogen in the center of the protostar became great enough for it to begin [[Physics:Nuclear fusion|thermonuclear fusion]].<ref name="Yi2001">{{Cite journal |last1=Yi |first1=Sukyoung |last2=Demarque |first2=Pierre |last3=Kim |first3=Yong-Cheol |last4=Lee |first4=Young-Wook |last5=Ree |first5=Chang H. |last6=Lejeune |first6=Thibault |last7=Barnes |first7=Sydney |date=2001 |title=Toward Better Age Estimates for Stellar Populations: The ''Y''<sup>2</sup> Isochrones for Solar Mixture |journal=Astrophysical Journal Supplement |volume=136 |issue=2 |pages=417–437 |arxiv=astro-ph/0104292 |bibcode=2001ApJS..136..417Y |doi=10.1086/321795 |s2cid=118940644}}</ref> As helium accumulates at its core the Sun is growing brighter;<ref name=":4">{{Cite journal |last=Gough |first=D. O. |date=November 1981 |title=Solar Interior Structure and Luminosity Variations |journal=Solar Physics |volume=74 |issue=1 |pages=21–34 |bibcode=1981SoPh...74...21G |doi=10.1007/BF00151270 |s2cid=120541081}}</ref> early in its main-sequence life its brightness was 70% that of what it is today.<ref>{{Cite journal |last=Shaviv |first=Nir J. |date=2003 |title=Towards a Solution to the Early Faint Sun Paradox: A Lower Cosmic Ray Flux from a Stronger Solar Wind |journal=[[Earth:Journal of Geophysical Research|Journal of Geophysical Research]] |volume=108 |issue=A12 |page=1437 |arxiv=astroph/0306477 |bibcode=2003JGRA..108.1437S |doi=10.1029/2003JA009997 |s2cid=11148141}}</ref> The temperature, reaction rate, pressure, and density increased until [[Astronomy:Hydrostatic equilibrium|hydrostatic equilibrium]] was achieved: the thermal pressure counterbalancing the force of gravity. At this point, the Sun became a [[Astronomy:Main sequence|main-sequence]] star.<ref>{{Cite journal |last1=Chrysostomou |first1=A. |last2=Lucas |first2=P. W. |date=2005 |title=The Formation of Stars |journal=[[Physics:Contemporary Physics|Contemporary Physics]] |volume=46 |issue=1 |pages=29–40 |bibcode=2005ConPh..46...29C |doi=10.1080/0010751042000275277 |s2cid=120275197}}</ref>

The main-sequence phase, from beginning to end, will last about 10&nbsp;billion years for the Sun compared to around two billion years for all other subsequent phases of the Sun's pre-remnant life combined.<ref name="mnras386_1">{{Cite journal |last1=Schröder |first1=K.-P. |last2=Connon Smith |first2=Robert |date=May 2008 |title=Distant future of the Sun and Earth revisited |journal=Monthly Notices of the Royal Astronomical Society |volume=386 |issue=1 |pages=155–163 |arxiv=0801.4031 |bibcode=2008MNRAS.386..155S |doi=10.1111/j.1365-2966.2008.13022.x |s2cid=10073988}}</ref> Solar wind from the Sun created the [[Astronomy:Heliosphere|heliosphere]] and swept away the remaining gas and dust from the protoplanetary disc into interstellar space.<ref name=":4" />

The Solar System will remain roughly as it is known today until the hydrogen in the core of the Sun has been entirely converted to helium, which will occur roughly 5&nbsp;billion years from now. This will mark the end of the Sun's main-sequence life. At that time, the core of the Sun will contract with hydrogen fusion occurring along a shell surrounding the inert helium, and the energy output will be greater than at present. The outer layers of the Sun will expand to roughly 260 times its current diameter, and the Sun will become a [[Astronomy:Red giant|red giant]]. Because of its increased surface area, the surface of the Sun will be cooler ({{Convert|2,600|K}} at its coolest) than it is on the main sequence.<ref name="mnras386_1"/>

[[File:Solar Life Cycle.svg|center|thumb|500x500px|Overview of the evolution of the Sun, a [[Astronomy:G-type main-sequence star|G-type main-sequence star]]. Around 11 billion years after being formed by the Solar System's protoplanetary disk, the Sun will expand to become a red giant; Mercury, Venus and possibly the Earth will be swallowed.]]

The expanding Sun is expected to vaporize Mercury as well as Venus, and render Earth uninhabitable (possibly destroying it as well). Eventually, the core will be hot enough for helium fusion; the Sun will burn helium for a fraction of the time it burned hydrogen in the core. The Sun is not massive enough to commence the fusion of heavier elements, and nuclear reactions in the core will dwindle. Its outer layers will be ejected into space, leaving behind a dense [[Astronomy:White dwarf|white dwarf]], half the original mass of the Sun but only the size of Earth.<ref name="mnras386_1"/> The ejected outer layers will form what is known as a [[Astronomy:Planetary nebula|planetary nebula]], returning some of the material that formed the Sun—but now enriched with [[Astronomy:Metallicity|heavier elements]] like carbon—to the interstellar medium.<ref>{{Cite web |last=Wendel |first=JoAnna |date=8 January 2022 |title=When will the Sun die? |url=https://www.space.com/14732-sun-burns-star-death.html |url-status=live |archive-url=https://web.archive.org/web/20220309172649/https://www.space.com/14732-sun-burns-star-death.html |archive-date=9 March 2022 |access-date=10 March 2022 |website=Space.com}}</ref><ref>{{Cite journal|url=https://www.nature.com/articles/s41550-018-0453-9.epdf?sharing_token=XozRTVzMDBR74HQBj2lbV9RgN0jAjWel9jnR3ZoTv0OzwWt8mLOdVW4Y_YiE39Le3Xp-8zVx5tUnLpAORu5j1mnJNZpxp_fWsbZgn60hEE3IHsu89UrtgD6uRRVi7jD74SBwEYsmB2RyB2RCfRqLbLr5EqTy1-rK2KrrLO-TxuHwLmapWXxYkuOn5Rgut4w4JuE1XKNeJeRNDNx_0juT0bPlXn9WB29_BzKx1pGlzEXtR677aZ3SUe5um8epWM4PgYT-VDXR6Jevm-M9SDszF4a2eWOeV0CdynDONJuE1n37sanK9itS1edHH_xrybrldJgWdACO4sxHnFn3DHdB0Q==|title=The mysterious age invariance of the planetary nebula luminosity function bright cut-off|first1=K.|last1=Gesicki|first2=A. A.|last2=Zijlstra|first3=M. M.|last3=Miller Bertolami|date=7 May 2018|journal=Nature Astronomy|volume=2|issue=7|pages=580–584|doi=10.1038/s41550-018-0453-9|arxiv=1805.02643 |bibcode=2018NatAs...2..580G |s2cid=256708667 }}</ref>

==Structure and composition==
The Sun is the dominant gravitational member of the Solar System, and its [[Astronomy:Planetary system|planetary system]] is maintained in a relatively stable, slowly evolving state by following isolated, gravitationally bound orbits around the Sun.<ref>{{Cite journal |last1=Malhotra |first1=R.|last2=Holman |first2=Matthew |last3=Ito |first3=Takashi |date=October 2001 |title=Chaos and stability of the solar system |journal=Proceedings of the National Academy of Sciences |volume=98 |issue=22 |pages=12342–12343 |bibcode=2001PNAS...9812342M |doi=10.1073/pnas.231384098 |pmc=60054 |pmid=11606772 |doi-access=free}}</ref> Although the Solar System has been fairly stable for billions of years, it is technically chaotic, and may eventually be disrupted (see [[Astronomy:Stability of the Solar System|Stability of the Solar System]]). There is also a small chance that another star will pass through the Solar System in the next billion years. Although this could destabilize the system and eventually lead millions of years later to expulsion of planets, collisions of planets, or planets hitting the Sun, it would most likely leave the Solar System much as it is today.<ref>{{cite journal|display-authors=0 |last1=Sean Raymond |title=Future trajectories of the Solar System: dynamical simulations of stellar encounters within 100 au |journal=Monthly Notices of the Royal Astronomical Society |date=Nov 27, 2023 |volume=527 |issue=3 |pages=6126–6138 |doi=10.1093/mnras/stad3604 |url=https://academic.oup.com/mnras/article/527/3/6126/7452883?login=false|doi-access=free |arxiv=2311.12171 }}</ref>

The overall structure of the charted regions of the Solar System consists of the Sun, four smaller inner planets surrounded by a belt of mostly rocky asteroids, and four giant planets surrounded by the Kuiper belt of mostly icy objects. Astronomers sometimes informally divide this structure into separate regions. The inner Solar System includes the four terrestrial planets and the asteroid belt. The outer Solar System is beyond the asteroids, including the four giant planets.<ref>{{Cite web |title=The Solar System |url=http://www.nineplanets.org/overview.html |url-status=live |archive-url=https://web.archive.org/web/20001017191001/http://www.nineplanets.org/overview.html |archive-date=17 October 2000 |access-date=15 February 2007 |website=Nine Planets}}</ref> Since the discovery of the Kuiper belt, the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune.<ref>{{Cite web |last=Alexander |first=Amir |date=2006 |title=New Horizons Set to Launch on 9-Year Voyage to Pluto and the Kuiper Belt |url=http://www.planetary.org/news/2006/0116_New_Horizons_Set_to_Launch_on_9_Year.html |url-status=dead |archive-url=https://web.archive.org/web/20060222080327/http://www.planetary.org/news/2006/0116_New_Horizons_Set_to_Launch_on_9_Year.html |archive-date=22 February 2006 |access-date=8 November 2006 |website=The Planetary Society}}</ref>[[File:Solar System true color (captions).jpg|center|thumb|600x600px|The Sun's, planets', dwarf planets' and moons' size to scale, labelled. Distance of objects is not to scale. The asteroid belt lies between the orbits of Mars and Jupiter, the Kuiper belt lies beyond Neptune's orbit.]]
=== Orbits ===
{{Multiple image
| align             = right
| direction         = vertical
| image1            = Solar system orrery inner planets.gif
| image2            = Solar system orrery outer planets.gif
| footer            = Animations of the Solar System's inner planets and outer planets orbiting; the latter animation is 100 times faster than the former. Jupiter is three times as far from the Sun as Mars.
| total_width       = 300
}}

The planets and other large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the [[Astronomy:Ecliptic|ecliptic]]. Smaller icy objects such as comets frequently orbit at significantly greater angles to this plane.<ref name="Levison2003">{{Cite journal |last1=Levison |first1=H.F. |last2=Morbidelli |first2=A. |date=27 November 2003 |title=The formation of the Kuiper belt by the outward transport of bodies during Neptune's migration |journal=Nature |volume=426 |issue=6965 |pages=419–421 |bibcode=2003Natur.426..419L |doi=10.1038/nature02120 |pmid=14647375 |s2cid=4395099}}</ref><ref>{{Cite journal |last1=Levison |first1=Harold F. |last2=Duncan |first2=Martin J. |date=1997 |title=From the Kuiper Belt to Jupiter-Family Comets: The Spatial Distribution of Ecliptic Comets |journal=Icarus |volume=127 |issue=1 |pages=13–32 |bibcode=1997Icar..127...13L |doi=10.1006/icar.1996.5637}}</ref> Most of the planets in the Solar System have secondary systems of their own, being orbited by natural satellites called moons. Many of the largest natural satellites are in synchronous rotation, with one face permanently turned toward their parent. The four giant planets have planetary rings, thin bands of tiny particles that orbit them in unison.<ref name="bennett_4.5">{{Cite book |last1=Bennett |first1=Jeffrey O. |title=The Cosmic Perspective |last2=Donahue |first2=Megan |last3=Schneider |first3=Nicholas |last4=Voit |first4=Mark |date=2020 |publisher=Pearson |isbn=978-0-134-87436-4 |edition=9th |location=Hoboken, NJ |chapter=4.5 Orbits, Tides, and the Acceleration of Gravity |oclc=1061866912 |author-link2=Megan Donahue}}</ref>

As a result of the [[Astronomy:Formation and evolution of the Solar System|formation of the Solar System]], planets and most other objects orbit the Sun in the same direction that the Sun is rotating. That is, counter-clockwise, as viewed from above Earth's north pole.<ref>{{Cite magazine |last=Grossman |first=Lisa |date=13 August 2009 |title=Planet found orbiting its star backwards for first time |url=https://www.newscientist.com/article/dn17603-planet-found-orbiting-its-star-backwards-for-first-time.html |url-status=live |magazine=New Scientist |archive-url=https://web.archive.org/web/20121017083955/http://www.newscientist.com/article/dn17603-planet-found-orbiting-its-star-backwards-for-first-time.html |archive-date=17 October 2012 |access-date=10 October 2009}}</ref> There are exceptions, such as [[Astronomy:Halley's Comet|Halley's Comet]].<ref>{{Cite web |last=Nakano |first=Syuichi |date=2001 |title=OAA computing section circular |url=http://www.oaa.gr.jp/~oaacs/nk/nk866.htm |url-status=live |archive-url=https://web.archive.org/web/20190921103057/http://www.oaa.gr.jp/~oaacs/nk/nk866.htm |archive-date=21 September 2019 |access-date=15 May 2007 |publisher=Oriental Astronomical Association}}</ref> Most of the larger moons orbit their planets in [[Astronomy:Retrograde and prograde motion|prograde]] direction, matching the planetary rotation; Neptune's moon [[Astronomy:Triton (moon)|Triton]] is the largest to orbit in the opposite, [[Astronomy:Retrograde and prograde motion|retrograde]] manner.<ref>{{Cite journal |last1=Agnor |first1=Craig B. |last2=Hamilton |first2=Douglas P. |date=May 2006 |title=Neptune's capture of its moon Triton in a binary–planet gravitational encounter |url=https://www.nature.com/articles/nature04792 |url-status=live |journal=Nature |language=en |volume=441 |issue=7090 |pages=192–194 |bibcode=2006Natur.441..192A |doi=10.1038/nature04792 |issn=1476-4687 |pmid=16688170 |s2cid=4420518 |archive-url=https://web.archive.org/web/20220415081402/https://www.nature.com/articles/nature04792 |archive-date=15 April 2022 |access-date=28 March 2022}}</ref> Most larger objects rotate around their own axes in the prograde direction relative to their orbit, though the rotation of Venus is retrograde.<ref>{{Cite book |last=Gallant |first=Roy A. |url=https://www.worldcat.org/oclc/6533014 |title=National Geographic Picture Atlas of Our Universe |date=1980 |publisher=National Geographic Society |isbn=0-87044-356-9 |editor-last=Sedeen |editor-first=Margaret |location=Washington, D.C. |pages=82 |oclc=6533014 |access-date=28 March 2022 |archive-url=https://web.archive.org/web/20220420161217/https://www.worldcat.org/title/national-geographic-picture-atlas-of-our-universe/oclc/6533014 |archive-date=20 April 2022 |url-status=live}}</ref>

To a good first approximation, [[Kepler's laws of planetary motion]] describe the orbits of objects around the Sun.<ref name=":0">{{Cite book |last1=Frautschi |first1=Steven C. |title=The Mechanical Universe: Mechanics and Heat |title-link=The Mechanical Universe |last2=Olenick |first2=Richard P. |last3=Apostol |first3=Tom M. |last4=Goodstein |first4=David L. |date=2007 |publisher=Cambridge University Press |isbn=978-0-521-71590-4 |edition=Advanced |location=Cambridge [Cambridgeshire] |oclc=227002144 |author-link3=Tom M. Apostol |author-link4=David L. Goodstein}}</ref>{{Rp|pages=433–437}} These laws stipulate that each object travels along an [[Ellipse|ellipse]] with the Sun at one [[Focus (geometry)|focus]], which causes the body's distance from the Sun to vary over the course of its year. A body's closest approach to the Sun is called its ''perihelion'', whereas its most distant point from the Sun is called its ''aphelion''.<ref name=":8">{{Cite book |last1=Feynman |first1=Richard P. |title=The Feynman Lectures on Physics, Volume 1 |title-link=The Feynman Lectures on Physics |last2=Leighton |first2=Robert B. |last3=Sands |first3=Matthew L. |date=1989 |publisher=Addison-Wesley Pub. Co |isbn=0-201-02010-6 |location=Reading, Mass. |oclc=531535 |author-link=Richard Feynman |author-link2=Robert B. Leighton |author-link3=Matthew Sands |orig-date=1965}}</ref>{{Rp|location=9-6}} With the exception of Mercury, the orbits of the planets are nearly circular, but many comets, asteroids, and Kuiper belt objects follow highly elliptical orbits. Kepler's laws only account for the influence of the Sun's gravity upon an orbiting body, not the gravitational pulls of different bodies upon each other. On a human time scale, these additional perturbations can be accounted for using [[Astronomy:Numerical model of the Solar System|numerical models]],<ref name=":8" />{{Rp|location=9-6}} but the planetary system can change chaotically over billions of years.<ref>{{Cite journal |last1=Lecar |first1=Myron |last2=Franklin |first2=Fred A. |last3=Holman |first3=Matthew J. |last4=Murray |first4=Norman J. |date=2001 |title=Chaos in the Solar System |journal=Annual Review of Astronomy and Astrophysics |volume=39 |pages=581–631 |arxiv=astro-ph/0111600 |bibcode=2001ARA&A..39..581L |doi=10.1146/annurev.astro.39.1.581 |s2cid=55949289}}</ref>

The [[Physics:Angular momentum|angular momentum]] of the Solar System is a measure of the total amount of orbital and rotational momentum possessed by all its moving components.<ref>{{Cite book |last=Piccirillo |first=Lucio |url=https://books.google.com/books?id=W0jpDwAAQBAJ&pg=PA210 |title=Introduction to the Maths and Physics of the Solar System |date=2020 |publisher=CRC Press |isbn=978-0429682803 |page=210 |access-date=10 May 2022 |archive-url=https://web.archive.org/web/20220730084321/https://www.google.com/books/edition/Introduction_to_the_Maths_and_Physics_of/W0jpDwAAQBAJ?gbpv=1&pg=PA210 |archive-date=30 July 2022 |url-status=live}}</ref> Although the Sun dominates the system by mass, it accounts for only about 2% of the angular momentum.<ref name="Marochnik1995">{{Cite conference |last1=Marochnik |first1=L. |last2=Mukhin |first2=L. |date=1995 |title=Is Solar System Evolution Cometary Dominated? |series=Astronomical Society of the Pacific Conference Series |volume=74 |page=83 |bibcode=1995ASPC...74...83M |isbn=0-937707-93-7 |book-title=Progress in the Search for Extraterrestrial Life |editor=Shostak, G.S.}}</ref><ref>{{Cite journal |last1=Bi |first1=S. L. |last2=Li |first2=T. D. |last3=Li |first3=L. H. |last4=Yang |first4=W. M. |year=2011 |title=Solar Models with Revised Abundance |journal=The Astrophysical Journal |volume=731 |issue=2 |pages=L42 |arxiv=1104.1032 |bibcode=2011ApJ...731L..42B |doi=10.1088/2041-8205/731/2/L42 |s2cid=118681206}}</ref> The planets, dominated by Jupiter, account for most of the rest of the angular momentum due to the combination of their mass, orbit, and distance from the Sun, with a possibly significant contribution from comets.<ref name="Marochnik1995" />

=== Composition ===
The principal component of the Solar System is the Sun, a low-mass star{{Efn|"Low-mass" is a relative term; the Sun is still more massive than 95% of stars in the galaxy.<ref>{{cite journal|journal=The Astrophysical Journal|title=A Comprehensive Comparison of the Sun to Other Stars: Searching for Self-Selection Effects|volume=684|year=2008|number=691|doi=10.1086/589985 |last1=Robles |first1=José A. |last2=Lineweaver |first2=Charles H. |last3=Grether |first3=Daniel |last4=Flynn |first4=Chris |last5=Egan |first5=Chas A. |last6=Pracy |first6=Michael B. |last7=Holmberg |first7=Johan |last8=Gardner |first8=Esko |pages=691–706 |arxiv=0805.2962 |bibcode=2008ApJ...684..691R }}</ref>}} that contains 99.86% of the system's known mass and dominates it gravitationally.<ref>{{Cite journal |last=Woolfson |first=M. |date=2000 |title=The origin and evolution of the solar system |journal=Astronomy & Geophysics |volume=41 |issue=1 |pages=1.12–1.19 |bibcode=2000A&G....41a..12W |doi=10.1046/j.1468-4004.2000.00012.x |doi-access=free}}</ref> The Sun's four largest orbiting bodies, the giant planets, account for 99% of the remaining mass, with Jupiter and Saturn together comprising more than 90%. The remaining objects of the Solar System (including the four terrestrial planets, the dwarf planets, moons, [[Astronomy:Asteroid|asteroid]]s, and comets) together comprise less than 0.002% of the Solar System's total mass.{{Refn |The mass of the Solar System excluding the Sun, Jupiter and Saturn can be determined by adding together all the calculated masses for its largest objects and using rough calculations for the masses of the Oort cloud (estimated at roughly 3 Earth masses),<ref>{{Cite arXiv |eprint=astro-ph/0512256 |first=Alessandro |last=Morbidelli |title=Origin and dynamical evolution of comets and their reservoirs |date=2005}}</ref> the Kuiper belt (estimated at 0.1 Earth mass)<ref name="Delsanti-Beyond_The_Planets"/> and the asteroid belt (estimated to be 0.0005 Earth mass)<ref name="Krasinsky2002"/> for a total, rounded upwards, of ~37 Earth masses, or 8.1% of the mass in orbit around the Sun. With the combined masses of Uranus and Neptune (~31 Earth masses) subtracted, the remaining ~6 Earth masses of material comprise 1.3% of the total orbiting mass.|name=footnoteD|group=lower-alpha}}

The Sun is composed of roughly 98% hydrogen and helium,<ref>{{Cite web |title=The Sun's Vital Statistics |url=http://solar-center.stanford.edu/vitalstats.html |url-status=live |archive-url=https://www.webcitation.org/6BOkQXma3?url=http://solar-center.stanford.edu/vitalstats.html |archive-date=14 October 2012 |access-date=29 July 2008 |publisher=Stanford Solar Center |postscript=,}} citing {{Cite book
|last=Eddy
|first=J.
|title=A New Sun: The Solar Results From Skylab
|url=https://history.nasa.gov/SP-402/contents.htm
|publisher=[[Organization:NASA|NASA]]
|date=1979
|page=37
|id=NASA SP-402
|access-date=12 July 2017
|archive-date=30 July 2021
|archive-url=https://web.archive.org/web/20210730024856/https://history.nasa.gov/SP-402/contents.htm
|url-status=live
}}</ref> as are Jupiter and Saturn.<ref>{{Cite web |last=Williams |first=David R. |date=7 September 2006 |title=Saturn Fact Sheet |url=http://nssdc.gsfc.nasa.gov/planetary/factsheet/saturnfact.html |url-status=dead |archive-url=https://web.archive.org/web/20110804224236/http://nssdc.gsfc.nasa.gov/planetary/factsheet/saturnfact.html |archive-date=4 August 2011 |access-date=31 July 2007 |publisher=NASA |df=dmy-all}}</ref><ref name="Williams-Jupiter" /> A composition gradient exists in the Solar System, created by heat and light pressure from the early Sun; those objects closer to the Sun, which are more affected by heat and light pressure, are composed of elements with high melting points. Objects farther from the Sun are composed largely of materials with lower melting points.<ref>{{Cite book |last1=Weissman |first1=Paul Robert |url=https://archive.org/details/encyclopediaofso0000unse_u6d1/page/615 |title=Encyclopedia of the solar system |last2=Johnson |first2=Torrence V. |date=2007 |publisher=Academic Press |isbn=978-0-12-088589-3 |page=[https://archive.org/details/encyclopediaofso0000unse_u6d1/page/615 615]}}</ref> The boundary in the Solar System beyond which those volatile substances could coalesce is known as the [[Physics:Frost line (astrophysics)|frost line]], and it lies at roughly five times the Earth's distance from the Sun.<ref name="Mumma" />

The objects of the inner Solar System are composed mostly of rocky materials,<ref name="Podolak Weizman et al. 1995">{{Cite journal |last1=Podolak |first1=M. |last2=Weizman |first2=A. |last3=Marley |first3=M. |date=December 1995 |title=Comparative models of Uranus and Neptune |journal=Planetary and Space Science |volume=43 |issue=12 |pages=1517–1522 |bibcode=1995P&SS...43.1517P |doi=10.1016/0032-0633(95)00061-5 |ref={{SfnRef|Podolak Weizman et al.|1995}}}}</ref> such as silicates, iron or nickel.<ref name="Podolak Podolak et al. 2000">{{Cite journal |last1=Podolak |first1=M. |last2=Podolak |first2=J. I. |last3=Marley |first3=M. S. |date=February 2000 |title=Further investigations of random models of Uranus and Neptune |url=https://zenodo.org/record/998024 |url-status=live |journal=Planetary and Space Science |volume=48 |issue=2–3 |pages=143–151 |bibcode=2000P&SS...48..143P |doi=10.1016/S0032-0633(99)00088-4 |archive-url=https://web.archive.org/web/20191221231229/https://zenodo.org/record/998024 |archive-date=21 December 2019 |access-date=25 August 2019 |ref={{SfnRef|Podolak Podolak et al.|2000}}}}</ref> Jupiter and Saturn are composed mainly of gases with extremely low melting points and high [[Physics:Vapor pressure|vapor pressure]], such as hydrogen, helium, and [[Chemistry:Neon|neon]].<ref name="Podolak Podolak et al. 2000" /> Ices, like water, methane, ammonia, [[Chemistry:Hydrogen sulfide|hydrogen sulfide]], and [[Chemistry:Carbon dioxide|carbon dioxide]],<ref name="Podolak Weizman et al. 1995" /> have a melting points of up to a few hundred kelvins.<ref name="Podolak Podolak et al. 2000" /> They can be found as ices, liquids, or gases in various places in the Solar System.<ref name="Podolak Podolak et al. 2000" /> Icy substances comprise the majority of the satellites of the giant planets, as well as most of Uranus and Neptune (the so-called "[[Astronomy:Ice giant|ice giant]]s") and the numerous small objects that lie beyond Neptune's orbit.<ref name="Podolak Weizman et al. 1995" /><ref name="zeilik">{{Cite book |last=Zellik |first=Michael |title=Astronomy: The Evolving Universe |date=2002 |publisher={{wipe|Cambridge University Press}} |isbn=978-0-521-80090-7 |edition=9th |page=240 |oclc=223304585}}</ref> Together, gases and ices are referred to as ''[[Earth:Volatile (astrogeology)|volatiles]]''.<ref name="Placxo">{{Cite book |last1=Placxo |first1=Kevin W. |url=https://books.google.com/books?id=2JuGDL144BEC&q=inventory%20volatiles%20hydrogen&pg=PA66 |title=Astrobiology: a brief introduction |last2=Gross |first2=Michael |date=2006 |publisher=JHU Press |isbn=978-0-8018-8367-5 |page=66 |access-date=12 November 2020 |archive-url=https://web.archive.org/web/20210806141003/https://books.google.com/books?id=2JuGDL144BEC&q=inventory%20volatiles%20hydrogen&pg=PA66 |archive-date=6 August 2021 |url-status=live}}</ref>

=== Distances and scales ===
[[File:Solar System distance to scale.svg|center|thumb|600x600px|To-scale diagram of distance between planets, with the white bar showing orbital variations. The size of the planets is not to scale.]]
[[File:Elements of astronomy- accompanied with numerous illustrations, a colored representation of the solar, stellar, and nebular spectra, and celestial charts of the northern and the southern hemisphere (14804687203).jpg|thumb|right| Apparent size of the Sun as seen from the Solar System's planets]]
The [[Astronomy:Astronomical unit|astronomical unit]] [AU] ({{Convert|1|AU|km mi|lk=off|disp=out|abbr=~}}) would be the distance from the Earth to the Sun if the planet's orbit were perfectly circular.<ref>{{Cite journal |last=Standish |first=E. M. |date=April 2005 |title=The Astronomical Unit now |journal=Proceedings of the International Astronomical Union |volume=2004 |pages=163–179 |bibcode=2005tvnv.conf..163S |doi=10.1017/S1743921305001365 |s2cid=55944238 |doi-access=free}}</ref> For comparison, the radius of the Sun is {{Cvt|0.0047|AU|km mi|sigfig=1|abbr=unit}}.<ref name="arxiv1203_4898">{{Cite journal |last1=Emilio |first1=Marcelo |last2=Kuhn |first2=Jeff R. |last3=Bush |first3=Rock I. |last4=Scholl |first4=Isabelle F. |year=2012 |title=Measuring the Solar Radius from Space during the 2003 and 2006 Mercury Transits |journal=The Astrophysical Journal |volume=750 |issue=2 |page=135 |arxiv=1203.4898 |bibcode=2012ApJ...750..135E |doi=10.1088/0004-637X/750/2/135 |s2cid=119255559}}</ref> Thus, the Sun occupies 0.00001% (10<sup>−5</sup> %) of the volume of a sphere with a radius the size of Earth's orbit, whereas Earth's volume is roughly one millionth (10<sup>−6</sup>) that of the Sun. Jupiter, the largest planet, is {{Convert|5.2|AU|km mi}} from the Sun and has a radius of {{Convert|71000|km|AU mi|abbr=on|sigfig=2}}, whereas the most distant planet, Neptune, is {{Convert|30|AU|km mi|abbr=on|sigfig=2}} from the Sun.<ref name="Williams-Jupiter">{{Cite web |last=Williams |first=David R. |date=23 December 2021 |title=Jupiter Fact Sheet |url=https://nssdc.gsfc.nasa.gov/planetary/factsheet/jupiterfact.html |url-status=live |archive-url=https://web.archive.org/web/20180122180353/https://nssdc.gsfc.nasa.gov/planetary/factsheet/jupiterfact.html |archive-date=22 January 2018 |access-date=28 March 2022 |website=NASA Goddard Space Flight Center}}</ref><ref>{{Cite web |last=Williams |first=David R. |date=23 December 2021 |title=Neptune Fact Sheet |url=https://nssdc.gsfc.nasa.gov/planetary/factsheet/neptunefact.html |url-status=live |archive-url=https://web.archive.org/web/20161119045252/http://nssdc.gsfc.nasa.gov/planetary/factsheet/neptunefact.html |archive-date=19 November 2016 |access-date=28 March 2022 |website=NASA Goddard Space Flight Center}}</ref>

With a few exceptions, the farther a planet or belt is from the Sun, the larger the distance between its orbit and the orbit of the next nearest object to the Sun. For example, Venus is approximately 0.33 AU farther out from the Sun than Mercury, whereas Saturn is 4.3 AU out from Jupiter, and Neptune lies 10.5 AU out from Uranus. Attempts have been made to determine a relationship between these orbital distances, like the [[Astronomy:Titius–Bode law|Titius–Bode law]]<ref>{{Cite journal |last=Jaki |first=Stanley L. |date=1 July 1972 |title=The Early History of the Titius-Bode Law |url=https://aapt.scitation.org/doi/abs/10.1119/1.1986734 |url-status=live |journal=American Journal of Physics |volume=40 |issue=7 |pages=1014–1023 |bibcode=1972AmJPh..40.1014J |doi=10.1119/1.1986734 |issn=0002-9505 |archive-url=https://web.archive.org/web/20220420161227/https://aapt.scitation.org/doi/abs/10.1119/1.1986734 |archive-date=20 April 2022 |access-date=2 April 2022}}</ref> and [[Unsolved:Mysterium Cosmographicum|Johannes Kepler's model]] based on the [[Platonic solid]]s,<ref>{{Cite journal |last=Phillips |first=J. P. |date=1965 |title=Kepler's Echinus |journal=Isis |volume=56 |issue=2 |pages=196–200 |doi=10.1086/349957 |issn=0021-1753 |jstor=227915 |s2cid=145268784}}</ref> but ongoing discoveries have invalidated these hypotheses.<ref name="Boss">{{Cite magazine |last=Boss |first=Alan |date=October 2006 |title=Is it a coincidence that most of the planets fall within the Titius-Bode law's boundaries? |url=https://astronomy.com/magazine/ask-astro/2006/10/is-it-a-coincidence-that-most-of-the-planets-fall-within-the-titius-bode-laws-boundaries |url-status=live |magazine=Astronomy |volume=30 |issue=10 |page=70 |archive-url=https://web.archive.org/web/20220316135255/https://astronomy.com/magazine/ask-astro/2006/10/is-it-a-coincidence-that-most-of-the-planets-fall-within-the-titius-bode-laws-boundaries |archive-date=16 March 2022 |access-date=9 April 2022 |series=Ask Astro}}</ref>

Some [[Astronomy:Solar System model|Solar System model]]s attempt to convey the relative scales involved in the Solar System in human terms. Some are small in scale (and may be mechanical—called [[Astronomy:Orrery|orreries]])—whereas others extend across cities or regional areas.<ref>{{Cite web |last=Ottewell |first=Guy |date=1989 |title=The Thousand-Yard Model: or, Earth as a Peppercorn |url=http://www.noao.edu/education/peppercorn/pcmain.html |url-status=dead |archive-url=https://web.archive.org/web/20160710065429/http://www.noao.edu/education/peppercorn/pcmain.html |archive-date=10 July 2016 |access-date=10 May 2012 |website=NOAO Educational Outreach Office}}</ref> The largest such scale model, the [[Astronomy:Sweden Solar System|Sweden Solar System]], uses the 110-metre (361&nbsp;ft) Avicii Arena in [[Place:Stockholm|Stockholm]] as its substitute Sun, and, following the scale, Jupiter is a 7.5-metre (25-foot) sphere at Stockholm Arlanda Airport, 40&nbsp;km (25&nbsp;mi) away, whereas the farthest current object, [[Astronomy:90377 Sedna|Sedna]], is a 10&nbsp;cm (4&nbsp;in) sphere in Luleå, 912&nbsp;km (567&nbsp;mi) away.<ref>{{Cite web |title=Tours of Model Solar Systems |url=http://internal.psychology.illinois.edu/~wbrewer/solarmodel.html |url-status=dead |archive-url=https://web.archive.org/web/20110412124455/http://internal.psychology.illinois.edu/~wbrewer/solarmodel.html |archive-date=12 April 2011 |access-date=10 May 2012 |publisher=University of Illinois}}</ref><ref name="Sedna">{{Cite web |title=Luleå är Sedna. I alla fall om vår sol motsvaras av Globen i Stockholm |url=http://www.kuriren.nu/arkiv/2005/11/17/Lokalt/1510647/Lule%C3%A5-%C3%A4r-Sedna.aspx |url-status=dead |archive-url=https://web.archive.org/web/20100715074955/http://www.kuriren.nu/arkiv/2005/11/17/Lokalt/1510647/Lule%C3%A5-%C3%A4r-Sedna.aspx |archive-date=15 July 2010 |access-date=10 May 2010 |publisher=Norrbotten Kuriren (in Swedish)}}</ref>

If the Sun–Neptune distance is scaled to {{Convert|100|m|4=-1}}, then the Sun would be about {{Convert|3|cm|abbr=on}} in diameter (roughly two-thirds the diameter of a golf ball), the giant planets would be all smaller than about {{Convert|3|mm|abbr=on}}, and Earth's diameter along with that of the other terrestrial planets would be smaller than a flea ({{Convert|0.3|mm|abbr=on|disp=or}}) at this scale.<ref>See, for example, {{Cite web |last=Office of Space Science |date=9 July 2004 |title=Solar System Scale |url=http://www.nasa.gov/audience/foreducators/5-8/features/F_Solar_System_Scale.html |url-status=live |archive-url=https://web.archive.org/web/20160827184323/http://www.nasa.gov/audience/foreducators/5-8/features/F_Solar_System_Scale.html |archive-date=27 August 2016 |access-date=2 April 2013 |website=NASA Educator Features}}</ref>

=== Interplanetary environment ===
[[File:Coronal mass ejection on 28 October 2021 ESA25032924.gif|thumb|Coronal mass ejections play a dominant role in the Solar System's environment and space weather in general.]]
The outermost layer of the Solar atmosphere is the [[Astronomy:Heliosphere|heliosphere]], which permeates much of the Solar [[Astronomy:Planetary system|planetary system]]. Along with [[Astronomy:Sunlight|light]], the Sun radiates a continuous stream of charged particles (a [[Physics:Plasma|plasma]]) called the [[Astronomy:Solar wind|solar wind]]. This stream of particles spreads outwards at speeds from {{Convert|900000|km/h|mph}} to {{Convert|2880000|km/h|mph}},<ref>{{Cite book |last=Kallenrode |first=May-Britt |url=https://www.worldcat.org/oclc/53443301 |title=Space Physics: An introduction to plasmas and particles in the heliosphere and magnetospheres |date=2004 |publisher=Springer |isbn=978-3-540-20617-0 |edition=3rd |location=Berlin |pages=150 |oclc=53443301 |access-date=1 April 2022 |archive-url=https://web.archive.org/web/20220420161223/https://www.worldcat.org/title/space-physics-an-introduction-to-plasmas-and-particles-in-the-heliosphere-and-magnetospheres/oclc/53443301 |archive-date=20 April 2022 |url-status=live}}</ref> filling the vacuum between the bodies of the Solar System. The result is a [[Physics:Vacuum|thin]], dusty atmosphere, called the [[Astronomy:Interplanetary medium|interplanetary medium]], which extends to at least {{Convert|100|AU|e9km e9mi|abbr=unit}}. Beyond the heliosphere, large objects remain gravitationally bound to the sun, but the flow of matter in the [[Astronomy:Interstellar medium|interstellar medium]] homogenizes the distribution of micro-scale objects {{See below|{{Slink||Farthest regions}}}}.<ref name="Voyager"/>

The interplanetary medium is home to at least two disc-like regions of [[Astronomy:Cosmic dust|cosmic dust]]. The first, the [[Astronomy:Interplanetary dust cloud|zodiacal dust cloud]], lies in the inner Solar System and causes the [[Astronomy:Zodiacal light|zodiacal light]]. It may have been formed by collisions within the asteroid belt brought on by gravitational interactions with the planets; a more recent proposed origin is the planet Mars.<ref>{{Cite journal |last1=Jorgensen |first1=J. L. |last2=Benn |first2=M. |last3=Connerney |first3=J. E. P. |last4=Denver |first4=T. |last5=Jorgensen |first5=P. S. |last6=Andersen |first6=A. C. |last7=Bolton |first7=S. J. |date=March 2021 |title=Distribution of Interplanetary Dust Detected by the Juno Spacecraft and Its Contribution to the Zodiacal Light |journal=Journal of Geophysical Research: Planets |language=en |volume=126 |issue=3 |bibcode=2021JGRE..12606509J |doi=10.1029/2020JE006509 |issn=2169-9097 |s2cid=228840132 |doi-access=free}}</ref> The second dust cloud extends from about {{Convert|10|AU|e9km e6mi|abbr=unit}} to about {{Convert|40|AU|e9km e9mi|abbr=unit}}, and was probably created by collisions within the Kuiper belt.<ref>{{Cite web |date=2003 |title=ESA scientist discovers a way to shortlist stars that might have planets |url=http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=29471 |url-status=live |archive-url=https://web.archive.org/web/20130502033116/http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=29471 |archive-date=2 May 2013 |access-date=3 February 2007 |website=ESA Science and Technology}}</ref><ref>{{Cite journal |last1=Landgraf |first1=M. |last2=Liou |first2=J.-C. |last3=Zook |first3=H. A. |last4=Grün |first4=E. |date=May 2002 |title=Origins of Solar System Dust beyond Jupiter |url=http://astron.berkeley.edu/~kalas/disksite/library/ladgraf02.pdf |url-status=live |journal=[[Astronomy:The Astronomical Journal|The Astronomical Journal]] |volume=123 |issue=5 |pages=2857–2861 |arxiv=astro-ph/0201291 |bibcode=2002AJ....123.2857L |doi=10.1086/339704 |s2cid=38710056 |archive-url=http://arquivo.pt/wayback/20160515115002/http://astron.berkeley.edu/~kalas/disksite/library/ladgraf02.pdf |archive-date=15 May 2016 |access-date=9 February 2007}}</ref>

Activity on the Sun's surface, such as [[Astronomy:Solar flare|solar flare]]s and [[Astronomy:Coronal mass ejection|coronal mass ejection]]s, disturbs the heliosphere, creating [[Astronomy:Space weather|space weather]] and causing [[Physics:Geomagnetic storm|geomagnetic storm]]s.<ref name="SunFlip">{{Cite web |last=Phillips |first=Tony |date=15 February 2001 |title=The Sun Does a Flip |url=https://science.nasa.gov/science-news/science-at-nasa/2001/ast15feb_1 |url-status=live |archive-url=https://web.archive.org/web/20220401050813/https://science.nasa.gov/science-news/science-at-nasa/2001/ast15feb_1 |archive-date=1 April 2022 |access-date=1 April 2022 |website=NASA Science: Share the Science}}</ref> Coronal mass ejections and similar events blow a magnetic field and huge quantities of material from the surface of the Sun. The interaction of this magnetic field and material with Earth's magnetic field funnels charged particles into Earth's upper atmosphere, where its interactions create aurorae seen near the [[Earth:Earth's magnetic field#Magnetic poles|magnetic poles]].<ref>{{Cite book |last1=Fraknoi |first1=Andrew |url=https://www.worldcat.org/oclc/961476196 |title=Astronomy |last2=Morrison |first2=David |last3=Wolff |first3=Sidney C. |last4=Beck |first4=John |date=2022 |publisher=OpenStax |isbn=978-1-947-17224-1 |location=Houston, Texas |chapter=15.4 Space weather |oclc=961476196 |display-authors=3 |access-date=9 March 2022 |orig-date=2016 |chapter-url=https://openstax.org/books/astronomy/pages/15-4-space-weather |archive-url=https://web.archive.org/web/20200719090803/http://worldcat.org/oclc/961476196 |archive-date=19 July 2020 |url-status=live}}</ref> The largest stable structure within the heliosphere is the [[Astronomy:Heliospheric current sheet|heliospheric current sheet]], a spiral form created by the actions of the Sun's rotating magnetic field on the interplanetary medium.<ref>{{Cite web |date=22 April 2003 |title=A Star with two North Poles |url=https://science.nasa.gov/science-news/science-at-nasa/2003/22apr_currentsheet |url-status=live |archive-url=https://web.archive.org/web/20220401192948/https://science.nasa.gov/science-news/science-at-nasa/2003/22apr_currentsheet |archive-date=1 April 2022 |access-date=1 April 2022 |website=NASA Science: Share the Science}}</ref><ref>{{Cite journal |last=Riley |first=Pete |date=2002 |title=Modeling the heliospheric current sheet: Solar cycle variations |journal=[[Earth:Journal of Geophysical Research|Journal of Geophysical Research]] |volume=107 |issue=A7 |page=1136 |bibcode=2002JGRA..107.1136R |doi=10.1029/2001JA000299 |doi-access=free}}</ref>

=== Habitability ===
{{Main|Unsolved:Planetary habitability in the Solar System}}
Besides solar energy, the primary characteristic of the Solar System enabling the presence of life is the heliosphere and planetary magnetic fields (for those planets that have them). These magnetic fields partially shield the Solar System from high-energy interstellar particles called [[Astronomy:Cosmic ray|cosmic ray]]s. The density of cosmic rays in the [[Astronomy:Interstellar medium|interstellar medium]] and the strength of the Sun's magnetic field change on very long timescales, so the level of cosmic-ray penetration in the Solar System varies, though by how much is unknown.<ref name="Langner_et_al_2005">{{Cite journal |last1=Langner |first1=U. W. |last2=Potgieter |first2=M. S. |date=2005 |title=Effects of the position of the solar wind termination shock and the heliopause on the heliospheric modulation of cosmic rays |journal=[[Astronomy:Advances in Space Research|Advances in Space Research]] |volume=35 |issue=12 |pages=2084–2090 |bibcode=2005AdSpR..35.2084L |doi=10.1016/j.asr.2004.12.005}}</ref>

[[Earth:Earth's magnetic field|Earth's magnetic field]] also stops its atmosphere from being stripped away by the solar wind.<ref>{{Cite web |last=Dooling |first=Dave |date=8 December 1998 |title=Solar Wind blows some of Earth's atmosphere into space |url=https://science.nasa.gov/science-news/science-at-nasa/1998/ast08dec98_1 |url-status=live |archive-url=https://web.archive.org/web/20220408025859/https://science.nasa.gov/science-news/science-at-nasa/1998/ast08dec98_1 |archive-date=8 April 2022 |access-date=1 April 2022 |website=NASA Science: Share the Science}}</ref> Venus and Mars do not have magnetic fields, and as a result the solar wind causes their atmospheres to gradually bleed away into space.<ref>{{Cite journal |last=Lundin |first=Richard |date=9 March 2001 |title=Erosion by the Solar Wind |journal=Science |volume=291 |issue=5510 |page=1909 |doi=10.1126/science.1059763 |pmid=11245195 |s2cid=128505404}}</ref>

The [[Astronomy:Circumstellar habitable zone|zone of habitability]] of the Solar System is conventionally located in the inner Solar System, where planetary surface or atmospheric temperatures admit the possibility of liquid water.<ref name="NASA-20150407">{{Cite web |last1=Dyches |first1=Preston |last2=Chou |first2=Felcia |date=7 April 2015 |title=The Solar System and Beyond is Awash in Water |url=http://www.nasa.gov/jpl/the-solar-system-and-beyond-is-awash-in-water |access-date=8 April 2015 |website=[[Organization:NASA|NASA]] |archive-date=10 April 2015 |archive-url=https://web.archive.org/web/20150410113514/http://www.nasa.gov/jpl/the-solar-system-and-beyond-is-awash-in-water/ |url-status=dead }}</ref> Habitability might also be possible in subsurface oceans of various outer Solar System moons.<ref>{{Cite book |last1=Robert T. Pappalardo |url=https://books.google.com/books?id=Jpcz2UoXejgC |title=Europa |last2=William B. McKinnon |last3=K. Khurana |publisher=University of Arizona Press |year=2009 |isbn=978-0-8165-2844-8 |page=658}} [https://books.google.com/books?id=Jpcz2UoXejgC&pg=PA658 Extract of page 658]</ref>

== Sun ==
{{Main|Astronomy:Sun}}
[[File:The Sun in white light.jpg|thumb|The Sun in true white color]]
The Sun is the Solar System's star and by far its most massive component. Its large mass (332,900 [[Astronomy:Earth mass|Earth mass]]es),<ref>{{Cite web |title=Sun: Facts & Figures |url=http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sun&Display=Facts&System=Metric |url-status=dead |archive-url=https://web.archive.org/web/20080102034758/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sun&Display=Facts&System=Metric |archive-date=2 January 2008 |access-date=14 May 2009 |publisher=NASA}}</ref> which comprises 99.86% of all the mass in the Solar System,<ref name="Woolfson00">{{Cite journal |last=Woolfson |first=M. |date=2000 |title=The origin and evolution of the solar system |journal=Astronomy & Geophysics |volume=41 |issue=1 |page=12 |bibcode=2000A&G....41a..12W |doi=10.1046/j.1468-4004.2000.00012.x |doi-access=free}}</ref> produces temperatures and densities in its [[Astronomy:Solar core|core]] high enough to sustain nuclear fusion of hydrogen into helium.<ref>{{Cite book |last=Zirker |first=Jack B. |url=https://archive.org/details/journeyfromcente0000zirk |title=Journey from the Center of the Sun |date=2002 |publisher=Princeton University Press |isbn=978-0-691-05781-1 |pages=[https://archive.org/details/journeyfromcente0000zirk/page/120 120–127] |url-access=registration}}</ref> This releases an enormous amount of [[Physics:Energy|energy]], mostly [[Physics:Radiant energy|radiated]] into [[Astronomy:Outer space|space]] as [[Physics:Electromagnetic radiation|electromagnetic radiation]] peaking in [[Physics:Visible light|visible light]].<ref>{{Cite news |title=What Color is the Sun? |work=Universe Today |url=http://www.universetoday.com/18689/color-of-the-sun |url-status=live |access-date=23 May 2016 |archive-url=https://web.archive.org/web/20160525215525/http://www.universetoday.com/18689/color-of-the-sun |archive-date=25 May 2016}}</ref><ref>{{Cite web |title=What Color is the Sun? |url=http://solar-center.stanford.edu/SID/activities/GreenSun.html |url-status=live |archive-url=https://web.archive.org/web/20171030154449/http://solar-center.stanford.edu/SID/activities/GreenSun.html |archive-date=30 October 2017 |access-date=23 May 2016 |publisher=[[Organization:Stanford University|Stanford]] Solar Center}}</ref>

Because the Sun fuses hydrogen into helium at its [[Astronomy:Stellar core|core]], it is a main-sequence star. More specifically, it is a [[Astronomy:G-type main-sequence star|G2-type main-sequence star]], where the type designation refers to its [[Astronomy:Effective temperature|effective temperature]]. Hotter main-sequence stars are more luminous but shorter lived. The Sun's temperature is intermediate between that of the [[Astronomy:O-type main-sequence star|hottest stars]] and that of the coolest stars. Stars brighter and hotter than the Sun are rare, whereas substantially dimmer and cooler stars, known as [[Astronomy:Red dwarf|red dwarf]]s, make up about 75% of the stars in the Milky Way.<ref name="bbcrd2013-04-11">{{Cite web |last=Palmer |first=Jason |date=6 February 2013 |title=Exoplanets near red dwarfs suggest another Earth nearer |url=https://www.bbc.co.uk/news/science-environment-21350899 |url-status=live |archive-url=https://web.archive.org/web/20220329003631/https://www.bbc.co.uk/news/science-environment-21350899 |archive-date=29 March 2022 |access-date=28 March 2022 |website=BBC News}}</ref><ref>{{Cite journal |last1=Mejías |first1=Andrea |last2=Minniti |first2=Dante |last3=Alonso-García |first3=Javier |last4=Beamín |first4=Juan Carlos |last5=Saito |first5=Roberto K. |last6=Solano |first6=Enrique |year=2022 |title=VVVX near-IR photometry for 99 low-mass stars in the Gaia EDR3 Catalog of Nearby Stars |journal=Astronomy & Astrophysics |volume=660 |pages=A131 |arxiv=2203.00786 |bibcode=2022A&A...660A.131M |doi=10.1051/0004-6361/202141759 |s2cid=246842719}}</ref>

The Sun is a population I star; it has a higher abundance of elements heavier than hydrogen and helium ("[[Astronomy:Metallicity|metals]]" in astronomical parlance) than the older population II stars.<ref>{{Cite journal |last1=van Albada |first1=T.S. |last2=Baker |first2=Norman |date=1973 |title=On the Two Oosterhoff Groups of Globular Clusters |journal=[[Astronomy:The Astrophysical Journal|The Astrophysical Journal]] |volume=185 |pages=477–498 |bibcode=1973ApJ...185..477V |doi=10.1086/152434 |doi-access=free}}</ref> Elements heavier than hydrogen and helium were formed in the cores of ancient and exploding stars, so the first generation of stars had to die before the [[Astronomy:Universe|universe]] could be enriched with these atoms. The oldest stars contain few metals, whereas stars born later have more. This higher [[Astronomy:Metallicity|metallicity]] is thought to have been crucial to the Sun's development of a [[Astronomy:Planetary system|planetary system]] because the planets form from the accretion of "metals".<ref>{{Cite journal |last=Lineweaver |first=Charles H. |date=9 March 2001 |title=An Estimate of the Age Distribution of Terrestrial Planets in the Universe: Quantifying Metallicity as a Selection Effect |journal=Icarus |volume=151 |issue=2 |pages=307–313 |arxiv=astro-ph/0012399 |bibcode=2001Icar..151..307L |citeseerx=10.1.1.254.7940 |doi=10.1006/icar.2001.6607 |s2cid=14077895}}</ref>

== Inner Solar System ==
[[File:Inner solar system objects top view for wiki.png|thumb|300px|Overview of the Inner Solar System up to the Jovian System]]

The inner Solar System is the region comprising the terrestrial planets and the asteroid belt.<ref>{{Cite web |title=Inner Solar System |url=https://science.nasa.gov/solar-system/focus-areas/inner-solar-system |url-status=live |archive-url=https://web.archive.org/web/20220410004501/https://science.nasa.gov/solar-system/focus-areas/inner-solar-system |archive-date=10 April 2022 |access-date=2 April 2022 |website=NASA Science: Share the Science}}</ref> Composed mainly of [[Chemistry:Silicate|silicate]]s and metals,<ref>{{Cite book |last1=Del Genio |first1=Anthony D. |title=Planetary Astrobiology |last2=Brain |first2=David |last3=Noack |first3=Lena |last4=Schaefer |first4=Laura |date=2020 |publisher=University of Arizona Press |isbn=978-0816540655 |editor-last=Meadows |editor-first=Victoria S. |page=420 |chapter=The Inner Solar System's Habitability Through Time |bibcode=2018arXiv180704776D |editor-last2=Arney |editor-first2=Giada N. |editor-last3=Schmidt |editor-first3=Britney |editor-last4=Des Marais |editor-first4=David J. |arxiv=1807.04776}}</ref> the objects of the inner Solar System are relatively close to the Sun; the radius of this entire region is less than the distance between the orbits of Jupiter and Saturn. This region is also within the [[Physics:Frost line (astrophysics)|frost line]], which is a little less than {{Convert|5|AU|e6km e6mi|abbr=unit|lk=off}} from the Sun.<ref name="Levison2003"/>

=== {{Anchor|Terrestrial planets}} Inner planets ===
{{Main|Astronomy:Terrestrial planet}}
[[File:Terrestrial planet sizes2.jpg|thumb|300px|The four terrestrial planets Mercury, Venus, Earth and Mars]]The four terrestrial or inner planets have dense, rocky compositions, few or no moons, and no ring systems. They are in hydrostatic equilibrium, forming a rounded shape, and have undergone planetary differentiation, causing chemical elements to accumulate at different radii. They are composed largely of refractory minerals such as silicates{{Mdash}}which form their crusts and mantles{{Mdash}}and metals such as iron and nickel which form their cores. Three of the four inner planets (Venus, Earth and Mars) have atmospheres substantial enough to generate weather; all have impact craters and tectonic surface features, such as [[Earth:Rift valley|rift valley]]s and volcanoes. The term ''inner planet'' should not be confused with ''inferior planet'', which designates those planets that are closer to the Sun than Earth (i.e. Mercury and Venus).<ref name=":2">{{Cite journal |last=Ryden |first=Robert |date=December 1999 |title=Astronomical Math |url=https://pubs.nctm.org/view/journals/mt/92/9/article-p786.xml |url-status=live |journal=The Mathematics Teacher |volume=92 |issue=9 |pages=786–792 |doi=10.5951/MT.92.9.0786 |issn=0025-5769 |jstor=27971203 |archive-url=https://web.archive.org/web/20220412010049/https://pubs.nctm.org/view/journals/mt/92/9/article-p786.xml |archive-date=12 April 2022 |access-date=29 March 2022}}</ref>

==== Mercury ====
{{Main|Astronomy:Mercury (planet)}}
Mercury ({{Convert|0.307|-|0.588|AU|e6km e6mi|abbr=unit}} from the Sun<ref name="nasa-factsheet">{{Cite web |last=Williams |first=David |date=27 December 2021 |title=Planetary Fact Sheet - Metric |url=https://nssdc.gsfc.nasa.gov/planetary/factsheet |access-date=11 December 2022 |website=NASA Goddard Space Flight Center}}</ref>) is the closest planet to the Sun. The smallest planet in the Solar System ({{Earth mass|0.055|link=yes}}), Mercury has no natural satellites. The dominant geological features are impact craters or basins with ejecta blankets, the remains of early volcanic activity including magma flows, and lobed ridges or [[Astronomy:Rupes|rupes]] that were probably produced by a period of contraction early in the planet's history.<ref>{{Cite journal |last1=Watters |first1=Thomas R. |last2=Solomon |first2=Sean C. |last3=Robinson |first3=Mark S. |last4=Head |first4=James W. |last5=André |first5=Sarah L. |last6=Hauck |first6=Steven A. |last7=Murchie |first7=Scott L. |date=August 2009 |title=The tectonics of Mercury: The view after MESSENGER's first flyby |journal=Earth and Planetary Science Letters |language=en |volume=285 |issue=3–4 |pages=283–296 |bibcode=2009E&PSL.285..283W |doi=10.1016/j.epsl.2009.01.025}}</ref> Mercury's very tenuous atmosphere consists of solar-wind particles trapped by Mercury's magnetic field, as well as atoms blasted off its surface by the solar wind.<ref>{{Cite journal |last1=Domingue |first1=Deborah L. |last2=Koehn |first2=Patrick L. |last3=Killen |first3=Rosemary M. |last4=Sprague |first4=Ann L. |last5=Sarantos |first5=Menelaos |last6=Cheng |first6=Andrew F. |last7=Bradley |first7=Eric T. |last8=McClintock |first8=William E. |display-authors=2 |date=2009 |title=Mercury's Atmosphere: A Surface-Bounded Exosphere |journal=Space Science Reviews |volume=131 |issue=1–4 |pages=161–186 |bibcode=2007SSRv..131..161D |doi=10.1007/s11214-007-9260-9 |s2cid=121301247 |quote=The composition of Mercury's exosphere, with its abundant H and He, clearly indicates a strong solar wind source. Once solar wind plasma and particles gain access to the magnetosphere, they predominantly precipitate to the surface, where solar wind species are neutralized, thermalized, and released again into the exosphere. Moreover, bombardment of the surface by solar wind particles, especially energetic ions, contributes to ejection of neutral species from the surface into the exosphere (via "sputtering") as well as other chemical and physical surface modification processes.}}</ref> Its relatively large iron core and thin mantle have not yet been adequately explained. Hypotheses include that its outer layers were stripped off by a giant impact, or that it was prevented from fully accreting by the young Sun's energy.<ref>{{Cite journal |last1=Benz |first1=W. |last2=Slattery |first2=W.L. |last3=Cameron |first3=A.G.W. |year=1988 |title=Collisional stripping of Mercury's mantle |url=https://zenodo.org/record/1253898 |url-status=live |journal=Icarus |type=Submitted manuscript |volume=74 |issue=3 |pages=516–528 |bibcode=1988Icar...74..516B |doi=10.1016/0019-1035(88)90118-2 |archive-url=https://web.archive.org/web/20190905034903/https://zenodo.org/record/1253898 |archive-date=5 September 2019 |access-date=25 August 2019}}</ref><ref>{{Cite journal |last=Cameron |first=A.G.W. |year=1985 |title=The partial volatilization of Mercury |journal=Icarus |volume=64 |issue=2 |pages=285–294 |bibcode=1985Icar...64..285C |doi=10.1016/0019-1035(85)90091-0}}</ref> There have been searches for "[[Astronomy:Vulcanoid|Vulcanoid]]s", asteroids in stable orbits between Mercury and the Sun, but none have been discovered.<ref>{{Cite journal |last1=Durda |first1=D .D. |last2=Stern |first2=S. A. |last3=Colwell |first3=W. B. |last4=Parker |first4=J. W. |last5=Levison |first5=H. F. |last6=Hassler |first6=D. M. |date=2004 |title=A New Observational Search for Vulcanoids in SOHO/LASCO Coronagraph Images |journal=Icarus |volume=148 |issue=1 |pages=312–315 |bibcode=2000Icar..148..312D |doi=10.1006/icar.2000.6520}}</ref><ref name="Steffl2013">{{Cite journal |last1=Steffl |first1=A. J. |last2=Cunningham |first2=N. J. |last3=Shinn |first3=A. B. |last4=Stern |first4=S. A. |date=2013 |title=A Search for Vulcanoids with the STEREO Heliospheric Imager |journal=Icarus |volume=233 |issue=1 |pages=48–56 |arxiv=1301.3804 |bibcode=2013Icar..223...48S |doi=10.1016/j.icarus.2012.11.031 |s2cid=118612132}}</ref>

==== Venus ====
{{Main|Astronomy:Venus}}
Venus ({{Convert|0.718|-|0.728|AU|e6km e6mi|abbr=unit}} from the Sun<ref name="nasa-factsheet"/>) is close in size to Earth ({{Earth mass|0.815}}) and, like Earth, has a thick silicate mantle around an iron core, a substantial atmosphere, and evidence of internal geological activity. It is much drier than Earth, and its atmosphere is ninety times as dense. Venus has no natural satellites. It is the hottest planet, with surface temperatures over {{Cvt|400|C|F}}, mainly due to the amount of [[Chemistry:Greenhouse gas|greenhouse gas]]es in the atmosphere.<ref>{{Cite thesis |last=Bullock |first=Mark Alan |title=The Stability of Climate on Venus |date=1997 |degree=PhD |publisher=Southwest Research Institute |url=http://www.boulder.swri.edu/~bullock/Homedocs/PhDThesis.pdf |access-date=26 December 2006 |url-status=dead |archive-url=https://web.archive.org/web/20070614202751/http://www.boulder.swri.edu/~bullock/Homedocs/PhDThesis.pdf |archive-date=14 June 2007}}</ref> The planet has no magnetic field that would prevent the depletion of its substantial atmosphere, which suggests that its atmosphere is being replenished by volcanic eruptions.<ref>{{Cite web |last=Rincon |first=Paul |date=1999 |title=Climate Change as a Regulator of Tectonics on Venus |url=http://www.boulder.swri.edu/~bullock/Homedocs/Science2_1999.pdf |url-status=dead |archive-url=https://web.archive.org/web/20070614202807/http://www.boulder.swri.edu/~bullock/Homedocs/Science2_1999.pdf |archive-date=14 June 2007 |access-date=19 November 2006 |website=Johnson Space Center Houston, TX, Institute of Meteoritics, University of New Mexico, Albuquerque, NM}}</ref> A relatively young planetary surface displays extensive evidence of volcanic activity, but is devoid of [[Earth:Plate tectonics|plate tectonics]]. It may undergo resurfacing episodes on a time scale of 700&nbsp;million years.<ref>{{Cite journal |last1=Elkins-Tanton |first1=L. T. |last2=Smrekar |first2=S. E. |last3=Hess |first3=P. C. |last4=Parmentier |first4=E. M. |date=March 2007 |title=Volcanism and volatile recycling on a one-plate planet: Applications to Venus |journal=Journal of Geophysical Research |volume=112 |issue=E4 |bibcode=2007JGRE..112.4S06E |doi=10.1029/2006JE002793 |id=E04S06 |doi-access=free}}</ref>

==== Earth ====
{{Main|Astronomy:Earth}}
Earth ({{Convert|0.983|-|1.017|AU|e6km e6mi|abbr=unit}} from the Sun) is the largest and densest of the inner planets, the only one known to have current geological activity, and the only place in the universe where life is known to exist.<ref name="life">{{Cite web |title=What are the characteristics of the Solar System that lead to the origins of life? |url=https://science.nasa.gov/planetary-science/big-questions/what-are-the-characteristics-of-the-solar-system-that-lead-to-the-origins-of-life-1 |url-status=dead |archive-url=https://web.archive.org/web/20100408055814/http://science.nasa.gov/planetary-science/big-questions/what-are-the-characteristics-of-the-solar-system-that-lead-to-the-origins-of-life-1 |archive-date=8 April 2010 |access-date=30 August 2011 |publisher=NASA Science (Big Questions)}}</ref> Its liquid [[Earth:Hydrosphere|hydrosphere]] is unique among the terrestrial planets, and it is the only planet where [[Earth:Plate tectonics|plate tectonics]] has been observed.<ref>{{Cite web |last=Hall |first=Shannon |date=20 July 2017 |title=Earth's Tectonic Activity May Be Crucial for Life—and Rare in Our Galaxy |url=https://www.scientificamerican.com/article/earths-tectonic-activity-may-be-crucial-for-life-and-rare-in-our-galaxy |url-status=live |archive-url=https://web.archive.org/web/20220512174249/https://www.scientificamerican.com/article/earths-tectonic-activity-may-be-crucial-for-life-and-rare-in-our-galaxy |archive-date=12 May 2022 |access-date=12 May 2022 |publisher=Scientific American}}</ref> Earth's atmosphere is radically different from those of the other planets, having been altered by the presence of life to contain 21% free [[Chemistry:Oxygen|oxygen]].<ref name=handbook>{{Cite book |title=[[Physics:CRC Handbook of Chemistry and Physics|CRC Handbook of Chemistry and Physics]] |publisher=CRC Press |year=2016–2017 |isbn=978-1-4987-5428-6 |editor-last=Haynes |editor-first=H. M. |edition=97th |page=14{{Hyphen}}3<!-- the page ref itself is hyphenated -->}}</ref><ref name="NYT-20131003">{{Cite news |last=Zimmer |first=Carl |date=3 October 2013 |title=Earth's Oxygen: A Mystery Easy to Take for Granted |work={{wipe|The New York Times}} |url=https://www.nytimes.com/2013/10/03/science/earths-oxygen-a-mystery-easy-to-take-for-granted.html |url-access=limited |access-date=3 October 2013 |archive-url=https://web.archive.org/web/20131003121909/http://www.nytimes.com/2013/10/03/science/earths-oxygen-a-mystery-easy-to-take-for-granted.html |archive-date=3 October 2013}}</ref> The planetary [[Astronomy:Magnetosphere|magnetosphere]] shields the surface from solar and cosmic radiation, limiting atmospheric stripping and maintaining habitability.<ref>{{Cite book |last=Pentreath |first=R. J. |url=https://books.google.com/books?id=avRVEAAAQBAJ&pg=PA94 |title=Radioecology: Sources and Consequences of Ionising Radiation in the Environment |date=2021 |publisher=Cambridge University Press |isbn=978-1009040334 |pages=94–97 |access-date=12 April 2022 |archive-url=https://web.archive.org/web/20220420161217/https://www.google.com/books/edition/Radioecology/avRVEAAAQBAJ?hl=en&gbpv=1&pg=PA94 |archive-date=20 April 2022 |url-status=live}}</ref> It has one natural satellite, the [[Astronomy:Moon|Moon]].<ref>{{Cite web |title=Facts About Earth - NASA Science |url=https://science.nasa.gov/earth/facts/ |access-date=2024-01-11 |website=NASA Science |language=en}}</ref>

==== Mars ====
{{Main|Astronomy:Mars}}
Mars ({{Convert|1.382|-|1.666|AU|e6km e6mi|abbr=unit}} from the Sun) is smaller than Earth and Venus ({{Earth mass|0.107}}). It has an atmosphere of mostly carbon dioxide with a surface pressure of {{Convert|6.1|mb|psi inHg}}; roughly 0.6% of that of Earth but sufficient to support weather phenomena.<ref>{{Cite book |last1=Gatling |first1=David C. |title=Encyclopaedia of the Solar System |last2=Leovy |first2=Conway |date=2007 |editor-last=Lucy-Ann McFadden |pages=301–314 |chapter=Mars Atmosphere: History and Surface Interactions |display-editors=etal}}</ref> Its surface, peppered with volcanoes, such as [[Astronomy:Olympus Mons|Olympus Mons]], and rift valleys, such as [[Astronomy:Valles Marineris|Valles Marineris]], shows geological activity that may have persisted until as recently as 2&nbsp;million years ago.<ref>{{Cite web |last=Noever |first=David |date=2004 |title=Modern Martian Marvels: Volcanoes? |url=https://www.astrobio.net/mars/modern-martian-marvels-volcanoes |url-status=usurped |archive-url=https://web.archive.org/web/20200314112555/https://www.astrobio.net/mars/modern-martian-marvels-volcanoes |archive-date=14 March 2020 |access-date=23 July 2006 |website=NASA Astrobiology Magazine}}</ref> Its red color comes from [[Chemistry:Iron(III) oxide|iron oxide]] (rust) in its soil,<ref>{{Cite journal |last=Peplow |first=Mark |date=6 May 2004 |title=How Mars got its rust |url=http://www.nature.com/articles/news040503-6 |url-status=live |journal=Nature |language=en |pages=news040503–6 |doi=10.1038/news040503-6 |issn=0028-0836 |archive-url=https://web.archive.org/web/20220407105832/https://www.nature.com/articles/news040503-6 |archive-date=7 April 2022 |access-date=9 April 2022}}</ref> while the polar regions show white ice caps consisting largely of water.<ref>{{Cite web |title=Polar Caps |url=https://marsed.asu.edu/mep/ice/polar-caps |access-date=2022-01-06 |website=Mars Education at Arizona State University}}</ref> Mars has two tiny natural satellites ([[Astronomy:Deimos (moon)|Deimos]] and [[Astronomy:Phobos (moon)|Phobos]]) thought to be either captured asteroids,<ref>{{Cite web |last1=Sheppard |first1=Scott S. |last2=Jewitt |first2=David |last3=Kleyna |first3=Jan |date=2004 |title=A Survey for Outer Satellites of Mars: Limits to Completeness |url=http://www2.ess.ucla.edu/~jewitt/papers/2004/SJK2004.pdf |url-status=live |archive-url=https://web.archive.org/web/20160303205126/http://www2.ess.ucla.edu/~jewitt/papers/2004/SJK2004.pdf |archive-date=3 March 2016 |access-date=26 December 2006 |website=Astronomical Journal}}</ref> or ejected debris from a massive impact early in Mars's history.<ref>{{Cite journal |last1=Rosenblatt |first1=Pascal |last2=Charnoz |first2=Sébastien |last3=Dunseath |first3=Kevin M. |last4=Terao-Dunseath |first4=Mariko |last5=Trinh |first5=Antony |last6=Hyodo |first6=Ryuki |last7=Genda |first7=Hidenori |last8=Toupin |first8=Stéven |year=2016 |title=Accretion of Phobos and Deimos in an extended debris disc stirred by transient moons |url=https://hal.archives-ouvertes.fr/hal-01350105/file/Letter.pdf |url-status=live |journal=Nature Geoscience |volume=9 |issue=8 |page=581 |bibcode=2016NatGe...9..581R |doi=10.1038/ngeo2742 |s2cid=133174714 |archive-url=https://web.archive.org/web/20200309100516/https://hal.archives-ouvertes.fr/hal-01350105/file/Letter.pdf |archive-date=9 March 2020 |access-date=25 August 2019}}</ref>

=== Asteroid belt ===
{{Main|Astronomy:Astronomy:Asteroid belt|Astronomy:Asteroid}}
[[File:Inner_solar_system_linear_map.png|thumb|Linear map of the inner Solar System, showing many asteroid populations]]
Asteroids except for the largest, Ceres, are classified as [[Astronomy:Small Solar System bodies|small Solar System bodies]]{{Refn|According to [[Astronomy:IAU definition of planet|IAU definitions]], objects orbiting the Sun are classified dynamically and physically into three categories: ''planets'', ''dwarf planets'', and ''small Solar System bodies''.
* A [[Astronomy:Planet|planet]] is any body orbiting the Sun whose [[Physics:Mass|mass]] is sufficient for gravity to have pulled it into a (near-)[[Sphere|spherical]] shape and that has [[Astronomy:Clearing the neighbourhood|cleared its immediate neighborhood]] of all smaller objects. By this definition, the Solar System has eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Because it has not cleared its neighborhood of other [[Astronomy:Kuiper belt|Kuiper belt]] objects, Pluto does not fit this definition.<ref name="FinalResolution"/>
* A [[Astronomy:Dwarf planet|dwarf planet]] is a body orbiting the Sun that is massive enough to be made near-spherical by its own gravity but that has not cleared [[Astronomy:Planetesimal|planetesimal]]s from its neighborhood and is also not a satellite.<ref name="FinalResolution"/> Dwarf planets are considered planets by some planetologists but not by the IAU.<ref name=planetarysociety/> The IAU has recognized four other bodies in the Solar System as dwarf planets: [[Astronomy:Ceres (dwarf planet)|Ceres]], [[Astronomy:Haumea|Haumea]], [[Astronomy:Makemake|Makemake]], and [[Astronomy:Eris (dwarf planet)|Eris]].<ref name="name">{{Cite web |title=Pluto and the Developing Landscape of Our Solar System |url=https://www.iau.org/public/themes/pluto |url-status=live |archive-url=https://web.archive.org/web/20160130022141/http://www.iau.org/public/themes/pluto |archive-date=30 January 2016 |access-date=9 April 2022 |website=International Astronomical Union}}</ref><ref>{{Cite web |last=Ekers |first=Ron |title=IAU Planet Definition Committee |url=http://www.iau.org/public_press/news/release/iau0601/newspaper |archive-url=https://web.archive.org/web/20090603001603/http://www.iau.org/public_press/news/release/iau0601/newspaper |archive-date=3 June 2009 |access-date=13 October 2008 |publisher=International Astronomical Union}}</ref> Other objects [[Astronomy:Dwarf planet#Most likely dwarf planets|commonly accepted as dwarf planets]] include [[Astronomy:225088 Gonggong|Gonggong]], [[Astronomy:90377 Sedna|Sedna]], [[Astronomy:90482 Orcus|Orcus]], and [[Astronomy:50000 Quaoar|Quaoar]]. In a reference to Pluto, other dwarf planets orbiting in the trans-Neptunian region are sometimes called "plutoids",<ref name="IAU0804">{{Cite news |date=11 June 2008 |title=Plutoid chosen as name for Solar System objects like Pluto |publisher=[[Organization:International Astronomical Union|International Astronomical Union]] |location=Paris |url=http://www.iau.org/public_press/news/release/iau0804 |access-date=11 June 2008 |archive-url=https://web.archive.org/web/20080613121232/http://www.iau.org/public_press/news/release/iau0804 |archive-date=13 June 2008}}</ref> though this term is seldom used.
* The remaining objects orbiting the Sun are known as [[Astronomy:Small Solar System body|small Solar System bodies]].<ref name="FinalResolution">{{Cite web |year=2006 |title=Resolutions B5 and B6: 'Definition of a Planet in the Solar System' and 'Pluto' |url=https://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf |url-status=live |archive-url=https://web.archive.org/web/20180106125750/https://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf |archive-date=6 January 2018 |access-date=9 April 2022 |website=Resolutions adopted at the General Assemblies |publisher=International Astronomical Union}}</ref>|name=footnoteB|group=lower-alpha}} and are composed mainly of [[Chemistry:Carbon|carbon]]aceous, refractory rocky and metallic minerals, with some ice.<ref>{{Cite web |date=2006 |title=IAU Planet Definition Committee |url=http://www.iau.org/public_press/news/release/iau0601/newspaper |url-status=dead |archive-url=https://web.archive.org/web/20090603001603/http://www.iau.org/public_press/news/release/iau0601/newspaper |archive-date=3 June 2009 |access-date=1 March 2009 |publisher=International Astronomical Union}}</ref><ref>{{Cite web |title=Are Kuiper Belt Objects asteroids? Are large Kuiper Belt Objects planets? |url=http://curious.astro.cornell.edu/question.php?number=601 |url-status=dead |archive-url=https://web.archive.org/web/20090103110110/http://curious.astro.cornell.edu/question.php?number=601 |archive-date=3 January 2009 |access-date=1 March 2009 |publisher=[[Organization:Cornell University|Cornell University]]}}</ref> They range from a few metres to hundreds of kilometres in size. Asteroids smaller than one meter are usually called [[Astronomy:Meteoroid|meteoroid]]s and micrometeoroids (grain-sized), with the exact division between the two categories being debated over the years.<ref>{{Cite journal |last1=Rubin |first1=Alan E. |last2=Grossman |first2=Jeffrey N. |date=February 2010 |title=Meteorite and meteoroid: new comprehensive definitions |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1945-5100.2009.01009.x |url-status=live |journal=Meteoritics and Planetary Science |language=en |volume=45 |issue=1 |page=114 |bibcode=2010M&PS...45..114R |doi=10.1111/j.1945-5100.2009.01009.x |s2cid=129972426 |archive-url=https://web.archive.org/web/20220325111938/https://onlinelibrary.wiley.com/doi/10.1111/j.1945-5100.2009.01009.x |archive-date=25 March 2022 |access-date=10 April 2022}}</ref> {{Asof|2017|post=,}} the IAU designates asteroids having a diameter between about 30 [[Micrometre|micrometre]]s and 1 metre as micrometeoroids, and terms smaller particles "dust".<ref>{{Cite web |last=Perlerin |first=Vincent |date=26 September 2017 |title=Definitions of terms in meteor astronomy (IAU) |url=https://www.imo.net/definitions-of-terms-in-meteor-astronomy-iau |url-status=live |archive-url=https://web.archive.org/web/20180123072313/https://www.imo.net/definitions-of-terms-in-meteor-astronomy-iau |archive-date=23 January 2018 |access-date=10 April 2022 |website=International Meteor Organization}}</ref>

The asteroid belt occupies the orbit between Mars and Jupiter, between {{Convert|2.3 and 3.3|AU|e6km e6mi|abbr=unit}} from the Sun. It is thought to be remnants from the Solar System's formation that failed to coalesce because of the gravitational interference of Jupiter.<ref>{{Cite journal |last1=Petit |first1=J.-M. |last2=Morbidelli |first2=A. |last3=Chambers |first3=J. |date=2001 |title=The Primordial Excitation and Clearing of the Asteroid Belt |url=http://www.gps.caltech.edu/classes/ge133/reading/asteroids.pdf |url-status=dead |journal=Icarus |volume=153 |issue=2 |pages=338–347 |bibcode=2001Icar..153..338P |doi=10.1006/icar.2001.6702 |archive-url=https://web.archive.org/web/20070221085835/http://www.gps.caltech.edu/classes/ge133/reading/asteroids.pdf |archive-date=21 February 2007 |access-date=22 March 2007}}</ref> The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometre in diameter.<ref>{{Cite journal |last1=Tedesco |first1=Edward F. |last2=Cellino |first2=Alberto |last3=Zappalá |first3=Vincenzo |date=June 2005 |title=The Statistical Asteroid Model. I. The Main-Belt Population for Diameters Greater than 1 Kilometer |journal=The Astronomical Journal |language=en |volume=129 |issue=6 |pages=2869–2886 |bibcode=2005AJ....129.2869T |doi=10.1086/429734 |issn=0004-6256 |s2cid=119906696 |doi-access=free}}</ref> Despite this, the total mass of the asteroid belt is unlikely to be more than a thousandth of that of Earth.<ref name="Krasinsky2002">{{Cite journal |last1=Krasinsky |first1=G. A. |last2=Pitjeva |first2=E. V. |author-link2=Elena V. Pitjeva |last3=Vasilyev |first3=M. V. |last4=Yagudina |first4=E. I. |date=July 2002 |title=Hidden Mass in the Asteroid Belt |journal=Icarus |volume=158 |issue=1 |pages=98–105 |bibcode=2002Icar..158...98K |doi=10.1006/icar.2002.6837}}</ref> The asteroid belt is very sparsely populated; spacecraft routinely pass through without incident.<ref>{{Cite web |date=14 April 2000 |title=Cassini Passes Through Asteroid Belt |url=https://solarsystem.nasa.gov/news/12195/cassini-passes-through-asteroid-belt |url-status=live |archive-url=https://web.archive.org/web/20210125180703/https://solarsystem.nasa.gov/news/12195/cassini-passes-through-asteroid-belt |archive-date=25 January 2021 |access-date=1 March 2021 |website=NASA}}</ref>

==== Ceres ====
{{Main|Astronomy:Ceres (dwarf planet)}}
Ceres ({{Convert|2.77|AU|e6km e6mi|abbr=unit}} from the Sun) is the largest asteroid, a [[Astronomy:Protoplanet|protoplanet]], and a dwarf planet.<ref group=lower-alpha name=footnoteB/> It has a diameter of slightly under {{Convert|1000|km|abbr=on}} and a mass large enough for its own gravity to pull it into a spherical shape. Ceres was considered a planet when it was discovered in 1801, but as further observations revealed additional asteroids, it became common to consider it as one of the minor rather than major planets.<ref>{{Cite web |date=26 June 2019 |title=Exploration: Ceres |url=https://solarsystem.nasa.gov/planets/dwarf-planets/ceres/exploration |url-status=live |archive-url=https://web.archive.org/web/20211030193414/https://solarsystem.nasa.gov/planets/dwarf-planets/ceres/exploration |archive-date=30 October 2021 |access-date=1 April 2022 |website=NASA Science: Solar System Exploration}}</ref> It was then reclassified again as a dwarf planet in 2006 when the [[Astronomy:IAU definition of planet|IAU definition of planet]] was established.<ref>{{Cite book |last=Brown |first=Mike |title=How I Killed Pluto and Why It Had It Coming |title-link=How I Killed Pluto and Why It Had It Coming |date=2010 |publisher=Spiegel & Grau |isbn=978-0-385-53108-5 |location=New York |oclc=495271396 |author-link=Mike Brown (astronomer)}}</ref>{{Rp|page=218}}

==== Pallas and Vesta ====
{{Main|Astronomy:2 Pallas|Astronomy:4 Vesta}}
[[Astronomy:2 Pallas|Pallas]] (2.77&nbsp;AU from the Sun) and [[Astronomy:4 Vesta|Vesta]] (2.36&nbsp;AU from the Sun) are the largest asteroids in the asteroid belt, after Ceres. They are the other two protoplanets that survive more or less intact. At about {{Convert|520|km|abbr=on}} in diameter, they were large enough to have developed planetary geology in the past, but both have suffered large impacts and been battered out of being round.<ref name="Carry2009">{{Cite journal |last=Carry |first=B. |display-authors=0 |date=2009 |title=Physical properties of (2) Pallas |journal=Icarus |volume=205 |issue=2 |pages=460–472 |arxiv=0912.3626 |bibcode=2010Icar..205..460C |doi=10.1016/j.icarus.2009.08.007 |s2cid=119194526}}</ref><ref name="Marsset2020">{{Cite journal |last1=Marsset |first1=M. |last2=Brož |first2=M. |last3=Vernazza |first3=P. |last4=Drouard |first4=A. |display-authors=3 |date=2020 |title=The violent collisional history of aqueously evolved (2) Pallas |url=https://orbi.uliege.be/bitstream/2268/246670/1/Pallas_Marsset.pdf |journal=Nature Astronomy |volume=4 |issue=6 |pages=569–576 |bibcode=2020NatAs...4..569M |doi=10.1038/s41550-019-1007-5 |s2cid=212927521 |hdl-access=free |hdl=10261/237549}}</ref><ref>{{Cite web |date=19 April 1995 |title=Asteroid or Mini-Planet? Hubble Maps the Ancient Surface of Vesta |url=http://hubblesite.org/contents/news-releases/1995/news-1995-20 |url-status=live |archive-url=https://web.archive.org/web/20220420161231/https://hubblesite.org/contents/news-releases/1995/news-1995-20.html |archive-date=20 April 2022 |access-date=10 April 2022 |website=HubbleSite.org |publisher=[[Organization:Space Telescope Science Institute|Space Telescope Science Institute]] |language=en}}</ref> Fragments from impacts upon these two bodies survive elsewhere in the asteroid belt, as the [[Astronomy:Pallas family|Pallas family]] and [[Astronomy:Vesta family|Vesta family]]. Both were considered planets upon their discoveries in 1802 and 1807 respectively, and like Ceres, eventually considered minor planets with the discovery of more asteroids. Some authors today have begun to consider Pallas and Vesta as planets again, along with Ceres, under geophysical definitions of the term.<ref name="planetarysociety">{{Cite web |last=Lakdawalla |first=Emily |display-authors=0 |date=21 April 2020 |title=What Is A Planet? |url=https://www.planetary.org/worlds/what-is-a-planet |url-status=live |archive-url=https://web.archive.org/web/20220122142140/https://www.planetary.org/worlds/what-is-a-planet |archive-date=22 January 2022 |access-date=3 April 2022 |website=The Planetary Society}}</ref>

==== Asteroid groups ====
Asteroids in the asteroid belt are divided into asteroid groups and [[Astronomy:Asteroid family|families]] based on their orbital characteristics. [[Astronomy:Kirkwood gap|Kirkwood gap]]s are sharp dips in the distribution of asteroid orbits that correspond to orbital resonances with Jupiter.<ref>{{Cite conference |last=Ferraz-Mello |first=S. |date=1994 |editor-last=Milani |editor-first=Andrea |editor2-last=Di Martino |editor2-first=Michel |editor3-last=Cellino |editor3-first=A. |title=Kirkwood Gaps and Resonant Groups |conference=Asteroids, Comets, Meteors 1993: Proceedings of the 160th Symposium of the International Astronomical Union, held in Belgirate, Italy, 14–18 June 1993 |publisher=Kluwer Academic Publishers |publication-place=Dordrecht |page=175 |bibcode=1994IAUS..160..175F}}</ref> Asteroid moons are asteroids that orbit larger asteroids. They are not as clearly distinguished as planetary moons, sometimes being almost as large as their partners (e.g. that of [[Astronomy:90 Antiope|90 Antiope]]). The asteroid belt includes [[Astronomy:Main-belt comet|main-belt comet]]s, which may have been the source of Earth's water.<ref>{{Cite journal |last1=Snodgrass |first1=Colin |last2=Agarwal |first2=Jessica |last3=Combi |first3=Michael |last4=Fitzsimmons |first4=Alan |last5=Guilbert-Lepoutre |first5=Aurelie |last6=Hsieh |first6=Henry H. |last7=Hui |first7=Man-To |last8=Jehin |first8=Emmanuel |last9=Kelley |first9=Michael S. P. |last10=Knight |first10=Matthew M. |last11=Opitom |first11=Cyrielle |date=November 2017 |title=The Main Belt Comets and ice in the Solar System |url=http://link.springer.com/10.1007/s00159-017-0104-7 |url-status=live |journal=The Astronomy and Astrophysics Review |language=en |volume=25 |issue=1 |page=5 |arxiv=1709.05549 |bibcode=2017A&ARv..25....5S |doi=10.1007/s00159-017-0104-7 |issn=0935-4956 |s2cid=7683815 |archive-url=https://web.archive.org/web/20220420161227/https://idp.springer.com/favicon.ico |archive-date=20 April 2022 |access-date=9 March 2022}}</ref>

[[Astronomy:Jupiter trojan|Jupiter trojan]]s are located in either of Jupiter's L<sub>4</sub> or L<sub>5</sub> points (gravitationally stable regions leading and trailing a planet in its orbit); the term {{Em|trojan}} is also used for small bodies in any other planetary or satellite [[Astronomy:Lagrange point|Lagrange point]]. [[Astronomy:Hilda asteroid|Hilda asteroid]]s are in a 2:3 resonance with Jupiter; that is, they go around the Sun three times for every two Jupiter orbits.<ref name="Barucci">{{Cite book |last1=Barucci |first1=M. A. |title=Asteroids III |last2=Kruikshank |first2=D. P. |last3=Mottola |first3=S. |last4=Lazzarin |first4=M. |date=2002 |publisher=University of Arizona Press |location=Tucson, Arizona |pages=273–287 |chapter=Physical Properties of Trojan and Centaur Asteroids}}</ref> The inner Solar System contains [[Astronomy:Near-Earth object|near-Earth asteroids]], many of which cross the orbits of the inner planets.<ref name="MorbidelliAstIII">{{Cite journal |last1=Morbidelli |first1=A. |last2=Bottke |first2=W.F. |last3=Froeschlé |first3=Ch. |last4=Michel |first4=P. |date=January 2002 |editor2-last=A. Cellino |editor3-last=P. Paolicchi |editor4-last=R.P. Binzel |title=Origin and Evolution of Near-Earth Objects |url=http://www.boulder.swri.edu/~bottke/Reprints/Morbidelli-etal_2002_AstIII_NEOs.pdf |url-status=live |journal=Asteroids III |pages=409–422 |bibcode=2002aste.book..409M |doi=10.2307/j.ctv1v7zdn4.33 |archive-url=https://web.archive.org/web/20170809014123/http://www.boulder.swri.edu/~bottke/Reprints/Morbidelli-etal_2002_AstIII_NEOs.pdf |archive-date=9 August 2017 |access-date=30 August 2009 |editor=W.F. Bottke Jr.}}</ref> Some of them are [[Astronomy:Potentially hazardous object|potentially hazardous object]]s.<ref name="CNEOS-Basics">{{Cite web |title=NEO Basics – Potentially Hazardous Asteroids (PHAs) |url=https://cneos.jpl.nasa.gov/about/neo_groups.html |url-status=live |archive-url=https://web.archive.org/web/20211111141623/https://cneos.jpl.nasa.gov/about/neo_groups.html |archive-date=11 November 2021 |access-date=10 March 2022 |publisher=CNEOS NASA/JPL}}</ref>

== Outer Solar System ==
[[File:Kuiper_belt_plot_objects_of_outer_solar_system.png|thumb|300x300px|Plot of objects around the Kuiper belt and other asteroid populations, the J, S, U and N denotes Jupiter, Saturn, Uranus and Neptune]]
The outer region of the Solar System is home to the [[Astronomy:Giant planet|giant planet]]s and their large moons. The [[Astronomy:Centaur (minor planet)|centaurs]] and many short-period comets also orbit in this region. Due to their greater distance from the Sun, the solid objects in the outer Solar System contain a higher proportion of volatiles, such as water, ammonia, and methane than those of the inner Solar System because the lower temperatures allow these compounds to remain solid, without significant rates of [[Sublimation (phase transition)|sublimation]].<ref name="bennett_8.2"/>

=== Outer planets ===
{{Main|Astronomy:Giant planet}}
[[File:Planet collage to scale.jpg|thumb|The outer planets Jupiter, Saturn, Uranus and Neptune, compared to the inner planets Earth, Venus, Mars, and Mercury at the bottom right]]
The four outer planets, also called giant planets or Jovian planets, collectively make up 99% of the mass known to orbit the Sun.<ref group=lower-alpha name=footnoteD/> Jupiter and Saturn are together more than 400 times the mass of Earth and consist overwhelmingly of the gases hydrogen and helium, hence their designation as [[Astronomy:Gas giant|gas giant]]s.<ref>{{Cite web |title=Gas Giant {{!}} Planet Types |url=https://exoplanets.nasa.gov/what-is-an-exoplanet/planet-types/gas-giant |url-status=live |archive-url=https://web.archive.org/web/20201128232046/https://exoplanets.nasa.gov/what-is-an-exoplanet/planet-types/gas-giant |archive-date=28 November 2020 |access-date=22 December 2020 |website=Exoplanet Exploration: Planets Beyond our Solar System}}</ref> Uranus and Neptune are far less massive{{Mdash}}less than 20 Earth masses ({{Earth mass}}) each{{Mdash}}and are composed primarily of ice. For these reasons, some astronomers suggest they belong in their own category, ice giants.<ref>{{Cite web |last1=Lissauer |first1=Jack J. |last2=Stevenson |first2=David J. |date=2006 |title=Formation of Giant Planets |url=http://www.gps.caltech.edu/uploads/File/People/djs/lissauer&stevenson(PPV).pdf |url-status=dead |archive-url=https://web.archive.org/web/20090326060004/http://www.gps.caltech.edu/uploads/File/People/djs/lissauer%26stevenson%28PPV%29.pdf |archive-date=26 March 2009 |access-date=16 January 2006 |website=NASA Ames Research Center; California Institute of Technology}}</ref> All four giant planets have rings, although only Saturn's ring system is easily observed from Earth. The term ''superior planet'' designates planets outside Earth's orbit and thus includes both the outer planets and Mars.<ref name=":2"/>

The ring–moon systems of Jupiter, Saturn, and Uranus are like miniature versions of the Solar System; that of Neptune is significantly different, having been disrupted by the capture of its largest moon Triton.<ref>{{Cite book |title=Physics and Chemistry of the Solar System |date=2004 |publisher=Elsevier |isbn=978-0-124-46744-6 |editor-last=Lewis |editor-first=John S. |edition=2nd |page=147 |oclc=475009299}}</ref>

==== Jupiter ====
{{Main|Astronomy:Jupiter}}
Jupiter ({{Convert|4.951|-|5.457|AU|e6km e6mi|abbr=unit}} from the Sun<ref name="nasa-factsheet"/>), at {{Earth mass|318}}, is 2.5 times the mass of all the other planets put together. It is composed largely of [[Chemistry:Hydrogen|hydrogen]] and [[Chemistry:Helium|helium]]. Jupiter's strong internal heat creates semi-permanent features in its atmosphere, such as cloud bands and the [[Astronomy:Great Red Spot|Great Red Spot]]. The planet possesses a 4.2–{{Val|14|ul=Gauss}} strength [[Astronomy:Magnetosphere|magnetosphere]] that spans 22–29&nbsp;million km, making it, in certain respects, the largest object in the Solar System.<ref>{{Cite book |last=Rogers |first=John H. |url=https://books.google.com/books?id=SO48AAAAIAAJ&pg=PA293 |title=The giant planet Jupiter |date=1995 |publisher=Cambridge University Press |isbn=978-0521410083 |page=293 |access-date=13 April 2022 |archive-url=https://web.archive.org/web/20220420161219/https://www.google.com/books/edition/The_Giant_Planet_Jupiter/SO48AAAAIAAJ?gbpv=1&pg=PA293 |archive-date=20 April 2022 |url-status=live}}</ref> Jupiter has [[Astronomy:Moons of Jupiter|95 known satellites]]. The four largest, [[Astronomy:Ganymede (moon)|Ganymede]], [[Astronomy:Callisto (moon)|Callisto]], [[Astronomy:Io (moon)|Io]], and [[Astronomy:Europa (moon)|Europa]], are called the [[Astronomy:Galilean moons|Galilean moons]]: they show similarities to the terrestrial planets, such as volcanism and internal heating.<ref>{{Cite web |last=Pappalardo |first=R. T. |date=1999 |title=Geology of the Icy Galilean Satellites: A Framework for Compositional Studies |url=http://www.agu.org/cgi-bin/SFgate/SFgate?&listenv=table&multiple=1&range=1&directget=1&application=fm99&database=%2Fdata%2Fepubs%2Fwais%2Findexes%2Ffm99%2Ffm99&maxhits=200&=%22P11C-10%22 |url-status=dead |archive-url=https://web.archive.org/web/20070930165551/http://www.agu.org/cgi-bin/SFgate/SFgate?&listenv=table&multiple=1&range=1&directget=1&application=fm99&database=%2Fdata%2Fepubs%2Fwais%2Findexes%2Ffm99%2Ffm99&maxhits=200&=%22P11C-10%22 |archive-date=30 September 2007 |access-date=16 January 2006 |website=Brown University}}</ref> Ganymede, the largest satellite in the Solar System, is larger than Mercury; Callisto is almost as large.<ref name="Allen_Cox_2000">{{Cite book |last=Allen |first=Clabon Walter |url=https://books.google.com/books?id=w8PK2XFLLH8C&pg=PA293 |title=Allen's Astrophysical Quantities |date=2000 |publisher=Springer |isbn=978-0-387-98746-0 |editor-last=Cox |editor-first=Arthur N. |pages=293–313 |access-date=1 May 2022 |archive-url=https://web.archive.org/web/20220501143333/https://books.google.com/books?id=w8PK2XFLLH8C&pg=PA293 |archive-date=1 May 2022 |url-status=live}}</ref>

==== Saturn ====
{{Main|Astronomy:Saturn}}
Saturn ({{Convert|9.075|-|10.07|AU|e9km e6mi|abbr=unit}} from the Sun<ref name="nasa-factsheet"/>), distinguished by its extensive [[Astronomy:Rings of Saturn|ring system]], has several similarities to Jupiter, such as its atmospheric composition and [[Astronomy:Magnetosphere|magnetosphere]]. Although Saturn has 60% of Jupiter's volume, it is less than a third as massive, at {{Earth mass|95}}. Saturn is the only planet of the Solar System that is less dense than water. The rings of Saturn are made up of small ice and rock particles.<ref>{{Cite web |date=18 August 2021 |title=In Depth: Saturn |url=https://solarsystem.nasa.gov/planets/saturn/in-depth |url-status=live |archive-url=https://web.archive.org/web/20180224210031/https://solarsystem.nasa.gov/planets/saturn/in-depth |archive-date=24 February 2018 |access-date=31 March 2022 |website=NASA Science: Solar System Exploration}}</ref> Saturn has [[Astronomy:Moons of Saturn|145 confirmed satellites]] composed largely of ice. Two of these, [[Astronomy:Titan (moon)|Titan]] and [[Astronomy:Enceladus|Enceladus]], show signs of geological activity;<ref>{{Cite journal |last=Kargel |first=J. S. |date=1994 |title=Cryovolcanism on the icy satellites |url=https://zenodo.org/record/1232444 |url-status=live |journal=Earth, Moon, and Planets |volume=67 |issue=1–3 |pages=101–113 |bibcode=1995EM&P...67..101K |doi=10.1007/BF00613296 |s2cid=54843498 |archive-url=https://web.archive.org/web/20200310185023/https://zenodo.org/record/1232444 |archive-date=10 March 2020 |access-date=25 August 2019}}</ref> they, as well as five other Saturnian moons (Iapetus, [[Astronomy:Rhea (moon)|Rhea]], Dione, Tethys, and Mimas), are large enough to be round. Titan, the second-largest moon in the Solar System, is bigger than Mercury and the only satellite in the Solar System to have a substantial atmosphere.<ref>{{Cite web |date=27 June 2019 |title=Overview: Titan |url=https://solarsystem.nasa.gov/moons/saturn-moons/titan/overview |url-status=live |archive-url=https://web.archive.org/web/20220331190711/https://solarsystem.nasa.gov/moons/saturn-moons/titan/overview |archive-date=31 March 2022 |access-date=31 March 2022 |website=NASA Science: Solar System Exploration}}</ref><ref>{{Cite journal |last=Hörst |first=Sarah M. |date=2017 |title=Titan's Atmosphere and Climate |journal=Journal of Geophysical Research: Planets |volume=122 |issue=3 |pages=432–482 |arxiv=1702.08611 |bibcode=2017JGRE..122..432H |doi=10.1002/2016JE005240 |s2cid=119482985}}</ref>

==== Uranus ====

==== Neptune ====
{{Main|Astronomy:Neptune}}
Neptune ({{Convert|29.89|-|30.47|AU|e9km e9mi|abbr=unit}} from the Sun<ref name="nasa-factsheet"/>), though slightly smaller than Uranus, is more massive ({{Earth mass|17}}) and hence more [[Physics:Density|dense]]. It radiates more internal heat than Uranus, but not as much as Jupiter or Saturn.<ref>{{Cite journal |last1=Podolak |first1=M. |last2=Reynolds |first2=R. T. |last3=Young |first3=R. |date=1990 |title=Post Voyager comparisons of the interiors of Uranus and Neptune |url=https://zenodo.org/record/1231394 |url-status=live |journal=Geophysical Research Letters |type=Submitted manuscript |volume=17 |issue=10 |pages=1737–1740 |bibcode=1990GeoRL..17.1737P |doi=10.1029/GL017i010p01737 |archive-url=https://web.archive.org/web/20201201234426/https://zenodo.org/record/1231394 |archive-date=1 December 2020 |access-date=8 September 2020}}</ref> Neptune has [[Astronomy:Moons of Neptune|14 known satellites]]. The largest, [[Astronomy:Triton (moon)|Triton]], is geologically active, with [[Earth:Geyser|geyser]]s of [[Chemistry:Liquid nitrogen|liquid nitrogen]].<ref name="Soderblom2">{{Cite journal |last1=Soderblom |first1=L. A. |last2=Kieffer |first2=S. W. |last3=Becker |first3=T. L. |last4=Brown |first4=R. H. |last5=Cook |first5=A. F. II |last6=Hansen |first6=C. J. |last7=Johnson |first7=T. V. |last8=Kirk |first8=R. L. |last9=Shoemaker |first9=E. M. |date=19 October 1990 |title=Triton's Geyser-Like Plumes: Discovery and Basic Characterization |url=https://www.geology.illinois.edu/~skieffer/papers/Truiton_Science_1990.pdf |url-status=live |journal=Science |volume=250 |issue=4979 |pages=410–415 |bibcode=1990Sci...250..410S |doi=10.1126/science.250.4979.410 |pmid=17793016 |s2cid=1948948 |archive-url=https://web.archive.org/web/20210831121844/https://geology.illinois.edu/~skieffer/papers/Truiton_Science_1990.pdf |archive-date=31 August 2021 |access-date=31 March 2022}}</ref> Triton is the only large satellite with a retrograde orbit, which indicates that it did not form with Neptune, but was probably captured from the Kuiper belt.<ref>{{Cite journal |last1=Agnor |first1=C. B. |last2=Hamilton |first2=D. P. |year=2006 |title=Neptune's capture of its moon Triton in a binary–planet gravitational encounter |url=http://extranet.on.br/rodney/curso2010/aula9/tritoncapt_hamilton.pdf |url-status=live |journal=Nature |volume=441 |issue=7090 |pages=192–194 |bibcode=2006Natur.441..192A |doi=10.1038/nature04792 |pmid=16688170 |s2cid=4420518 |archive-url=https://web.archive.org/web/20161014134243/http://extranet.on.br/rodney/curso2010/aula9/tritoncapt_hamilton.pdf |archive-date=14 October 2016 |access-date=31 March 2022}}</ref> Neptune is accompanied in its orbit by several [[Astronomy:Minor planet|minor planet]]s, termed [[Astronomy:Neptune trojan|Neptune trojan]]s, that either lead or trail the planet by about one-sixth of the way around the Sun, positions known as [[Astronomy:Lagrange point|Lagrange point]]s.<ref>{{Cite journal |last1=Sheppard |first1=Scott S. |author-link=Scott S. Sheppard |last2=Trujillo |first2=Chadwick A. |author-link2=Chad Trujillo |date=10 September 2010 |title=Detection of a Trailing (L5) Neptune Trojan |journal=Science |language=en |volume=329 |issue=5997 |page=1304 |bibcode=2010Sci...329.1304S |doi=10.1126/science.1189666 |issn=0036-8075 |pmid=20705814 |s2cid=7657932 |doi-access=free}}</ref>

=== Centaurs ===
{{Main|Astronomy:Centaur (small Solar System body)}}
The centaurs are icy comet-like bodies whose orbits have semi-major axes greater than Jupiter's ({{Convert|5.5|AU|e6km e6mi|abbr=unit}}) and less than Neptune's ({{Convert|30|AU|e9km e9mi|abbr=unit}}). These are former Kuiper belt and scattered disc objects that were gravitationally [[Astronomy:Perturbation|perturbed]] closer to the Sun by the outer planets, and are expected to become comets or get ejected out of the Solar System.<ref name="Delsanti-Beyond_The_Planets"/> While most centaurs are inactive and asteroid-like, some exhibit clear cometary activity, such as the first centaur discovered, [[Astronomy:2060 Chiron|2060 Chiron]], which has been classified as a comet (95P) because it develops a coma just as comets do when they approach the Sun.<ref>{{Cite web |last=Vanouplines |first=Patrick |date=1995 |title=Chiron biography |url=http://www.vub.ac.be/STER/www.astro/chibio.htm |url-status=dead |archive-url=https://web.archive.org/web/20090502122306/http://www.vub.ac.be/STER/www.astro/chibio.htm |archive-date=2 May 2009 |access-date=23 June 2006 |website=Vrije Universitiet Brussel}}</ref> The largest known centaur, [[Astronomy:10199 Chariklo|10199 Chariklo]], has a diameter of about {{Convert|250|km|abbr=on}} and is one of the only few minor planets known to possess a ring system.<ref name="spitzer">{{Cite conference |last1=Stansberry |first1=John |last2=Grundy |first2=Will |last3=Brown |first3=Mike |last4=Cruikshank |first4=Dale |last5=Spencer |first5=John |last6=Trilling |first6=David |last7=Margot |first7=Jean-Luc |date=2007 |title=Physical Properties of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space Telescope |page=161 |arxiv=astro-ph/0702538 |bibcode=2008ssbn.book..161S |book-title=The Solar System Beyond Neptune}}</ref><ref name="Braga-Ribas-2014">{{Cite journal |last=Braga-Ribas |first=F. |display-authors=0 |date=April 2014 |title=A ring system detected around the Centaur (10199) Chariklo |journal=Nature |volume=508 |issue=7494 |pages=72–75 |arxiv=1409.7259 |bibcode=2014Natur.508...72B |doi=10.1038/nature13155 |issn=0028-0836 |pmid=24670644 |s2cid=4467484}}</ref>

== Comets ==
{{Main|Astronomy:Comet}}
[[File:Comet_Hale-Bopp_1995O1.jpg|thumb|150px|[[Astronomy:Comet Hale–Bopp|Comet Hale–Bopp]] seen in 1997]]
Comets are small Solar System bodies,<ref group="lower-alpha" name="footnoteB"/> typically only a few kilometres across, composed largely of volatile ices. They have highly eccentric orbits, generally a perihelion within the orbits of the inner planets and an aphelion far beyond Pluto. When a comet enters the inner Solar System, its proximity to the Sun causes its icy surface to sublimate and [[Physics:Ion|ion]]ise, creating a [[Astronomy:Coma (cometary)|coma]]: a long tail of gas and dust often visible to the naked eye.<ref>{{Cite web |date=19 December 2019 |title=In Depth: Comets |url=https://solarsystem.nasa.gov/asteroids-comets-and-meteors/comets/in-depth |url-status=live |archive-url=https://web.archive.org/web/20220331200633/https://solarsystem.nasa.gov/asteroids-comets-and-meteors/comets/in-depth |archive-date=31 March 2022 |access-date=31 March 2022 |website=NASA Science: Solar System Exploration}}</ref>

Short-period comets have orbits lasting less than two hundred years. Long-period comets have orbits lasting thousands of years. Short-period comets are thought to originate in the Kuiper belt, whereas long-period comets, such as [[Astronomy:Comet Hale–Bopp|Hale–Bopp]], are thought to originate in the Oort cloud. Many comet groups, such as the Kreutz sungrazers, formed from the breakup of a single parent.<ref>{{Cite journal |last=Sekanina |first=Zdeněk |date=2001 |title=Kreutz sungrazers: the ultimate case of cometary fragmentation and disintegration? |journal=Publications of the Astronomical Institute of the Academy of Sciences of the Czech Republic |volume=89 |pages=78–93 |bibcode=2001PAICz..89...78S}}</ref> Some comets with [[Astronomy:Hyperbolic trajectory|hyperbolic]] orbits may originate outside the Solar System, but determining their precise orbits is difficult.<ref name="hyperbolic">{{Cite journal |last=Królikowska |first=M. |date=2001 |title=A study of the original orbits of ''hyperbolic'' comets |journal=Astronomy & Astrophysics |volume=376 |issue=1 |pages=316–324 |bibcode=2001A&A...376..316K |doi=10.1051/0004-6361:20010945 |doi-access=free}}</ref> Old comets whose volatiles have mostly been driven out by solar warming are often categorised as asteroids.<ref>{{Cite journal |last=Whipple |first=Fred L. |date=1992 |title=The activities of comets related to their aging and origin |journal=[[Astronomy:Celestial Mechanics and Dynamical Astronomy|Celestial Mechanics and Dynamical Astronomy]] |volume=54 |issue=1–3 |pages=1–11 |bibcode=1992CeMDA..54....1W |doi=10.1007/BF00049540 |s2cid=189827311}}</ref>

== Trans-Neptunian region ==
[[File:TheTransneptunians 73AU.svg|thumb|Distribution and size of trans-Neptunian objects. The horizontal axis stand for the semi-major axis of the body, the vertical axis stands for the inclination of the orbit, and the size of the circle stands for the relative size of the object.|400x400px]] thumb|Size comparison of some large [[trans-Neptunian object|TNOs with Earth: {{Dp|Pluto}} and its moons, {{Dp|Eris}}, {{Dp|Makemake}}, {{Dp|Haumea}}, Sedna, Gonggong, Quaoar, Orcus, {{Dp|Salacia}}, and {{Mpl|2002 MS|4}}.]]

Beyond the orbit of Neptune lies the area of the "[[Astronomy:Trans-Neptunian object|trans-Neptunian region]]", with the doughnut-shaped Kuiper belt, home of Pluto and several other dwarf planets, and an overlapping disc of scattered objects, which is [[Astronomy:Orbital inclination|tilted toward the plane]] of the Solar System and reaches much further out than the Kuiper belt. The entire region is still [[Astronomy:Timeline of Solar System exploration|largely unexplored]]. It appears to consist overwhelmingly of many thousands of small worlds—the largest having a diameter only a fifth that of Earth and a mass far smaller than that of the Moon—composed mainly of rock and ice. This region is sometimes described as the "third zone of the Solar System", enclosing the inner and the outer Solar System.<ref>{{Cite web |last=Stern |first=Alan |date=February 2015 |title=Journey to the Solar System's Third Zone |url=https://www.americanscientist.org/article/journey-to-the-solar-systems-third-zone |url-status=live |archive-url=https://web.archive.org/web/20181026222414/https://www.americanscientist.org/article/journey-to-the-solar-systems-third-zone |archive-date=26 October 2018 |access-date=26 October 2018 |website=American Scientist}}</ref>

=== Kuiper belt ===

The Kuiper belt can be roughly divided into the "[[Astronomy:Classical Kuiper belt object|classical]]" belt and the resonant trans-Neptunian objects.<ref name=physical/> The latter have orbits whose periods are in a simple ratio to that of Neptune: for example, going around the Sun twice for every three times that Neptune does, or once for every two. The classical belt consists of objects having no resonance with Neptune, and extends from roughly {{Convert|39.4 to 47.7|AU|e9km e9mi|abbr=unit}}.<ref>{{Cite journal |last1=Buie |first1=M. W. |last2=Millis |first2=R. L. |last3=Wasserman |first3=L. H. |last4=Elliot |first4=J. L. |last5=Kern |first5=S. D. |last6=Clancy |first6=K. B. |last7=Chiang |first7=E. I. |last8=Jordan |first8=A. B. |last9=Meech |first9=K. J. |last10=Wagner |first10=R. M. |last11=Trilling |first11=D. E. |date=2005 |title=Procedures, Resources and Selected Results of the Deep Ecliptic Survey |journal=Earth, Moon, and Planets |volume=92 |issue=1 |pages=113–124 |arxiv=astro-ph/0309251 |bibcode=2003EM&P...92..113B |doi=10.1023/B:MOON.0000031930.13823.be |s2cid=14820512}}</ref> Members of the classical Kuiper belt are sometimes called "cubewanos", after the first of their kind to be discovered, originally designated [[Astronomy:15760 Albion|1992 QB<sub>1</sub>]]; they are still in near primordial, low-eccentricity orbits.<ref>{{Cite journal |last1=Dotto |first1=E. |last2=Barucci |first2=M. A. |last3=Fulchignoni |first3=M. |date=1 January 2003 |title=Beyond Neptune, the new frontier of the Solar System |url=http://sait.oat.ts.astro.it/MSAIS/3/PDF/20.pdf |url-status=live |journal=Memorie della Societa Astronomica Italiana Supplementi |volume=3 |page=20 |bibcode=2003MSAIS...3...20D |issn=0037-8720 |archive-url=https://web.archive.org/web/20140825122005/http://sait.oat.ts.astro.it/MSAIS/3/PDF/20.pdf |archive-date=25 August 2014 |access-date=26 December 2006}}</ref>

==== Pluto and Charon ====
{{Main|Astronomy:Pluto|Astronomy:Charon (moon)}}
The dwarf planet Pluto (with an average orbit of {{Convert|39|AU|e9km e9mi|abbr=unit}} from the Sun) is the largest known object in the Kuiper belt. When discovered in 1930, it was considered to be the ninth planet; this changed in 2006 with the adoption of a formal [[Astronomy:Definition of planet|definition of planet]]. Pluto has a relatively eccentric orbit inclined 17 degrees to the ecliptic plane and ranging from {{Convert|29.7|AU|e9km e9mi|abbr=unit}} from the Sun at perihelion (within the orbit of Neptune) to {{Convert|49.5|AU|e9km e9mi|abbr=unit}} at aphelion. Pluto has a 2:3 resonance with Neptune, meaning that Pluto orbits twice round the Sun for every three Neptunian orbits. Kuiper belt objects whose orbits share this resonance are called [[Astronomy:Plutino|plutino]]s.<ref name="Fajans_et_al_2001">{{Cite journal |last1=Fajans |first1=J. |last2=Frièdland |first2=L. |date=October 2001 |title=Autoresonant (nonstationary) excitation of pendulums, Plutinos, plasmas, and other nonlinear oscillators |url=http://ist-socrates.berkeley.edu/~fajans/pub/pdffiles/AutoPendAJP.pdf |url-status=dead |journal=[[Physics:American Journal of Physics|American Journal of Physics]] |volume=69 |issue=10 |pages=1096–1102 |bibcode=2001AmJPh..69.1096F |doi=10.1119/1.1389278 |archive-url=https://web.archive.org/web/20110607210435/http://ist-socrates.berkeley.edu/~fajans/pub/pdffiles/AutoPendAJP.pdf |archive-date=7 June 2011 |access-date=26 December 2006}}</ref>

Charon, the largest of [[Astronomy:Moons of Pluto|Pluto's moons]], is sometimes described as part of a binary system with Pluto, as the two bodies orbit a [[Barycenter|barycenter]] of gravity above their surfaces (i.e. they appear to "orbit each other"). Beyond Charon, four much smaller moons, Styx, Nix, Kerberos, and Hydra, orbit Pluto.<ref>{{Cite web |date=6 August 2021 |title=In Depth: Pluto |url=https://solarsystem.nasa.gov/planets/dwarf-planets/pluto/in-depth |url-status=live |archive-url=https://web.archive.org/web/20220331112026/https://solarsystem.nasa.gov/planets/dwarf-planets/pluto/in-depth |archive-date=31 March 2022 |access-date=31 March 2022 |website=NASA Science: Solar System Exploration}}</ref>

==== Others ====

Besides Pluto, astronomers generally agree that at least four other Kuiper belt objects are dwarf planets,<ref name=Grundy2019/> though there is some doubt for Orcus,<ref name=JWST>{{cite arXiv|last1=Emery|first1=J. P. |first2=I. |last2=Wong |first3=R. |last3=Brunetto |first4=J. C. |last4=Cook |first5=N. |last5=Pinilla-Alonso |first6=J. A. |last6=Stansberry |first7=B. J. |last7=Holler |first8=W. M. |last8=Grundy |first9=S. |last9=Protopapa |first10=A. C. |last10=Souza-Feliciano |first11=E. |last11=Fernández-Valenzuela |first12=J. I. |last12=Lunine |first13=D. C. |last13=Hines |date=26 September 2023|title=A Tale of 3 Dwarf Planets: Ices and Organics on Sedna, Gonggong, and Quaoar from JWST Spectroscopy|eprint=2309.15230|class=astro-ph.EP}}</ref> and additional bodies have also been proposed:<ref name="Tancredi2008">{{Cite journal |last1=Tancredi |first1=G. |last2=Favre |first2=S. A. |year=2008 |title=Which are the dwarfs in the Solar System? |journal=Icarus |volume=195 |issue=2 |pages=851–862 |bibcode=2008Icar..195..851T |doi=10.1016/j.icarus.2007.12.020}}</ref>

* [[Astronomy:Makemake|Makemake]] (45.79&nbsp;AU average from the Sun), although smaller than Pluto, is the largest known object in the ''classical'' Kuiper belt (that is, a Kuiper belt object not in a confirmed resonance with Neptune). Makemake is the brightest object in the Kuiper belt after Pluto. Discovered in 2005, it was officially named in 2009.<ref>{{Cite web |date=19 July 2009 |title=Fourth dwarf planet named Makemake |url=https://www.iau.org/news/pressreleases/detail/iau0806 |url-status=live |archive-url=https://web.archive.org/web/20170730222925/https://www.iau.org/news/pressreleases/detail/iau0806 |archive-date=30 July 2017 |access-date=9 April 2022 |website=International Astronomical Union}}</ref> Its orbit is far more inclined than Pluto's, at 29°.<ref name="Buie136472">{{Cite web |last=Buie |first=Marc W. |date=5 April 2008 |title=Orbit Fit and Astrometric record for 136472 |url=http://www.boulder.swri.edu/~buie/kbo/astrom/136472.html |url-status=live |archive-url=https://web.archive.org/web/20200527191044/https://www.boulder.swri.edu/~buie/kbo/astrom/136472.html |archive-date=27 May 2020 |access-date=15 July 2012 |publisher=SwRI (Space Science Department)}}</ref> It has one known moon.<ref name="ParkerA2016">{{Cite journal |last1=Parker |first1=A. H. |last2=Buie |first2=M. W. |last3=Grundy |first3=W. M. |last4=Noll |first4=K. S. |date=25 April 2016 |title=Discovery of a Makemakean Moon |journal=The Astrophysical Journal |volume=825 |issue=1 |page=L9 |arxiv=1604.07461 |bibcode=2016ApJ...825L...9P |doi=10.3847/2041-8205/825/1/L9 |s2cid=119270442 |doi-access=free}}</ref>
* [[Astronomy:Haumea|Haumea]] (43.13&nbsp;AU average from the Sun) is in an orbit similar to Makemake, except that it is in a temporary 7:12 orbital resonance with Neptune.<ref name="brownlargest">{{Cite book |last=Brown |first=Mike |title=The Solar System Beyond Neptune |publisher=University of Arizona Press |year=2008 |isbn=978-0-816-52755-7 |editor-last=Barucci |editor-first=M. Antonietta |pages=335–344 |chapter=The largest Kuiper belt objects |oclc=1063456240 |access-date=9 April 2022 |chapter-url=http://www.gps.caltech.edu/~mbrown/papers/ps/kbochap.pdf |archive-url=https://web.archive.org/web/20121113114533/http://www.gps.caltech.edu/~mbrown/papers/ps/kbochap.pdf |archive-date=13 November 2012 |url-status=live}}</ref> Like Makemake, it was discovered in 2005.<ref>{{Cite web |date=17 September 2008 |title=IAU names fifth dwarf planet Haumea |url=https://www.iau.org/news/pressreleases/detail/iau0807 |url-status=live |archive-url=https://web.archive.org/web/20140425065601/http://iau.org/public_press/news/detail/iau0807 |archive-date=25 April 2014 |access-date=9 April 2022 |website=International Astronomical Union}}</ref> Uniquely among the dwarf planets, Haumea possess a ring system, two known moons named Hiʻiaka and Namaka, and rotates so quickly (once every 3.9 hours) that it is stretched into an [[Ellipsoid|ellipsoid]]. It is part of a [[Engineering:Collisional family|collisional family]] of Kuiper belt objects that share similar orbits, which suggests a giant collision took place on Haumea and ejected its fragments into space billions of years ago.<ref name="Noviello2022">{{Cite journal |last1=Noviello |first1=Jessica L. |last2=Desch |first2=Stephen J. |last3=Neveu |first3=Marc |last4=Proudfoot |first4=Benjamin C. N. |last5=Sonnett |first5=Sarah |date=September 2022 |title=Let It Go: Geophysically Driven Ejection of the Haumea Family Members |journal=The Planetary Science Journal |volume=3 |issue=9 |page=19 |bibcode=2022PSJ.....3..225N |doi=10.3847/PSJ/ac8e03 |s2cid=252620869 |id=225 |doi-access=free}}</ref>
* {{Dp|Quaoar}} (43.69&nbsp;AU average from the Sun) is the second-largest known object in the classical Kuiper belt, after Makemake. Its orbit is significantly less eccentric and inclined than those of Makemake or Haumea.<ref name="brownlargest"/> It possesses a ring system and one known moon, Weywot.<ref name="Morgado2023">{{Cite Q|Q116754015|display-authors=1}}</ref>
* {{Dp|Orcus}} (39.40&nbsp;AU average from the Sun) is in the same 2:3 orbital resonance with Neptune as Pluto, and is the largest such object after Pluto itself.<ref name="brownlargest"/> Its eccentricity and inclination are similar to Pluto's, but its perihelion lies about 120° from that of Pluto. Thus, the [[Physics:Phase (waves)#Phase difference|phase]] of Orcus's orbit is opposite to Pluto's: Orcus is at aphelion (most recently in 2019) around when Pluto is at perihelion (most recently in 1989) and vice versa.<ref name="MPC2004-D15">{{Cite web |date=20 February 2004 |title=MPEC 2004-D15 : 2004 DW |url=http://www.minorplanetcenter.net/mpec/K04/K04D15.html |url-status=live |archive-url=https://web.archive.org/web/20160303232947/http://www.minorplanetcenter.net/mpec/K04/K04D15.html |archive-date=3 March 2016 |access-date=5 July 2011 |publisher=Minor Planet Center}}</ref> For this reason, it has been called the ''anti-Pluto''.<ref name="MBP">{{Cite web |last=Michael E. Brown |date=23 March 2009 |title=S/2005 (90482) 1 needs your help |url=http://www.mikebrownsplanets.com/2009/03/s1-90482-2005-needs-your-help.html |url-status=live |archive-url=https://web.archive.org/web/20090328012339/http://www.mikebrownsplanets.com/2009/03/s1-90482-2005-needs-your-help.html |archive-date=28 March 2009 |access-date=25 March 2009 |publisher=Mike Brown's Planets (blog)}}</ref><ref>{{Cite book |last=Moltenbrey |first=Michael |url=https://www.worldcat.org/oclc/926914921 |title=Dawn of Small Worlds: Dwarf planets, asteroids, comets |date=2016 |publisher=Springer |isbn=978-3-319-23003-0 |location=Cham |page=171 |oclc=926914921 |access-date=9 April 2022 |archive-url=https://web.archive.org/web/20220420161222/https://www.worldcat.org/title/dawn-of-small-worlds-dwarf-planets-asteroids-comets/oclc/926914921 |archive-date=20 April 2022 |url-status=live}}</ref> It has one known moon, [[Astronomy:Vanth (moon)|Vanth]].<ref name="IAUC8812">{{Cite web |last=Green |first=Daniel W. E. |date=22 February 2007 |title=IAUC 8812: Sats OF 2003 AZ_84, (50000), (55637),, (90482) |url=http://www.cbat.eps.harvard.edu/iauc/08800/08812.html |url-status=live |archive-url=https://web.archive.org/web/20120314060043/http://cbat.eps.harvard.edu/iauc/08800/08812.html |archive-date=14 March 2012 |access-date=4 July 2011 |publisher=International Astronomical Union Circular}}</ref>

=== Scattered disc ===
{{Main|Astronomy:Scattered disc}}
[[File:TheKuiperBelt Projections 100AU Classical SDO.svg|thumb|The orbital eccentricities and inclinations of the scattered disc population compared to the classical and resonant Kuiper belt objects]]
The scattered disc, which overlaps the Kuiper belt but extends out to near 500 AU, is thought to be the source of short-period comets. Scattered-disc objects are believed to have been perturbed into erratic orbits by the gravitational influence of [[Astronomy:Formation and evolution of the Solar System#Planetary migration|Neptune's early outward migration]]. Most scattered disc objects (SDOs) have perihelia within the Kuiper belt but aphelia far beyond it (some more than 150&nbsp;AU from the Sun). SDOs' orbits can also be inclined up to 46.8° from the ecliptic plane.<ref>{{Cite book |last1=Gomes |first1=R. S. |url=https://www.lpi.usra.edu/books/ssbn2008/7003.pdf |title=The Solar System Beyond Neptune |last2=Fernández |first2=J. A. |last3=Gallardo |first3=T. |last4=Brunini |first4=A. |date=2008 |publisher=University of Arizona Press |isbn=978-0816527557 |pages=259–273 |chapter=The Scattered Disk: Origins, Dynamics, and End States |access-date=12 May 2022 |archive-url=https://web.archive.org/web/20220121172507/https://www.lpi.usra.edu/books/ssbn2008/7003.pdf |archive-date=21 January 2022 |url-status=live}}</ref> Some astronomers consider the scattered disc to be merely another region of the Kuiper belt and describe scattered-disc objects as "scattered Kuiper belt objects".<ref>{{Cite web |last=Jewitt |first=David |date=2005 |title=The 1,000 km Scale KBOs |url=http://www2.ess.ucla.edu/~jewitt/kb/big_kbo.html |url-status=live |archive-url=https://web.archive.org/web/20140609134900/http://www2.ess.ucla.edu/~jewitt/kb/big_kbo.html |archive-date=9 June 2014 |access-date=16 July 2006 |website=University of Hawaii}}</ref> Some astronomers also classify centaurs as inward-scattered Kuiper belt objects along with the outward-scattered residents of the scattered disc.<ref>{{Cite web |title=List of Centaurs and Scattered-Disk Objects |url=http://www.minorplanetcenter.org/iau/lists/Centaurs.html |url-status=live |archive-url=https://web.archive.org/web/20170629210646/http://www.minorplanetcenter.org/iau/lists/Centaurs.html |archive-date=29 June 2017 |access-date=2 April 2007 |website=IAU: Minor Planet Center}}</ref>

==== Eris and Gonggong ====
{{Dp|Eris}} (67.78&nbsp;AU average from the Sun) is the largest known scattered disc object, and caused a debate about what constitutes a planet, because it is 25% more massive than Pluto<ref name="Brown Schaller 2007">{{Cite journal |last1=Brown |first1=Michael E. |last2=Schaller |first2=Emily L. |date=15 June 2007 |title=The Mass of Dwarf Planet Eris |journal=Science |volume=316 |issue=5831 |page=1585 |bibcode=2007Sci...316.1585B |doi=10.1126/science.1139415 |pmid=17569855 |s2cid=21468196|url=https://resolver.caltech.edu/CaltechAUTHORS:20121001-135149660 }}</ref> and about the same diameter. It is the most massive of the known dwarf planets. It has one known moon, [[Astronomy:Dysnomia (moon)|Dysnomia]]. Like Pluto, its orbit is highly eccentric, with a perihelion of 38.2 AU (roughly Pluto's distance from the Sun) and an aphelion of 97.6 AU, and steeply inclined to the ecliptic plane at an angle of 44°.<ref>{{Cite journal |last1=Dumas |first1=C. |last2=Merlin |first2=F. |last3=Barucci |first3=M. A. |last4=de Bergh |first4=C. |last5=Hainault |first5=O. |last6=Guilbert |first6=A. |last7=Vernazza |first7=P. |last8=Doressoundiram |first8=A. |date=August 2007 |title=Surface composition of the largest dwarf planet 136199 Eris (2003 UB{313}) |journal=Astronomy and Astrophysics |volume=471 |issue=1 |pages=331–334 |bibcode=2007A&A...471..331D |doi=10.1051/0004-6361:20066665 |doi-access=free}}</ref>

{{Dp|Gonggong}} (67.38&nbsp;AU average from the Sun) is another dwarf planet in a comparable orbit to Eris, except that it is in a 3:10 resonance with Neptune.<ref name="jpldata">{{Cite web |date=10 April 2017 |title=JPL Small-Body Database Browser: 225088 Gonggong (2007 OR10) |url=https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2225088 |url-status=live |archive-url=https://web.archive.org/web/20200610013703/https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2225088 |archive-date=10 June 2020 |access-date=20 February 2020 |publisher=[[Organization:Jet Propulsion Laboratory|Jet Propulsion Laboratory]] |type=20 September 2015 last obs.}}</ref> It has one known moon, Xiangliu.<ref name="Kissetal2017">{{Cite journal |last1=Kiss |first1=Csaba |last2=Marton |first2=Gábor |last3=Farkas-Takács |first3=Anikó |last4=Stansberry |first4=John |last5=Müller |first5=Thomas |last6=Vinkó |first6=József |last7=Balog |first7=Zoltán |last8=Ortiz |first8=Jose-Luis |last9=Pál |first9=András |date=16 March 2017 |title=Discovery of a Satellite of the Large Trans-Neptunian Object (225088) 2007 OR<sub>10</sub> |journal=The Astrophysical Journal Letters |volume=838 |page=5 |arxiv=1703.01407 |bibcode=2017ApJ...838L...1K |doi=10.3847/2041-8213/aa6484 |s2cid=46766640 |id=L1 |doi-access=free |number=1}}</ref>

== Farthest regions ==
The point at which the Solar System ends and interstellar space begins is not precisely defined because its outer boundaries are shaped by two forces: the solar wind and the Sun's gravity. The limit of the solar wind's influence is roughly four times Pluto's distance from the Sun; this ''heliopause'', the outer boundary of the [[Astronomy:Heliosphere|heliosphere]], is considered the beginning of the [[Astronomy:Interstellar medium|interstellar medium]].<ref name="Voyager"/> The Sun's [[Astronomy:Hill sphere|Hill sphere]], the effective range of its gravitational dominance, is thought to extend up to a thousand times farther and encompasses the hypothetical [[Astronomy:Oort cloud|Oort cloud]].<ref name="Littmann">{{Cite book |last=Littmann |first=Mark |url=https://archive.org/details/planetsbeyonddis00mlit |title=Planets Beyond: Discovering the Outer Solar System |date=2004 |publisher=Courier Dover Publications |isbn=978-0-486-43602-9 |pages=[https://archive.org/details/planetsbeyonddis00mlit/page/n92 162]–163 |url-access=limited}}</ref>

=== {{Anchor|Heliopause}} Edge of the heliosphere ===
[[File:Ibexheliosphererevised.jpg|thumb|300x300px|Artistic depiction of the Solar System's heliosphere]]
The Sun's [[Astronomy:Stellar-wind bubble|stellar-wind bubble]], the [[Astronomy:Heliosphere|heliosphere]], a region of space dominated by the Sun, has its boundary at the ''termination shock'', which is roughly 80–100&nbsp;AU from the Sun upwind of the interstellar medium and roughly 200&nbsp;AU from the Sun downwind.<ref name="fahr"/> Here the solar wind collides with the interstellar medium<ref>{{Cite web |last=Hatfield |first=Miles |date=3 June 2021 |title=The Heliopedia |url=http://www.nasa.gov/mission_pages/sunearth/the-heliopedia |url-status=live |archive-url=https://web.archive.org/web/20220325142928/https://www.nasa.gov/mission_pages/sunearth/the-heliopedia |archive-date=25 March 2022 |access-date=29 March 2022 |website=NASA}}</ref> and dramatically slows, condenses and becomes more turbulent,<ref name="fahr"/> forming a great oval structure known as the ''heliosheath''. This structure has been theorized to look and behave very much like a comet's tail, extending outward for a further 40&nbsp;AU on the upwind side but tailing many times that distance downwind.<ref>{{Cite journal |last1=Baranov |first1=V. B. |last2=Malama |first2=Yu. G. |date=1993 |title=Model of the solar wind interaction with the local interstellar medium: Numerical solution of self-consistent problem |url=http://doi.wiley.com/10.1029/93JA01171 |journal=Journal of Geophysical Research |language=en |volume=98 |issue=A9 |page=15157 |bibcode=1993JGR....9815157B |doi=10.1029/93JA01171 |issn=0148-0227 |access-date=9 April 2022}}</ref> Evidence from the ''Cassini'' and [[Astronomy:Interstellar Boundary Explorer|Interstellar Boundary Explorer]] spacecraft has suggested that it is forced into a bubble shape by the constraining action of the interstellar magnetic field,<ref>{{Cite web |date=19 November 2009 |title=Cassini's Big Sky: The View from the Center of Our Solar System |url=https://www.jpl.nasa.gov/news/cassinis-big-sky-the-view-from-the-center-of-our-solar-system |url-status=live |archive-url=https://web.archive.org/web/20220409213721/https://www.jpl.nasa.gov/news/cassinis-big-sky-the-view-from-the-center-of-our-solar-system |archive-date=9 April 2022 |access-date=9 April 2022 |website=Jet Propulsion Laboratory}}</ref><ref>{{Cite journal |last1=Kornbleuth |first1=M. |last2=Opher |first2=M. |last3=Baliukin |first3=I. |last4=Gkioulidou |first4=M. |last5=Richardson |first5=J. D. |last6=Zank |first6=G. P. |last7=Michael |first7=A. T. |last8=Tóth |first8=G. |last9=Tenishev |first9=V. |last10=Izmodenov |first10=V. |last11=Alexashov |first11=D. |date=1 December 2021 |title=The Development of a Split-tail Heliosphere and the Role of Non-ideal Processes: A Comparison of the BU and Moscow Models |journal=The Astrophysical Journal |volume=923 |issue=2 |page=179 |arxiv=2110.13962 |bibcode=2021ApJ...923..179K |doi=10.3847/1538-4357/ac2fa6 |issn=0004-637X |s2cid=239998560 |doi-access=free}}</ref> but the actual shape remains unknown.<ref>{{Cite journal |last1=Reisenfeld |first1=Daniel B. |last2=Bzowski |first2=Maciej |last3=Funsten |first3=Herbert O. |last4=Heerikhuisen |first4=Jacob |last5=Janzen |first5=Paul H. |last6=Kubiak |first6=Marzena A. |last7=McComas |first7=David J. |last8=Schwadron |first8=Nathan A. |last9=Sokół |first9=Justyna M. |last10=Zimorino |first10=Alex |last11=Zirnstein |first11=Eric J. |date=1 June 2021 |title=A Three-dimensional Map of the Heliosphere from IBEX |journal=The Astrophysical Journal Supplement Series |volume=254 |issue=2 |page=40 |bibcode=2021ApJS..254...40R |doi=10.3847/1538-4365/abf658 |issn=0067-0049 |osti=1890983 |s2cid=235400678 |doi-access=free}}</ref>

The outer boundary of the heliosphere, the ''heliopause'', is the point at which the solar wind finally terminates and is the beginning of interstellar space.<ref name="Voyager">{{Cite web |last=Steigerwald |first=Bill |date=24 May 2005 |title=Voyager Enters Solar System's Final Frontier |url=http://www.nasa.gov/vision/universe/solarsystem/voyager_agu.html |url-status=live |archive-url=https://web.archive.org/web/20200516082547/https://www.nasa.gov/vision/universe/solarsystem/voyager_agu.html |archive-date=16 May 2020 |access-date=2 April 2007 |website=NASA}}</ref> ''[[Astronomy:Voyager 1|Voyager 1]]'' and ''[[Astronomy:Voyager 2|Voyager 2]]'' passed the termination shock and entered the heliosheath at 94 and 84&nbsp;AU from the Sun, respectively.<ref>{{Cite journal |last1=Stone |first1=E. C. |last2=Cummings |first2=A. C. |last3=McDonald |first3=F. B. |last4=Heikkila |first4=B. C. |last5=Lal |first5=N. |last6=Webber |first6=W. R. |date=September 2005 |title=Voyager 1 explores the termination shock region and the heliosheath beyond |journal=Science |volume=309 |issue=5743 |pages=2017–2020 |bibcode=2005Sci...309.2017S |doi=10.1126/science.1117684 |pmid=16179468 |s2cid=34517751}}</ref><ref>{{Cite journal |last1=Stone |first1=E. C. |last2=Cummings |first2=A. C. |last3=McDonald |first3=F. B. |last4=Heikkila |first4=B. C. |last5=Lal |first5=N. |last6=Webber |first6=W. R. |date=July 2008 |title=An asymmetric solar wind termination shock |journal=Nature |volume=454 |issue=7200 |pages=71–74 |bibcode=2008Natur.454...71S |doi=10.1038/nature07022 |pmid=18596802 |s2cid=4431329}}</ref> ''Voyager 1'' was reported to have crossed the heliopause in August 2012, and ''Voyager 2'' in December 2018.<ref name="NASA-20130912">{{Cite web |last1=Cook |first1=Jia-Rui C. |last2=Agle |first2=D. C. |last3=Brown |first3=Dwayne |date=12 September 2013 |title=NASA Spacecraft Embarks on Historic Journey into Interstellar Space |url=http://www.nasa.gov/mission_pages/voyager/voyager20130912.html |url-status=live |archive-url=https://web.archive.org/web/20200611233345/https://www.nasa.gov/mission_pages/voyager/voyager20130912.html |archive-date=11 June 2020 |access-date=12 September 2013 |website=[[Organization:NASA|NASA]]}}</ref><ref>{{Cite web |last=Lakdawalla |first=Emily |date=10 December 2018 |title=News brief: Voyager 2 has passed beyond the heliopause |url=https://www.planetary.org/articles/voyager-2-heliopause |url-status=live |archive-url=https://web.archive.org/web/20210615225732/https://www.planetary.org/articles/voyager-2-heliopause |archive-date=15 June 2021 |access-date=10 April 2022 |website=The Planetary Society |language=en}}</ref>

The shape and form of the outer edge of the heliosphere is likely affected by the [[Physics:Fluid dynamics|fluid dynamics]] of interactions with the interstellar medium as well as solar magnetic fields prevailing to the south, e.g. it is bluntly shaped with the northern hemisphere extending 9&nbsp;AU farther than the southern hemisphere.<ref name="fahr">{{Cite journal |last1=Fahr |first1=H. J. |last2=Kausch |first2=T. |last3=Scherer |first3=H. |date=2000 |title=A 5-fluid hydrodynamic approach to model the Solar System-interstellar medium interaction |url=http://aa.springer.de/papers/0357001/2300268.pdf |url-status=dead |journal=Astronomy & Astrophysics |volume=357 |page=268 |bibcode=2000A&A...357..268F |archive-url=https://web.archive.org/web/20170808135422/http://aa.springer.de/papers/0357001/2300268.pdf |archive-date=8 August 2017 |access-date=24 August 2008}} See Figures 1 and 2.</ref> Beyond the heliopause, at around 230 AU, lies the [[Astronomy:Bow shock|bow shock]]: a plasma "wake" left by the Sun as it travels through the [[Astronomy:Milky Way|Milky Way]].<ref>{{Cite APOD | date=24 June 2002 |title=The Sun's Heliosphere & Heliopause |access-date=23 June 2006}}</ref>

=== Detached objects ===
[[File:Oort cloud Sedna orbit.svg|thumb|300x300px|The detached object Sedna and its orbit within the Solar System]]

=== Oort cloud ===
{{Main|Astronomy:Oort cloud}}
The Oort cloud is a hypothetical spherical cloud of up to a trillion icy objects that is thought to be the source for all long-period comets and to surround the Solar System at roughly 50,000&nbsp;AU (around 1&nbsp;[[Astronomy:Light-year|light-year]] (ly)) from the Sun, and possibly to as far as 100,000&nbsp;AU (1.87&nbsp;ly). It is thought to be composed of comets that were ejected from the inner Solar System by gravitational interactions with the outer planets. Oort cloud objects move very slowly, and can be perturbed by infrequent events, such as collisions, the gravitational effects of a passing star, or the [[Astronomy:Galactic tide|galactic tide]], the [[Physics:Tidal force|tidal force]] exerted by the Milky Way.<ref>{{Cite journal |vauthors=Stern SA, Weissman PR |date=2001 |title=Rapid collisional evolution of comets during the formation of the Oort cloud |journal=Nature |volume=409 |issue=6820 |pages=589–591 |bibcode=2001Natur.409..589S |doi=10.1038/35054508 |pmid=11214311 |s2cid=205013399}}</ref><ref>{{Cite web |last=Arnett |first=Bill |date=2006 |title=The Kuiper Belt and the Oort Cloud |url=http://www.nineplanets.org/kboc.html |url-status=live |archive-url=https://web.archive.org/web/20190807064224/http://nineplanets.org/kboc.html |archive-date=7 August 2019 |access-date=23 June 2006 |website=Nine Planets}}</ref>

=== Boundaries ===
Much of the Solar System is still unknown. The Sun's gravitational field is estimated to dominate the gravitational forces of [[Astronomy:List of nearest stars and brown dwarfs|surrounding stars]] out to about two light-years (125,000 AU). Lower estimates for the radius of the Oort cloud, by contrast, do not place it farther than 50,000 AU.<ref name="Encrenaz_et_al_2004">{{Cite book |last1=Encrenaz |first1=T. |title=The Solar System |last2=Bibring |first2=J. P. |last3=Blanc |first3=M. |last4=Barucci |first4=M. A. |last5=Roques |first5=F. |last6=Zarka |first6=P. H. |date=2004 |publisher=Springer |edition=3rd |page=1 |author-link=Thérèse Encrenaz}}</ref> Most of the mass is orbiting in the region between 3,000 and 100,000 AU.<ref>{{Cite journal |last1=Torres |first1=S. |last2=Cai |first2=M. X. |last3=Brown |first3=A. G. A. |last4=Portegies Zwart |first4=S. |date=September 2019 |title=Galactic tide and local stellar perturbations on the Oort cloud: creation of interstellar comets |journal=Astronomy & Astrophysics |volume=629 |page=13 |arxiv=1906.10617 |bibcode=2019A&A...629A.139T |doi=10.1051/0004-6361/201935330 |s2cid=195584070 |id=A139}}</ref> Despite discoveries such as Sedna, the region between the Kuiper belt and the Oort cloud, an area tens of thousands of AU in radius, is still virtually unmapped. Learning about this region of space is difficult, because it depends upon inferences from those few objects whose orbits happen to be perturbed such that they fall closer to the Sun, and even then, detecting these objects has often been possible only when they happened to become bright enough to register as comets.<ref>{{Cite journal |last1=Bernardinelli |first1=Pedro H. |last2=Bernstein |first2=Gary M. |last3=Montet |first3=Benjamin T. |last4=Weryk |first4=Robert |last5=Wainscoat |first5=Richard |last6=Aguena |first6=M. |last7=Allam |first7=S. |last8=Andrade-Oliveira |first8=F. |last9=Annis |first9=J. |last10=Avila |first10=S. |last11=Bertin |first11=E. |display-authors=3 |date=1 November 2021 |title=C/2014 UN 271 (Bernardinelli-Bernstein): The Nearly Spherical Cow of Comets |journal=The Astrophysical Journal Letters |volume=921 |issue=2 |page=L37 |arxiv=2109.09852 |bibcode=2021ApJ...921L..37B |doi=10.3847/2041-8213/ac32d3 |issn=2041-8205 |s2cid=237581632 |doi-access=free}}</ref> Objects may yet be discovered in the Solar System's uncharted regions.<ref>{{Cite web |last=Loeffler |first=John |date=1 October 2021 |title=Our solar system may have a hidden planet beyond Neptune – no, not that one |url=https://www.msn.com/en-us/news/technology/our-solar-system-may-have-a-hidden-planet-beyond-neptune-no-not-that-one/ar-AAP3b0l?ocid=msedgntp |url-status=live |archive-url=https://web.archive.org/web/20211001203656/https://www.msn.com/en-us/news/technology/our-solar-system-may-have-a-hidden-planet-beyond-neptune-no-not-that-one/ar-AAP3b0l?ocid=msedgntp |archive-date=1 October 2021 |access-date=7 April 2022 |website=MSN}}</ref> The furthest known objects, such as [[Astronomy:Comet West|Comet West]], have aphelia around 70,000 AU from the Sun.<ref>{{Cite web |last=Norman |first=Neil |date=May 2020 |title=10 great comets of recent times |url=https://www.skyatnightmagazine.com/space-science/greatest-comets-of-recent-times |url-status=live |archive-url=https://web.archive.org/web/20220125042109/https://www.skyatnightmagazine.com/space-science/greatest-comets-of-recent-times |archive-date=25 January 2022 |access-date=10 April 2022 |website=BBC Sky at Night Magazine |language=en}}</ref>

== Location ==

=== Celestial neighborhood ===
[[File:The Local Interstellar Cloud and neighboring G-cloud complex.svg|thumb|Diagram of the Local Interstellar Cloud, the G-Cloud and surrounding stars. As of 2022, the precise location of the Solar System in the clouds is an open question in astronomy.<ref>{{Cite journal |last1=Swaczyna |first1=Paweł |last2=Schwadron |first2=Nathan A. |last3=Möbius |first3=Eberhard |last4=Bzowski |first4=Maciej |last5=Frisch |first5=Priscilla C. |last6=Linsky |first6=Jeffrey L. |last7=McComas |first7=David J. |last8=Rahmanifard |first8=Fatemeh |last9=Redfield |first9=Seth |last10=Winslow |first10=Réka M. |last11=Wood |first11=Brian E. |last12=Zank |first12=Gary P. |date=2022-10-01 |title=Mixing Interstellar Clouds Surrounding the Sun |journal=The Astrophysical Journal Letters |volume=937 |issue=2 |pages=L32:1–2 |arxiv=2209.09927 |bibcode=2022ApJ...937L..32S |doi=10.3847/2041-8213/ac9120 |issn=2041-8205 |doi-access=free}}</ref>]]

The Solar System is surrounded by the Local Interstellar Cloud, although it is not clear if it is embedded in the Local Interstellar Cloud or if it lies just outside the cloud's edge.<ref>{{Cite web |date=5 June 2013 |title=Our Local Galactic Neighborhood |url=http://interstellar.jpl.nasa.gov/interstellar/probe/introduction/neighborhood.html |url-status=dead |archive-url=https://web.archive.org/web/20131121061128/http://interstellar.jpl.nasa.gov/interstellar/probe/introduction/neighborhood.html |archive-date=21 November 2013 |publisher=NASA}}</ref><ref name=":1">{{Cite journal |last1=Linsky |first1=Jeffrey L. |last2=Redfield |first2=Seth |last3=Tilipman |first3=Dennis |date=20 November 2019 |title=The Interface between the Outer Heliosphere and the Inner Local ISM: Morphology of the Local Interstellar Cloud, Its Hydrogen Hole, Strömgren Shells, and 60 Fe Accretion* |journal=The Astrophysical Journal |volume=886 |issue=1 |page=41 |arxiv=1910.01243 |bibcode=2019ApJ...886...41L |doi=10.3847/1538-4357/ab498a |issn=0004-637X |s2cid=203642080 |doi-access=free}}</ref> Multiple other [[Astronomy:Interstellar cloud|interstellar cloud]]s also exist in the region within 300 light-years of the Sun, known as the [[Astronomy:Local Bubble|Local Bubble]].<ref name=":1"/> The latter feature is an hourglass-shaped cavity or [[Astronomy:Superbubble|superbubble]] in the interstellar medium roughly 300 light-years across. The bubble is suffused with high-temperature plasma, suggesting that it may be the product of several recent supernovae.<ref>{{Cite journal |last1=Zucker |first1=Catherine |last2=Goodman |first2=Alyssa A. |last3=Alves |first3=João |last4=Bialy |first4=Shmuel |last5=Foley |first5=Michael |last6=Speagle |first6=Joshua S. |last7=Großschedl |first7=Josefa |last8=Finkbeiner |first8=Douglas P. |last9=Burkert |first9=Andreas |last10=Khimey |first10=Diana |last11=Swiggum |first11=Cameren |display-authors=3 |date=January 2022 |title=Star formation near the Sun is driven by expansion of the Local Bubble |url=https://www.nature.com/articles/s41586-021-04286-5 |url-status=live |journal=Nature |language=en |volume=601 |issue=7893 |pages=334–337 |arxiv=2201.05124 |bibcode=2022Natur.601..334Z |doi=10.1038/s41586-021-04286-5 |issn=1476-4687 |pmid=35022612 |s2cid=245906333 |archive-url=https://web.archive.org/web/20220417115600/https://www.nature.com/articles/s41586-021-04286-5 |archive-date=17 April 2022 |access-date=1 April 2022}}</ref>

The Local Bubble is a small superbubble compared to the neighboring wider Radcliffe Wave and ''Split'' linear structures (formerly [[Astronomy:Gould Belt|Gould Belt]]), each of which are some thousands of light-years in length.<ref name="Alves Zucker Goodman Speagle 2020">{{Cite journal |last1=Alves |first1=João |last2=Zucker |first2=Catherine |last3=Goodman |first3=Alyssa A. |last4=Speagle |first4=Joshua S. |last5=Meingast |first5=Stefan |last6=Robitaille |first6=Thomas |last7=Finkbeiner |first7=Douglas P. |last8=Schlafly |first8=Edward F. |last9=Green |first9=Gregory M. |date=23 January 2020 |title=A Galactic-scale gas wave in the Solar Neighborhood |journal=Nature |volume=578 |issue=7794 |pages=237–239 |arxiv=2001.08748v1 |bibcode=2020Natur.578..237A |doi=10.1038/s41586-019-1874-z |pmid=31910431 |s2cid=210086520}}</ref> All these structures are part of the Orion Arm, which contains most of the stars in the Milky Way that are visible to the unaided eye. The density of all matter in the local neighborhood is {{Val|0.097|0.013|ul=solar mass}}·pc<sup>−3</sup>.<ref>{{Cite journal |last1=McKee |first1=Christopher F. |last2=Parravano |first2=Antonio |last3=Hollenbach |first3=David J. |date=November 2015 |title=Stars, Gas, and Dark Matter in the Solar Neighborhood |journal=The Astrophysical Journal |volume=814 |issue=1 |pages=24 |arxiv=1509.05334 |bibcode=2015ApJ...814...13M |doi=10.1088/0004-637X/814/1/13 |s2cid=54224451 |id=13}}</ref>

[[Astronomy:List of nearest stars and brown dwarfs|Within ten light-years of the Sun]] there are relatively few stars, the closest being the triple star system [[Astronomy:Alpha Centauri|Alpha Centauri]], which is about 4.4 light-years away and may be in the Local Bubble's [[Astronomy:G-Cloud|G-Cloud]].<ref>{{Cite journal |last1=Linsky |first1=Jeffrey L. |last2=Redfield |first2=Seth |last3=Tilipman |first3=Dennis |date=November 2019 |title=The Interface between the Outer Heliosphere and the Inner Local ISM: Morphology of the Local Interstellar Cloud, Its Hydrogen Hole, Strömgren Shells, and 60Fe Accretion |journal=The Astrophysical Journal |volume=886 |issue=1 |page=19 |arxiv=1910.01243 |bibcode=2019ApJ...886...41L |doi=10.3847/1538-4357/ab498a |s2cid=203642080 |id=41 |doi-access=free}}</ref> Alpha Centauri A and B are a closely tied pair of Sun-like stars, whereas the closest star to Earth, the small [[Astronomy:Red dwarf|red dwarf]] [[Astronomy:Proxima Centauri|Proxima Centauri]], orbits the pair at a distance of 0.2 light-year. In 2016, a potentially habitable [[Astronomy:Exoplanet|exoplanet]] was found to be orbiting Proxima Centauri, called [[Astronomy:Proxima Centauri b|Proxima Centauri b]], the closest confirmed exoplanet to the Sun.<ref name="proxima b discovery paper">{{Cite journal |last1=Anglada-Escudé |first1=Guillem |last2=Amado |first2=Pedro J. |last3=Barnes |first3=John |last4=Berdiñas |first4=Zaira M. |last5=Butler |first5=R. Paul |last6=Coleman |first6=Gavin A. L. |last7=de la Cueva |first7=Ignacio |last8=Dreizler |first8=Stefan |last9=Endl |first9=Michael |last10=Giesers |first10=Benjamin |last11=Jeffers |first11=Sandra V. |last12=Jenkins |first12=James S. |last13=Jones |first13=Hugh R. A. |last14=Kiraga |first14=Marcin |last15=Kürster |first15=Martin |display-authors=3 |year=2016 |title=A terrestrial planet candidate in a temperate orbit around Proxima Centauri |url=https://www.nature.com/articles/nature19106 |url-status=live |journal=Nature |volume=536 |issue=7617 |pages=437–440 |arxiv=1609.03449 |bibcode=2016Natur.536..437A |doi=10.1038/nature19106 |pmid=27558064 |s2cid=4451513 |archive-url=https://web.archive.org/web/20211003204853/https://www.nature.com/articles/nature19106 |archive-date=3 October 2021 |access-date=11 September 2021 |last16=López-González |first16=María J. |last17=Marvin |first17=Christopher J. |last18=Morales |first18=Nicolás |last19=Morin |first19=Julien |last20=Nelson |first20=Richard P. |last21=Ortiz |first21=José L. |last22=Ofir |first22=Aviv |last23=Paardekooper |first23=Sijme-Jan |last24=Reiners |first24=Ansgar |last25=Rodríguez |first25=Eloy |last26=Rodríguez-López |first26=Cristina |last27=Sarmiento |first27=Luis F. |last28=Strachan |first28=John P. |last29=Tsapras |first29=Yiannis |last30=Tuomi |first30=Mikko |first31=Mathias |last31=Zechmeister}}</ref>

The next closest known [[Astronomy:Fusor|fusors]] to the Sun are the red dwarfs [[Astronomy:Barnard's Star|Barnard's Star]] (at 5.9&nbsp;ly), [[Astronomy:Wolf 359|Wolf 359]] (7.8&nbsp;ly), and [[Astronomy:Lalande 21185|Lalande 21185]] (8.3&nbsp;ly).<ref name="RECONS">{{Cite web |date=17 September 2007 |title=The One Hundred Nearest Star Systems |url=http://www.astro.gsu.edu/RECONS/TOP100.posted.htm |url-status=live |archive-url=https://web.archive.org/web/20071112173559/http://www.chara.gsu.edu/RECONS/TOP100.posted.htm |archive-date=12 November 2007 |access-date=1 May 2022 |website=Georgia State University Astronomy Department |publisher=Research Consortium on Nearby Stars (RECONS)}}</ref> The nearest [[Astronomy:Brown dwarf|brown dwarf]]s belong to the binary [[Astronomy:Luhman 16|Luhman 16]] system (6.6&nbsp;ly), and the closest known [[Astronomy:Rogue planet|rogue]] or free-floating [[Astronomy:Planetary-mass object|planetary-mass object]] at less than 10 Jupiter masses is the [[Astronomy:Sub-brown dwarf|sub-brown dwarf]] [[Astronomy:WISE 0855−0714|WISE 0855−0714]] (7.4&nbsp;ly).<ref>{{Cite journal |last=Luhman |first=K. L. |year=2014 |title=Discovery of a ~250 K Brown Dwarf at 2 pc from the Sun |journal=The Astrophysical Journal |volume=786 |issue=2 |page=L18 |arxiv=1404.6501 |bibcode=2014ApJ...786L..18L |doi=10.1088/2041-8205/786/2/L18 |s2cid=119102654}}</ref>

Just beyond at 8.6&nbsp;ly lies [[Astronomy:Sirius|Sirius]], the [[Astronomy:List of brightest stars|brightest star]] in Earth's [[Astronomy:Night sky|night sky]], with roughly twice the Sun's mass, orbited by the closest [[Astronomy:White dwarf|white dwarf]] to Earth, Sirius B. Other stars within ten light-years are the binary red-dwarf system [[Astronomy:Gliese 65|Gliese 65]] (8.7&nbsp;ly) and the solitary red dwarf [[Astronomy:Ross 154|Ross 154]] (9.7&nbsp;ly).<ref>{{Cite book |url=https://www.worldcat.org/oclc/51003837 |title=Fundamental Astronomy |date=2003 |publisher=Springer |isbn=978-3-540-00179-9 |editor-last=Karttunen |editor-first=Hannu |edition=4th |location=Berlin |page=414 |oclc=51003837 |access-date=1 April 2022 |editor-last2=Oja |editor-first2=Heikki |editor-last3=Donner |editor-first3=Karl Johan |editor-last4=Poutanen |editor-first4=Markku |editor-last5=Kröger |editor-first5=Pekka |archive-url=https://web.archive.org/web/20220420161228/https://www.worldcat.org/title/fundamental-astronomy/oclc/51003837 |archive-date=20 April 2022 |url-status=live}}</ref><ref>{{Cite journal |last=van Leeuwen |first=F. |date=November 2007 |title=Validation of the new Hipparcos reduction |journal=Astronomy and Astrophysics |volume=474 |issue=2 |pages=653–664 |arxiv=0708.1752 |bibcode=2007A&A...474..653V |doi=10.1051/0004-6361:20078357 |s2cid=18759600}}</ref> The closest solitary Sun-like star to the Solar System is [[Astronomy:Tau Ceti|Tau Ceti]] at 11.9 light-years. It has roughly 80% of the Sun's mass but only about half of its luminosity.<ref name="aaa494">{{Cite journal |last1=Teixeira |first1=T. C. |last2=Kjeldsen |first2=H. |last3=Bedding |first3=T. R. |last4=Bouchy |first4=F. |last5=Christensen-Dalsgaard |first5=J. |author-link5=Jørgen Christensen-Dalsgaard |last6=Cunha |first6=M. S. |last7=Dall |first7=T. |last8=Frandsen |first8=S. |last9=Karoff |first9=C. |last10=Monteiro |first10=M. J. P. F. G. |last11=Pijpers |first11=F. P. |display-authors=6 |date=January 2009 |title=Solar-like oscillations in the G8 V star τ Ceti |journal=Astronomy and Astrophysics |volume=494 |issue=1 |pages=237–242 |arxiv=0811.3989 |bibcode=2009A&A...494..237T |doi=10.1051/0004-6361:200810746 |s2cid=59353134}}</ref>

The nearest and unaided-visible group of stars beyond the immediate celestial neighborhood is the [[Astronomy:Ursa Major moving group|Ursa Major moving group]] at roughly 80 light-years, which is within the Local Bubble, like the nearest as well as unaided-visible [[Astronomy:Star cluster|star cluster]] the [[Astronomy:Hyades (star cluster)|Hyades]], which lie at its edge. The closest star-forming regions are the Corona Australis Molecular Cloud, the [[Astronomy:Rho Ophiuchi cloud complex|Rho Ophiuchi cloud complex]] and the [[Astronomy:Taurus molecular cloud|Taurus molecular cloud]]; the latter lies just beyond the Local Bubble and is part of the Radcliffe wave.<ref>{{Cite journal |last1=Alves |first1=João |last2=Zucker |first2=Catherine |last3=Goodman |first3=Alyssa A. |last4=Speagle |first4=Joshua S. |last5=Meingast |first5=Stefan |last6=Robitaille |first6=Thomas |last7=Finkbeiner |first7=Douglas P. |last8=Schlafly |first8=Edward F. |last9=Green |first9=Gregory M. |display-authors=3 |year=2020 |title=A Galactic-scale gas wave in the solar neighborhood |journal=Nature |volume=578 |issue=7794 |pages=237–239 |arxiv=2001.08748 |bibcode=2020Natur.578..237A |doi=10.1038/s41586-019-1874-z |pmid=31910431 |s2cid=210086520}}</ref>

=== Galactic position and orbit ===
[[File:Milky Way side view.png|thumb|300x300px|Diagram of the Milky Way, with galactic features and the relative position of the Solar System labelled.]]

The Solar System is located in the [[Astronomy:Milky Way|Milky Way]], a [[Astronomy:Barred spiral galaxy|barred spiral galaxy]] with a diameter of about 100,000 [[Astronomy:Light-year|light-year]]s containing more than 100&nbsp;billion stars.<ref name="Lang2013">{{Cite book |last=Lang |first=Kenneth R. |url=https://books.google.com/books?id=MN-UCkUK9pcC&pg=PA264 |title=The Life and Death of Stars |date=2013 |publisher=Cambridge University Press |isbn=978-1107016385 |page=264 |access-date=8 April 2022 |archive-url=https://web.archive.org/web/20220420161220/https://www.google.com/books/edition/The_Life_and_Death_of_Stars/MN-UCkUK9pcC?gbpv=1&pg=PA264 |archive-date=20 April 2022 |url-status=live}}</ref> The Sun is part of one of the Milky Way's outer spiral arms, known as the Orion–Cygnus Arm or Local Spur.<ref>{{Cite journal |last1=Drimmel |first1=R. |last2=Spergel |first2=D. N. |date=2001 |title=Three Dimensional Structure of the Milky Way Disk |journal=[[Astronomy:The Astrophysical Journal|The Astrophysical Journal]] |volume=556 |issue=1 |pages=181–202 |arxiv=astro-ph/0101259 |bibcode=2001ApJ...556..181D |doi=10.1086/321556 |s2cid=15757160}}</ref>

The Sun orbits close to circular the [[Galactic Center]] (where the [[Astronomy:Supermassive black hole|supermassive black hole]] [[Astronomy:Sagittarius A*|Sagittarius A*]] resides) at a distance of 26,660 light-years,<ref>{{Cite journal |last1=Abuter |first1=R. |last2=Amorim |first2=A. |last3=Bauböck |first3=M. |last4=Berger |first4=J. P. |last5=Bonnet |first5=H. |last6=Brandner |first6=W. |last7=Clénet |first7=Y. |last8=Coudé du Foresto |first8=V. |last9=de Zeeuw |first9=P. T. |last10=Dexter |first10=J. |display-authors=6 |date=May 2019 |title=A geometric distance measurement to the Galactic center black hole with 0.3% uncertainty |url=https://www.aanda.org/10.1051/0004-6361/201935656 |url-status=live |journal=Astronomy & Astrophysics |volume=625 |page=L10 |arxiv=1904.05721 |bibcode=2019A&A...625L..10G |doi=10.1051/0004-6361/201935656 |issn=0004-6361 |s2cid=119190574 |archive-url=https://web.archive.org/web/20220420161243/https://www.aanda.org/articles/aa/full_html/2019/05/aa35656-19/aa35656-19.html |archive-date=20 April 2022 |access-date=1 April 2022}}</ref> orbiting at roughly the same speed as that of the spiral arms.<ref name="astrobiology"/><ref>{{Cite journal |last=Gerhard |first=O. |date=2011 |title=Pattern speeds in the Milky Way |journal=Memorie della Societa Astronomica Italiana, Supplementi |volume=18 |page=185 |arxiv=1003.2489 |bibcode=2011MSAIS..18..185G}}</ref> Therefore, the Sun passes through arms only rarely. 

Its speed around the center of the Milky Way is about 220&nbsp;km/s, so that it completes one revolution every 240&nbsp;million years.<ref name="Lang2013"/> This revolution is known as the Solar System's [[Astronomy:Galactic year|galactic year]].<ref>{{Cite web |last=Leong |first=Stacy |date=2002 |title=Period of the Sun's Orbit around the Galaxy (Cosmic Year) |url=http://hypertextbook.com/facts/2002/StacyLeong.shtml |url-status=live |archive-url=https://web.archive.org/web/20190107010909/https://hypertextbook.com/facts/2002/StacyLeong.shtml |archive-date=7 January 2019 |access-date=2 April 2007 |website=The Physics Factbook}}</ref> The [[Astronomy:Solar apex|solar apex]], the direction of the Sun's path through interstellar space, is near the constellation [[Astronomy:Hercules (constellation)|Hercules]] in the direction of the current location of the bright star [[Astronomy:Vega|Vega]].<ref>{{Cite book |last=Greiner |first=Walter |url=https://www.worldcat.org/oclc/56727455 |title=Classical Mechanics: Point particles and relativity |date=2004 |publisher=Springer |isbn=978-0-387-21851-9 |location=New York |page=323 |oclc=56727455 |access-date=29 March 2022 |archive-url=https://web.archive.org/web/20220420161222/https://www.worldcat.org/title/classical-mechanics-point-particles-and-relativity/oclc/56727455 |archive-date=20 April 2022 |url-status=live}}</ref> The plane of the ecliptic lies at an angle of about 60° to the [[Astronomy:Galactic plane|galactic plane]].{{Refn |group=lower-alpha |1=If <math>\psi</math> is the angle between the north pole of the ecliptic and the north galactic pole then:
<br/>{{Big|1=<math>\cos\psi=\cos(\beta_g)\cos(\beta_e)\cos(\alpha_g-\alpha_e)+\sin(\beta_g)\sin(\beta_e)</math>}}<br/>
where <math>\beta_g</math> = 27° 07′ 42.01″ and <math>\alpha_g</math> = 12h 51m 26.282s are the declination and right ascension of the north galactic pole,<ref>{{Cite journal |last1=Reid |first1=M. J. |last2=Brunthaler |first2=A. |date=2004 |title=The Proper Motion of Sagittarius A* |journal=[[Astronomy:The Astrophysical Journal|The Astrophysical Journal]] |volume=616 |issue=2 |pages=872–884 |arxiv=astro-ph/0408107 |bibcode=2004ApJ...616..872R |doi=10.1086/424960 |s2cid=16568545}}</ref> whereas <math>\beta_e</math> = 66° 33′ 38.6″ and <math>\alpha_e</math> = 18h 0m 00s are those for the north pole of the ecliptic. (Both pairs of coordinates are for [[Astronomy:J2000|J2000]] epoch.) The result of the calculation is 60.19°.}}

====Habitability of galactic position and orbit====
The Solar System's location in the Milky Way is a factor in the [[Biology:Evolutionary history of life|evolutionary history of life]] on Earth. Spiral arms are home to a far larger concentration of [[Astronomy:Supernova|supernova]]e, gravitational instabilities, and radiation that could disrupt the Solar System, but since Earth stays in the Local Spur and therefore does not pass frequently through spiral arms, this has given Earth long periods of stability for life to evolve.<ref name="astrobiology">{{Cite web |last=Mullen |first=Leslie |date=18 May 2001 |title=Galactic Habitable Zones |url=http://www.astrobio.net/news-exclusive/galactic-habitable-zones |url-status=live |archive-url=https://web.archive.org/web/20110807024530/http://www.astrobio.net/exclusive/139/galactic-habitable-zones |archive-date=7 August 2011 |access-date=1 June 2020 |website=Astrobiology Magazine}}</ref> However, the changing position of the Solar System relative to other parts of the Milky Way could explain periodic extinction events on Earth, according to the [[Unsolved:Shiva hypothesis|Shiva hypothesis]] or related theories, but this remains controversial.<ref>{{Cite journal |last=Bailer-Jones |first=C. A. L. |date=1 July 2009 |title=The evidence for and against astronomical impacts on climate change and mass extinctions: a review |url=https://ui.adsabs.harvard.edu/abs/2009IJAsB...8..213B |url-status=live |journal=International Journal of Astrobiology |volume=8 |issue=3 |pages=213–219 |arxiv=0905.3919 |bibcode=2009IJAsB...8..213B |doi=10.1017/S147355040999005X |s2cid=2028999 |archive-url=https://web.archive.org/web/20220401231355/https://ui.adsabs.harvard.edu/abs/2009IJAsB...8..213B |archive-date=1 April 2022 |access-date=1 April 2022}}</ref><ref>{{Cite journal |last=Racki |first=Grzegorz |date=December 2012 |title=The Alvarez Impact Theory of Mass Extinction; Limits to its Applicability and the "Great Expectations Syndrome" |url=https://www.app.pan.pl/article/item/app20110058.html |url-status=live |journal=Acta Palaeontologica Polonica |language=en |volume=57 |issue=4 |pages=681–702 |doi=10.4202/app.2011.0058 |issn=0567-7920 |s2cid=54021858 |archive-url=https://web.archive.org/web/20220401214314/https://www.app.pan.pl/article/item/app20110058.html |archive-date=1 April 2022 |access-date=1 April 2022 |doi-access=free|hdl=20.500.12128/534 |hdl-access=free }}</ref>

The Solar System lies well outside the star-crowded environs of the [[Galactic Center]]. Near the center, gravitational tugs from nearby stars could perturb bodies in the Oort cloud and send many comets into the inner Solar System, producing collisions with potentially catastrophic implications for life on Earth. The intense radiation of the Galactic Center could also interfere with the development of complex life.<ref name="astrobiology"/> Stellar flybys that pass within {{Convert|0.25|pc|ly|1|abbr=off|disp=out}} of the Sun occur roughly once every 100,000&nbsp;years. The [[Astronomy:List of nearest stars and brown dwarfs#Distant future and past encounters|closest well-measured approach]] was [[Astronomy:Scholz's Star|Scholz's Star]], which approached to {{Val|52|23|14|u=kAU|fmt=commas}} of the Sun some {{Val|70|15|10|ul=kya}}, likely passing through the outer Oort cloud.<ref>{{Cite journal |last1=Mamajek |first1=Eric E. |last2=Barenfeld |first2=Scott A. |last3=Ivanov |first3=Valentin D. |last4=Kniazev |first4=Alexei Y. |last5=Väisänen |first5=Petri |last6=Beletsky |first6=Yuri |last7=Boffin |first7=Henri M. J. |date=February 2015 |title=The Closest Known Flyby of a Star to the Solar System |journal=The Astrophysical Journal Letters |volume=800 |issue=1 |page=4 |arxiv=1502.04655 |bibcode=2015ApJ...800L..17M |doi=10.1088/2041-8205/800/1/L17 |s2cid=40618530 |id=L17}}</ref>

== Comparison with extrasolar systems ==
[[File:Planetary habitable zones of the Solar System and the Gliese 581.jpg|alt=|thumb|300x300px|Comparison between the [[Astronomy:Habitable zone|habitable zones]] of the Solar System and Gliese 581 (planet d was later found to not exist). The habitable zone is highly dependent on parent star's luminosity.]]
Compared to many extrasolar systems, the Solar System stands out in lacking planets interior to the orbit of Mercury.<ref name="Martin082015">{{Cite journal |last1=Martin |first1=Rebecca G. |last2=Livio |first2=Mario |year=2015 |title=The Solar System as an Exoplanetary System |journal=The Astrophysical Journal |volume=810 |issue=2 |page=105 |arxiv=1508.00931 |bibcode=2015ApJ...810..105M |doi=10.1088/0004-637X/810/2/105 |s2cid=119119390}}</ref><ref>{{Cite journal |last=Kohler |first=Susanna |date=25 September 2015 |title=How Normal is Our Solar System? |url=https://aasnova.org/2015/09/25/how-normal-is-our-solar-system |url-status=live |journal=Aas Nova Highlights |publisher=American Astronomical Society |page=313 |bibcode=2015nova.pres..313K |archive-url=https://web.archive.org/web/20220407043952/https://aasnova.org/2015/09/25/how-normal-is-our-solar-system |archive-date=7 April 2022 |access-date=31 March 2022}}</ref> The known Solar System also lacks [[Astronomy:Super-Earth|super-Earth]]s, planets between one and ten times as massive as the Earth,<ref name="Martin082015"/> although the hypothetical [[Astronomy:Planet Nine|Planet Nine]], if it does exist, could be a super-Earth orbiting in the outer Solar System.<ref>{{Cite journal |last1=Sheppard |first1=Scott S. |last2=Trujillo |first2=Chadwick |author-link2=Chad Trujillo |date=7 December 2016 |title=New extreme trans-Neptunian objects: Toward a super-Earth in the outer solar system |journal=The Astronomical Journal |volume=152 |issue=6 |page=221 |arxiv=1608.08772 |bibcode=2016AJ....152..221S |doi=10.3847/1538-3881/152/6/221 |issn=1538-3881 |s2cid=119187392 |doi-access=free}}</ref> Uncommonly, it has only small rocky planets and large gas giants; elsewhere planets of intermediate size are typical—both rocky and gas—so there is no "gap" as seen between the size of Earth and of Neptune (with a radius 3.8 times as large). As many of these super-Earths are closer to their respective stars than Mercury is to the Sun, a hypothesis has arisen that all planetary systems start with many close-in planets, and that typically a sequence of their collisions causes consolidation of mass into few larger planets, but in case of the Solar System the collisions caused their destruction and ejection.<ref name="Martin082015"/><ref>{{Cite journal |last1=Volk |first1=Kathryn |last2=Gladman |first2=Brett |year=2015 |title=Consolidating and Crushing Exoplanets: Did it happen here? |journal=The Astrophysical Journal Letters |volume=806 |page=L26 |arxiv=1502.06558 |bibcode=2015ApJ...806L..26V |doi=10.1088/2041-8205/806/2/L26 |s2cid=118052299 |number=2}}</ref>

The orbits of Solar System planets are nearly circular. Compared to other systems, they have smaller [[Astronomy:Orbital eccentricity|orbital eccentricity]].<ref name="Martin082015"/> Although there are attempts to explain it partly with a bias in the [[Astronomy:Doppler spectroscopy|radial-velocity detection method]] and partly with long interactions of a quite high number of planets, the exact causes remain undetermined.<ref name="Martin082015"/><ref>{{Cite journal |last1=Goldreich |first1=Peter |last2=Lithwick |first2=Yoram |last3=Sari |first3=Re'em |year=2004 |title=Final Stages of Planet Formation |journal=The Astrophysical Journal |volume=614 |issue=1 |pages=497–507 |arxiv=astro-ph/0404240 |bibcode=2004ApJ...614..497G |doi=10.1086/423612 |s2cid=16419857}}</ref>

== Humanity's perspective ==
{{Main|Astronomy:Discovery and exploration of the Solar System}}
[[File:Apparent_retrograde_motion_of_Mars_in_2003.gif|thumb|The motion of 'lights' moving across the sky is the basis of the classical definition of planets: wandering stars.]]
Humanity's knowledge of the Solar System has grown incrementally over the centuries. Up to the [[History:Late Middle Ages|Late Middle Ages]]–[[Philosophy:Renaissance|Renaissance]], astronomers from Europe to India believed Earth to [[Geocentric model|be stationary at the center]] of the universe<ref>{{Cite book |last=Orrell |first=David |url=https://books.google.com/books?id=mNMsa18vTpsC&pg=PA25 |title=Truth Or Beauty: Science and the Quest for Order |date=2012 |publisher=Yale University Press |isbn=978-0300186611 |pages=25–27 |access-date=13 May 2022 |archive-url=https://web.archive.org/web/20220730084322/https://www.google.com/books/edition/Truth_Or_Beauty/mNMsa18vTpsC?gbpv=1&pg=PA25 |archive-date=30 July 2022 |url-status=live}}</ref> and categorically different from the divine or ethereal objects that moved through the sky. Although the {{wipe|Ancient Greece}} philosopher [[Biography:Aristarchus of Samos|Aristarchus of Samos]] had speculated on a heliocentric reordering of the cosmos, [[Biography:Nicolaus Copernicus|Nicolaus Copernicus]] was the first person known to have developed [[Astronomy:Copernican heliocentrism|a mathematically predictive heliocentric system]].<ref>{{Cite magazine |last=Rufus |first=W. C. |date=1923 |title=The astronomical system of Copernicus |magazine=Popular Astronomy |volume=31 |page=510 |bibcode=1923PA.....31..510R}}</ref><ref>{{Cite book |last=Weinert |first=Friedel |url=https://archive.org/details/copernicusdarwin00wein |title=Copernicus, Darwin, & Freud: revolutions in the history and philosophy of science |date=2009 |publisher=[[Company:Wiley-Blackwell|Wiley-Blackwell]] |isbn=978-1-4051-8183-9 |page=[https://archive.org/details/copernicusdarwin00wein/page/n29 21] |url-access=limited}}</ref> Heliocentrism did not triumph immediately over geocentrism, but the work of Copernicus had its champions, notably [[Biography:Johannes Kepler|Johannes Kepler]]. Using a heliocentric model that improved upon Copernicus by allowing orbits to be elliptical, and the precise observational data of [[Biography:Tycho Brahe|Tycho Brahe]], Kepler produced the ''[[Astronomy:Rudolphine Tables|Rudolphine Tables]]'', which enabled accurate computations of the positions of the then-known planets. [[Biography:Pierre Gassendi|Pierre Gassendi]] used them to predict a [[Astronomy:Transit of Mercury|transit of Mercury]] in 1631, and Jeremiah Horrocks did the same for a [[Astronomy:Transit of Venus|transit of Venus]] in 1639. This provided a strong vindication of heliocentrism and Kepler's elliptical orbits.<ref>{{Cite book |last=LoLordo |first=Antonia |url=https://www.worldcat.org/oclc/182818133 |title=Pierre Gassendi and the Birth of Early Modern Philosophy |date=2007 |publisher=Cambridge University Press |isbn=978-0-511-34982-9 |location=New York |pages=12, 27 |oclc=182818133 |access-date=1 April 2022 |archive-url=https://web.archive.org/web/20220420161223/https://www.worldcat.org/title/pierre-gassendi-and-the-birth-of-early-modern-philosophy/oclc/182818133 |archive-date=20 April 2022 |url-status=live}}</ref><ref>{{Cite journal |last1=Athreya |first1=A. |last2=Gingerich |first2=O. |date=December 1996 |title=An Analysis of Kepler's Rudolphine Tables and Implications for the Reception of His Physical Astronomy |journal=Bulletin of the American Astronomical Society |volume=28 |issue=4 |page=1305 |bibcode=1996AAS...189.2404A}}<!--|accessdate=26 December 2013--></ref>

In the 17th century, [[Biography:Galileo Galilei|Galileo]] publicized the use of the telescope in astronomy; he and Simon Marius independently discovered that Jupiter had four satellites in orbit around it.<ref>{{Cite journal |last=Pasachoff |first=Jay M. |date=May 2015 |title=Simon Marius's Mundus Iovialis: 400th Anniversary in Galileo's Shadow |url=http://journals.sagepub.com/doi/10.1177/0021828615585493 |url-status=live |journal=Journal for the History of Astronomy |language=en |volume=46 |issue=2 |pages=218–234 |bibcode=2015JHA....46..218P |doi=10.1177/0021828615585493 |issn=0021-8286 |s2cid=120470649 |archive-url=https://web.archive.org/web/20211127213209/https://journals.sagepub.com/doi/10.1177/0021828615585493 |archive-date=27 November 2021 |access-date=1 April 2022}}</ref> [[Biography:Christiaan Huygens|Christiaan Huygens]] followed on from these observations by discovering Saturn's moon [[Astronomy:Titan (moon)|Titan]] and the shape of the [[Astronomy:Rings of Saturn|rings of Saturn]].<ref>{{Cite web |date=8 December 2012 |title=Christiaan Huygens: Discoverer of Titan |url=https://www.esa.int/About_Us/ESA_history/Christiaan_Huygens_Discoverer_of_Titan |url-status=live |archive-url=https://web.archive.org/web/20191206001920/http://www.esa.int/About_Us/ESA_history/Christiaan_Huygens_Discoverer_of_Titan |archive-date=6 December 2019 |access-date=27 October 2010 |website=ESA Space Science |publisher=The European Space Agency}}</ref> In 1677, [[Biography:Edmond Halley|Edmond Halley]] observed a transit of Mercury across the Sun, leading him to realize that observations of the solar parallax of a planet (more ideally using the transit of Venus) could be used to [[Trigonometry|trigonometrically]] determine the distances between Earth, [[Astronomy:Venus|Venus]], and the Sun.<ref>{{Cite conference |last=Chapman |first=Allan |date=April 2005 |editor-last=Kurtz |editor-first=D. W. |title=Jeremiah Horrocks, William Crabtree, and the Lancashire observations of the transit of Venus of 1639 |conference=Transits of Venus: New Views of the Solar System and Galaxy, Proceedings of IAU Colloquium #196, held 7–11 June 2004 in Preston, U.K. |publisher=Cambridge University Press |publication-place=Cambridge |volume=2004 |pages=3–26 |bibcode=2005tvnv.conf....3C |doi=10.1017/S1743921305001225 |doi-access=free |journal=Proceedings of the International Astronomical Union}}</ref> Halley's friend [[Biography:Isaac Newton|Isaac Newton]], in his magisterial ''[[Physics:Philosophiæ Naturalis Principia Mathematica|Principia Mathematica]]'' of 1687, demonstrated that celestial bodies are not quintessentially different from Earthly ones: the same [[Physics:Newton's laws of motion|laws of motion]] and of [[Physics:Newton's law of universal gravitation|gravity]] apply on Earth and in the skies.<ref name=":0"/>{{Rp|page=142}}
[[File:The Solar System, with the orbits of 5 remarkable comets. LOC 2013593161.jpg|thumb|True-scale Solar System poster made by Emanuel Bowen in 1747. At that time, Uranus, Neptune, nor the asteroid belts have been discovered yet.]]
The term "Solar System" entered the English language by 1704, when [[Biography:John Locke|John Locke]] used it to refer to the Sun, planets, and comets.<ref>See, for example:

*{{Cite web |title=solar |url=https://www.etymonline.com/word/solar |url-status=live |archive-url=https://web.archive.org/web/20220318002833/https://www.etymonline.com/word/solar |archive-date=18 March 2022 |access-date=17 March 2022 |website=Online Etymology Dictionary}}
*{{Cite OED|solar system}}
*{{Cite book |last=Locke |first=John |url=https://books.google.com/books?id=Ni9bAAAAcAAJ |title=Elements of Natural Philosophy ... To which are added. Some Thoughts concerning Reading and Study for a Gentleman. By the same author. With prefatory remarks by P. Des Maizeaux |date=1754 |publisher=R. Taylor |page=8 |language=en |orig-date=1720 |access-date=18 March 2022 |archive-date=18 March 2022 |archive-url=https://web.archive.org/web/20220318005707/https://books.google.com/books?id=Ni9bAAAAcAAJ&newbks=0 |url-status=live }} Posthumous publication.</ref> In 1705, Halley realized that repeated sightings of [[Astronomy:Halley's Comet|a comet]] were of the same object, returning regularly once every 75–76 years. This was the first evidence that anything other than the planets repeatedly orbited the Sun,<ref>{{Cite book |last1=Festou |first1=M. C. |title=Comets II |last2=Keller |first2=H. U. |last3=Weaver |first3=H. A. |date=2004 |publisher=University of Arizona Press |isbn=978-0816524501 |publication-place=Tucson |pages=3–16 |chapter=A brief conceptual history of cometary science |bibcode=2004come.book....3F |access-date=7 April 2022 |chapter-url=https://books.google.com/books?id=ehA8EAAAQBAJ&pg=PA4 |archive-url=https://web.archive.org/web/20220420161222/https://www.google.com/books/edition/Comets_II/ehA8EAAAQBAJ?gbpv=1&pg=PA4 |archive-date=20 April 2022 |url-status=live}}</ref> though [[Biography:Seneca the Younger|Seneca]] had theorized this about comets in the 1st century.<ref>{{Cite book |last1=Sagan |first1=Carl |url=https://books.google.com/books?id=LhkoowKFaTsC |title=Comet |last2=Druyan |first2=Ann |publisher=Random House |year=1997 |isbn=978-0-3078-0105-0 |location=New York |pages=26–27, 37–38 |author-link=Carl Sagan |author-link2=Ann Druyan |access-date=28 June 2021 |archive-url=https://web.archive.org/web/20210615020250/https://books.google.com/books?id=LhkoowKFaTsC |archive-date=15 June 2021 |url-status=live}}</ref> Careful observations of the 1769 transit of Venus allowed astronomers to calculate the average Earth–Sun distance as {{Convert|93726900|mi|km}}, only 0.8% greater than the modern value.<ref>{{Cite journal |last=Teets |first=Donald |date=December 2003 |title=Transits of Venus and the Astronomical Unit |url=http://www.maa.org/sites/default/files/pdf/pubs/mm_dec03-Venus.pdf |url-status=live |journal=Mathematics Magazine |volume=76 |pages=335–348 |doi=10.1080/0025570X.2003.11953207 |jstor=3654879 |s2cid=54867823 |archive-url=https://web.archive.org/web/20220203080207/https://www.maa.org/sites/default/files/pdf/pubs/mm_dec03-Venus.pdf |archive-date=3 February 2022 |access-date=3 April 2022 |number=5}}</ref> [[Astronomy:Uranus|Uranus]], having occasionally been observed since antiquity, was recognized to be a planet orbiting beyond Saturn by 1783.<ref>{{Cite journal |last=Bourtembourg |first=René |date=2013 |title=Was Uranus Observed by Hipparchos? |journal=Journal for the History of Astronomy |volume=44 |issue=4 |pages=377–387 |bibcode=2013JHA....44..377B |doi=10.1177/002182861304400401 |s2cid=122482074}}</ref> In 1838, Friedrich Bessel successfully measured a [[Astronomy:Stellar parallax|stellar parallax]], an apparent shift in the position of a star created by Earth's motion around the Sun, providing the first direct, experimental proof of heliocentrism.<ref>{{Cite book |last=Di Bari |first=Pasquale |url=https://books.google.com/books?id=hPm7DwAAQBAJ&pg=PA4 |title=Cosmology and the Early Universe |date=2018 |publisher=CRC Press |isbn=978-1351020138 |pages=3–4}}</ref> [[Astronomy:Neptune|Neptune]] was identified as a planet some years later, in 1846, thanks to its gravitational pull causing a slight but detectable variation in the orbit of Uranus.<ref>{{Cite journal |last1=Bhatnagar |first1=Siddharth |last2=Vyasanakere |first2=Jayanth P. |last3=Murthy |first3=Jayant |date=May 2021 |title=A geometric method to locate Neptune |url=https://aapt.scitation.org/doi/10.1119/10.0003349 |url-status=live |journal=American Journal of Physics |language=en |volume=89 |issue=5 |pages=454–458 |arxiv=2102.04248 |bibcode=2021AmJPh..89..454B |doi=10.1119/10.0003349 |issn=0002-9505 |s2cid=231846880 |archive-url=https://web.archive.org/web/20211129125826/https://aapt.scitation.org/doi/10.1119/10.0003349 |archive-date=29 November 2021 |access-date=1 April 2022}}</ref>

In the 20th century, humans began their space exploration around the Solar System, starting with placing [[Astronomy:Space telescope|telescopes in space]].<ref>{{Cite web |last=Garner |first=Rob |date=10 December 2018 |title=50th Anniversary of OAO 2: NASA's 1st Successful Stellar Observatory |url=http://www.nasa.gov/feature/goddard/2018/nasa-s-first-stellar-observatory-oao-2-turns-50 |url-status=live |archive-url=https://web.archive.org/web/20211229231948/https://www.nasa.gov/feature/goddard/2018/nasa-s-first-stellar-observatory-oao-2-turns-50 |archive-date=29 December 2021 |access-date=20 April 2022 |website=NASA}}</ref> Since then, humans have landed on the Moon during the [[Astronomy:Apollo program|Apollo program]]; the [[Engineering:Apollo 13|Apollo 13]] mission marked the furthest any human has been away from Earth at {{Convert|400171|km|sp=us}}.<ref>{{Cite book |url=https://archive.org/details/guinnessworldrec00vari/page/13 |title=Guinness World Records 2010 |publisher=Bantam Books |year=2010 |isbn=978-0-553-59337-2 |editor-last=Glenday |editor-first=Craig |location=New York}}</ref> All eight planets and two dwarf planets have been visited by space probes. This began with [[Astronomy:Mariner 2|Mariner 2]]'s fly-by of Venus in 1962, while [[Astronomy:Mariner 9|Mariner 9]] mission to Mars was the first to orbit another planet in 1971. The outer planets were first visited by [[Astronomy:Pioneer 10|Pioneer 10]]'s encounter with Jupiter, and [[Astronomy:Pioneer 11|Pioneer 11]]'s encounter with Saturn. The remaining gas giants were first visited by the ''Voyager'' spacecraft, one of which (''[[Astronomy:Voyager 1|Voyager 1]]'') is the furthest object made by humankind and the first in interstellar space.<ref>{{Cite web |last=Rao |first=Rahul |date=28 April 2021 |title=How the most distant object ever made by humans is spending its dying days |url=https://www.popsci.com/story/space/nasa-voyager-1-final-days |url-status=live |archive-url=https://web.archive.org/web/20220128180719/https://www.popsci.com/story/space/nasa-voyager-1-final-days |archive-date=28 January 2022 |access-date=20 April 2022 |website=Popular Science |language=en-US}}</ref> In addition, probes have also returned samples from comets<ref>{{Cite magazine |last=Woo |first=Marcus |date=20 November 2014 |title=This Is What It Sounded Like When We Landed on a Comet |url=https://www.wired.com/2014/11/sounded-like-landed-comet |magazine=Wired |archive-url=https://web.archive.org/web/20141123021050/https://www.wired.com/2014/11/sounded-like-landed-comet |archive-date=23 November 2014 |access-date=20 April 2022}}</ref> and asteroids,<ref>{{Cite web |last=Marks |first=Paul |date=3 December 2014 |title=Hayabusa 2 probe begins journey to land on an asteroid |url=https://www.newscientist.com/article/dn26650-hayabusa-2-probe-begins-journey-to-land-on-an-asteroid |url-status=live |archive-url=https://web.archive.org/web/20220211062123/https://www.newscientist.com/article/dn26650-hayabusa-2-probe-begins-journey-to-land-on-an-asteroid |archive-date=11 February 2022 |access-date=20 April 2022 |website=New Scientist |language=en-US}}</ref> as well as flown through the Sun's corona<ref>{{Cite web |date=14 December 2021 |title=NASA's Parker Solar Probe becomes first spacecraft to 'touch' the sun |url=https://www.cnn.com/2021/12/14/world/nasa-parker-solar-probe-sun-scn/index.html |url-status=live |archive-url=https://web.archive.org/web/20211214235239/https://www.cnn.com/2021/12/14/world/nasa-parker-solar-probe-sun-scn/index.html |archive-date=14 December 2021 |access-date=15 December 2021 |website=CNN}}</ref> and made fly-bys of Kuiper belt objects.<ref>{{Cite news |last1=Corum |first1=Jonathan |last2=Gröndahl |first2=Mika |last3=Parshina-Kottas |first3=Yuliya |date=13 July 2015 |title=New Horizons' Pluto Flyby |language=en-US |work=The New York Times |url=https://www.nytimes.com/interactive/2015/07/14/science/space/pluto-flyby.html,%20https://www.nytimes.com/interactive/2015/07/14/science/space/pluto-flyby.html |access-date=20 April 2022 |issn=0362-4331}}</ref> Six of the planets (all but Uranus and Neptune) have or had a dedicated orbiter.<ref>{{Cite web |last=Daines |first=Gary |date=11 March 2015 |title=Solar System Missions |url=http://www.nasa.gov/content/solar-missions-list |url-status=live |archive-url=https://web.archive.org/web/20220412132605/https://www.nasa.gov/content/solar-missions-list |archive-date=12 April 2022 |access-date=20 April 2022 |website=NASA}}</ref>
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== See also ==
{{Div col|colwidth=20em}}
* List of gravitationally rounded objects of the Solar System
* [[Astronomy:List of Solar System objects by size|List of Solar System objects by size]]
* Lists of geological features of the Solar System
* [[Astronomy:List of Solar System extremes|List of Solar System extremes]]
* [[Astronomy:Outline of the Solar System|Outline of the Solar System]]
* {{annotated link|Astronomy:Planetary mnemonic}}
{{Div col end}}

== Notes ==
{{Reflist|group=lower-alpha}}

== References ==
{{Reflist}}

== External links ==
{{Library resources box
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* [http://www.joshworth.com/a-tediously-accurate-map-of-the-solar-system/ If the Moon were only 1 Pixel: A Tediously Accurate Map of the Solar System (web based scroll map scaled to the Moon being 1 pixel)]
* [https://eyes.nasa.gov/apps/solar-system NASA's Eyes on the Solar System]
* [https://solarsystem.nasa.gov/ NASA's Solar System Exploration]
* [http://space.jpl.nasa.gov NASA's Solar System Simulator]

{{Solar system|state=expanded}}
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{{Nearest systems|1}}
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[[Category:Solar System| ]]
[[Category:Planetary science]]
[[Category:Space science]]

{{Sourceattribution|Solar System}}