Astronomy:Solar System

Short description: The Sun, its planets and their moons
Age The Sun and planets(distances not to scale) 4.568 billion years Local Interstellar Cloud, Local Bubble, Orion–Cygnus Arm, Milky Way 1.0014 Solar masses Proxima Centauri (4.25 ly)Alpha Centauri (4.37 ly) Proxima Centauri system (4.25 ly) 30.10 AU(4.5 bill. km; 2.8 bill. mi) 50 AU 1 (Sun) 8 declared by IAU: MercuryVenusEarthMarsJupiterSaturnUranusNeptune 9 by general consensus: CeresOrcusPlutoHaumeaQuaoarMakemakeGonggongErisSedna 575 (185 planetary390 minor planetary)[1][2] 796,354[lower-alpha 1][3] 4,143[lower-alpha 1][3] 19 60.19° (ecliptic) 27,000 ± 1,000 ly 220 km/s; 136 mi/s 225–250 myr G2V ≈5 AU[4] ≈120 AU ≈1–3 ly

The Solar System[lower-alpha 2] is the gravitationally bound system of the Sun and the objects that orbit it. Of the bodies that orbit the Sun directly, the largest are the four gas and ice giants and the four terrestrial planets, followed by an unknown number of dwarf planets and innumerable small Solar System bodies. Of the bodies that orbit the Sun indirectly—the natural satellites—two are larger than Mercury, the smallest terrestrial planet, and one is nearly as large.[lower-alpha 3]

The Solar System formed 4.6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud. The vast majority of the system's mass is in the Sun, with the majority of the remaining mass contained in Jupiter. The four inner system planets—Mercury, Venus, Earth and Mars—are terrestrial planets, being composed primarily of rock and metal. The four giant planets of the outer system are substantially more massive than the terrestrials. The two largest planets, Jupiter and Saturn, are gas giants, being composed mainly of hydrogen and helium; the next two, Uranus and Neptune, are ice giants, being composed mostly of substances with relatively high melting points compared with hydrogen and helium, called volatiles, such as water, ammonia and methane. All eight have nearly circular orbits that lie close to the plane of the Earth's orbit, called the ecliptic.

The Solar System also contains smaller objects.[lower-alpha 4] The asteroid belt, which lies between the orbits of Mars and Jupiter, contains objects composed of rock, metal and ice. Beyond Neptune's orbit lie the Kuiper belt and scattered disc, which are populations of objects composed mostly of ice and rock, and beyond them lies a class of minor planets called detached objects. Within these populations, some objects are large enough to have rounded under their own gravity and thus to be planets under some definitions, though there is considerable debate as to how many such objects there will prove to be.[9][10] Such objects are categorized as dwarf planets. Astronomers generally accept about nine objects as dwarf planets: the asteroid Ceres, the Kuiper-belt objects Pluto, Orcus, Haumea, Quaoar and Makemake, the scattered-disk objects Gonggong and Eris, and Sedna.[lower-alpha 4] Various small-body populations, including comets, centaurs and interplanetary dust clouds, freely travel between the regions of the Solar System. Six of the major planets, the six largest possible dwarf planets, and many of the smaller bodies are orbited by natural satellites, commonly called "moons" after the Moon. Each of the giant planets and some smaller bodies are encircled by planetary rings of ice, dust and moonlets.

The solar wind, a stream of charged particles flowing outwards from the Sun, creates a bubble-like region of interplanetary medium in the interstellar medium known as the heliosphere. The heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the interstellar medium; it extends out to the edge of the scattered disc. The Oort cloud, which is thought to be the source for long-period comets, may also exist at a distance roughly a thousand times further than the heliosphere. The Solar System is located 26,000 light-years from the center of the Milky Way galaxy in the Orion Arm, which contains most of the visible stars in the night sky. The nearest stars are within the so-called Local Bubble, with the closest, Proxima Centauri, at 4.25 light-years.

Structure and composition

The principal component of the Solar System is the Sun, a G2 main-sequence star that contains 99.86% of the system's known mass and dominates it gravitationally.[11] 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, asteroids, and comets) together comprise less than 0.002% of the Solar System's total mass.[lower-alpha 5]

Most large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the ecliptic. The planets are very close to the ecliptic, whereas comets and Kuiper belt objects are frequently at significantly greater angles to it.[15][16] As a result of the formation of the Solar System, planets (and most other objects) orbit the Sun in the same direction that the Sun is rotating (counter-clockwise, as viewed from above Earth's north pole).[17] There are exceptions, such as Halley's Comet.[18] Most of the larger moons orbit their planets in this prograde direction; Neptune's moon Triton is the largest to orbit in the opposite, retrograde manner.[19] Most larger objects rotate around their own axes in the prograde direction, though the rotation of Venus is retrograde.[20]

The overall structure of the charted regions of the Solar System consists of the Sun, four relatively small 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.[21] 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.[22]

Most of the planets in the Solar System have secondary systems of their own, being orbited by planetary objects called natural satellites, or moons (two of which, Titan and Ganymede, are larger than the planet Mercury). The four giant planets have planetary rings, thin bands of tiny particles that orbit them in unison. Most of the largest natural satellites are in synchronous rotation, with one face permanently turned toward their parent.[23]

All planets of the Solar System lie very close to the ecliptic. The closer they are to the Sun, the faster they travel (inner planets on the left, all planets except Neptune on the right).

To a good first approximation, Kepler's laws of planetary motion describe the orbits of objects about the Sun.[24](pp433–437) Kepler's first law states that each object travels along an ellipse with the Sun at one focus. On an elliptical orbit, a body's distance from the Sun varies 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. The orbits of the planets are nearly circular, but many comets, asteroids, and Kuiper belt objects follow highly elliptical orbits. Kepler's second law states that the angular momentum of an object remains constant as it orbits the Sun, meaning that an object will speed up as it approaches the Sun and slow down as it moves farther away, in a quantitatively predictable manner. Kepler's third law states that for an object in an elliptical orbit, the time it takes to go around in that orbit is proportional to the three-halves power of the orbit's semi-major axis. 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. These additional perturbations can be accounted for using numerical models.[25]:{{{1}}}

Although the Sun dominates the system by mass, it accounts for only about 2% of the angular momentum.[26][27] 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.[26]

The Sun, which comprises nearly all the matter in the Solar System, is composed of roughly 98% hydrogen and helium.[28] Jupiter and Saturn, which comprise nearly all the remaining matter, are also primarily composed of hydrogen and helium.[29][30] A composition gradient exists in the Solar System, created by heat and light pressure from the 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.[31] The boundary in the Solar System beyond which those volatile substances could condense is known as the frost line, and it lies at roughly 5 astronomical units (750×106 km; 460×106 mi) from the Sun.[4]

The objects of the inner Solar System are composed mostly of rock,[32] the collective name for compounds with high melting points, such as silicates, iron or nickel, that remained solid under almost all conditions in the protoplanetary nebula.[33] Jupiter and Saturn are composed mainly of gases, the astronomical term for materials with extremely low melting points and high vapour pressure, such as hydrogen, helium, and neon, which were always in the gaseous phase in the nebula.[33] Ices, like water, methane, ammonia, hydrogen sulfide, and carbon dioxide,[32] have melting points up to a few hundred kelvins.[33] They can be found as ices, liquids, or gases in various places in the Solar System, whereas in the nebula they were either in the solid or gaseous phase.[33] Icy substances comprise the majority of the satellites of the giant planets, as well as most of Uranus and Neptune (the so-called "ice giants") and the numerous small objects that lie beyond Neptune's orbit.[32][34] Together, gases and ices are referred to as volatiles.[35]

Distances and scales

Size comparison of the Sun and the planets (clickable)

The distance from Earth to the Sun is 1 Astronomy:astronomical unit|astronomical unit [AU] (150,000,000 km; 93,000,000 mi). For comparison, the radius of the Sun is 0.0047 AU (700,000 km; 400,000 mi).[36] Thus, the Sun occupies 0.00001% (10−5 %) of the volume of a sphere with a radius the size of Earth's orbit, whereas Earth's volume is roughly one millionth (10−6) that of the Sun. Jupiter, the largest planet, is 5.2 Astronomy:astronomical unit|astronomical units (780,000,000 km; 480,000,000 mi) from the Sun and has a radius of 71,000 km (0.00047 AU; 44,000 mi), whereas the most distant planet, Neptune, is 30 AU (4.5×109 km; 2.8×109 mi) from the Sun.[37][38]

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 nearer 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 (for example, the Titius–Bode law),[39] but no such theory has been accepted.[40]

Some Solar System models attempt to convey the relative scales involved in the Solar System on human terms. Some are small in scale (and may be mechanical—called orreries)—whereas others extend across cities or regional areas.[41] The largest such scale model, the Sweden Solar System, uses the 110-metre (361 ft) Ericsson Globe in Stockholm as its substitute Sun, and, following the scale, Jupiter is a 7.5-metre (25-foot) sphere at Stockholm Arlanda Airport, 40 km (25 mi) away, whereas the farthest current object, Sedna, is a 10 cm (4 in) sphere in Luleå, 912 km (567 mi) away.[42][43]

If the Sun–Neptune distance is scaled to 100 metres (330 ft), then the Sun would be about 3 cm (1.2 in) in diameter (roughly two-thirds the diameter of a golf ball), the giant planets would be all smaller than about 3 mm (0.12 in), and Earth's diameter along with that of the other terrestrial planets would be smaller than a flea (0.3 mm or 0.012 in) at this scale.[44]

The Solar System. Distances are to scale, objects are not.

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Distances of selected bodies of the Solar System from the Sun. The left and right edges of each bar correspond to the perihelion and aphelion of the body, respectively, hence long bars denote high orbital eccentricity. The radius of the Sun is 0.7 million km, and the radius of Jupiter (the largest planet) is 0.07 million km, both too small to resolve on this image.

Table of Size and Distance
Body Diameter (km) Average distance from the sun (km)
Sun 1,391,000 0
Mercury 4,880 57,910,000
Venus 12,104 108,200,000
Earth 12,756 149,600,000
Mars 6,794 227,940,000
Jupiter 142,984 778,330,000
Saturn 120,536 1,429,400,000
Uranus 51,118 2,870,990,000
Neptune 49,532 4,504,000,000

Formation and evolution

Main page: Astronomy:Formation and evolution of the Solar System
Artist's conception of a protoplanetary disk

The Solar System formed 4.568 billion years ago from the gravitational collapse of a region within a large molecular cloud.[lower-alpha 6] This initial cloud was likely several light-years across and probably birthed several stars.[46] 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. As the region that would become the Solar System, known as the pre-solar nebula,[47] collapsed, conservation of angular momentum caused it to rotate faster. The centre, where most of the mass collected, became increasingly hotter than the surrounding disc.[46] As the contracting nebula rotated faster, it began to flatten into a protoplanetary disc with a diameter of roughly 200 AU (30 billion km; 19 billion mi)[46] and a hot, dense protostar at the centre.[48][49] The planets formed by accretion from this disc,[50] 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, leaving the planets, dwarf planets, and leftover minor bodies.[51]

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, and these 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. 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. Leftover debris that never became planets congregated in regions such as the asteroid belt, Kuiper belt, and Oort cloud.[51] The Nice model is an explanation for the creation of these regions and how the outer planets could have formed in different positions and migrated to their current orbits through various gravitational interactions.[52]

Within 50 million years, the pressure and density of hydrogen in the centre of the protostar became great enough for it to begin thermonuclear fusion.[53] The temperature, reaction rate, pressure, and density increased until hydrostatic equilibrium was achieved: the thermal pressure equalled the force of gravity. At this point, the Sun became a main-sequence star.[54] The main-sequence phase, from beginning to end, will last about 10 billion years for the Sun compared to around two billion years for all other phases of the Sun's pre-remnant life combined.[55] Solar wind from the Sun created the heliosphere and swept away the remaining gas and dust from the protoplanetary disc into interstellar space, ending the planetary formation process. The Sun is growing brighter; early in its main-sequence life its brightness was 70% that of what it is today.[56]

The Helix Nebula, a planetary nebula similar to what the Sun will create when it enters its white dwarf stage.

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 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 much 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 red giant. Because of its vastly increased surface area, the surface of the Sun will be considerably cooler (2,600 K (2,330 °C; 4,220 °F) at its coolest) than it is on the main sequence.[55] The expanding Sun is expected to vaporize Mercury and render Earth uninhabitable. 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 move away into space, leaving a white dwarf, an extraordinarily dense object, half the original mass of the Sun but only the size of Earth.[57] The ejected outer layers will form what is known as a planetary nebula, returning some of the material that formed the Sun—but now enriched with heavier elements like carbon—to the interstellar medium.[58]

Sun

Main page: Astronomy:Sun

The Sun is the Solar System's star and by far its most massive component. Its large mass (332,900 Earth masses),[59] which comprises 99.86% of all the mass in the Solar System,[60] produces temperatures and densities in its core high enough to sustain nuclear fusion of hydrogen into helium.[61] This releases an enormous amount of energy, mostly radiated into space as electromagnetic radiation peaking in visible light.[62][63]

Because the Sun fuses hydrogen into helium, it is a main-sequence star. More specifically, it is a G2-type main-sequence star, where the type designation refers to its effective temperature. Hotter main-sequence stars are more luminous. The Sun's temperature is intermediate between that of the 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 red dwarfs, make up about 75% of the stars in the Milky Way.[64][65]

The Sun is a population I star; it has a higher abundance of elements heavier than hydrogen and helium ("metals" in astronomical parlance) than the older population II stars.[66] 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 universe could be enriched with these atoms. The oldest stars contain few metals, whereas stars born later have more. This high metallicity is thought to have been crucial to the Sun's development of a planetary system because the planets form from the accretion of "metals".[67]

Interplanetary medium

Main pages: Astronomy:Interplanetary medium and Astronomy:Solar wind
The heliospheric current sheet

The vast majority of the Solar System consists of a near-vacuum known as the interplanetary medium. Along with light, the Sun radiates a continuous stream of charged particles (a plasma) known as the solar wind. This stream of particles spreads outwards at speeds from 900,000 kilometres per hour (560,000 mph) to 2,880,000 kilometres per hour (1,790,000 mph),[68] creating a tenuous atmosphere that permeates the interplanetary medium out to at least 100 AU (15 billion km; 9.3 billion mi) (see § Heliosphere).[69] Activity on the Sun's surface, such as solar flares and coronal mass ejections, disturbs the heliosphere, creating space weather and causing geomagnetic storms.[70] The largest structure within the heliosphere is the heliospheric current sheet, a spiral form created by the actions of the Sun's rotating magnetic field on the interplanetary medium.[71][72]

Earth's magnetic field stops its atmosphere from being stripped away by the solar wind.[73] Venus and Mars do not have magnetic fields, and as a result the solar wind is causing their atmospheres to gradually bleed away into space.[74] 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 magnetic poles.[75]

The heliosphere and planetary magnetic fields (for those planets that have them) partially shield the Solar System from high-energy interstellar particles called cosmic rays. The density of cosmic rays in the 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.[76]

The interplanetary medium is home to at least two disc-like regions of cosmic dust. The first, the zodiacal dust cloud, lies in the inner Solar System and causes the 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.[77] The second dust cloud extends from about 10 AU (1.5 billion km; 930 million mi) to about 40 AU (6.0 billion km; 3.7 billion mi), and was probably created by collisions within the Kuiper belt.[78][79]

Inner Solar System

The inner Solar System is the region comprising the terrestrial planets and the asteroid belt.[80] Composed mainly of silicates and metals, 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 frost line, which is a little less than 5 AU (750 million km; 460 million mi) from the Sun.[15]

Inner planets

Main page: Astronomy:Terrestrial planet
The inner planets. From top to bottom rightwards: Earth, Mars, Venus, and Mercury (sizes to scale).
Orrery showing the motions of the inner four planets. The small spheres represent the position of each planet on every two Julian days, beginning August 3, 2020 and ending June 21, 2022 (Mars at perihelion).

The four terrestrial or inner planets have dense, rocky compositions, few or no moons, and no ring systems. They are composed largely of refractory minerals such as the silicates—which form their crusts and mantles—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 rift valleys 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 is (i.e. Mercury and Venus).[81]

Mercury

Main page: Astronomy:Mercury (planet)

Mercury (0.4 AU (60 million km; 37 million mi) from the Sun) is the closest planet to the Sun. The smallest planet in the Solar System (0.055 M), Mercury has no natural satellites. Besides impact craters, its only known geological features are lobed ridges or rupes that were probably produced by a period of contraction early in its history.[82] Mercury's very tenuous atmosphere consists of atoms blasted off its surface by the solar wind.[83] 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.[84][85]

Venus

Main page: Astronomy:Venus

Venus (0.7 AU (100 million km; 65 million mi) from the Sun) is close in size to Earth (0.815 M) 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 400 °C (752 °F), most likely due to the amount of greenhouse gases in the atmosphere.[86] No definitive evidence of current geological activity has been detected on Venus, but it has no magnetic field that would prevent depletion of its substantial atmosphere, which suggests that its atmosphere is being replenished by volcanic eruptions.[87]

Earth

Main page: Astronomy:Earth

Earth (1 AU (150 million km; 93 million mi) 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 where life is known to exist.[88] Its liquid hydrosphere is unique among the terrestrial planets, and it is the only planet where plate tectonics has been observed. Earth's atmosphere is radically different from those of the other planets, having been altered by the presence of life to contain 21% free oxygen.[89][90] It has one natural satellite, the Moon, the only large satellite of a terrestrial planet in the Solar System.

Mars

Main page: Astronomy:Mars

Mars (1.5 AU (220 million km; 140 million mi) from the Sun) is smaller than Earth and Venus (0.107 M). It has an atmosphere of mostly carbon dioxide with a surface pressure of 6.1 millibars (0.088 psi; 0.18 inHg) (roughly 0.6% of that of Earth).[91] Its surface, peppered with vast volcanoes, such as Olympus Mons, and rift valleys, such as Valles Marineris, shows geological activity that may have persisted until as recently as 2 million years ago.[92] Its red colour comes from iron oxide (rust) in its soil.[93] Mars has two tiny natural satellites (Deimos and Phobos) thought to be either captured asteroids,[94] or ejected debris from a massive impact early in Mars's history.[95]

Asteroid belt

Main page: Astronomy:Asteroid belt
The donut-shaped asteroid belt is located between the orbits of Mars and Jupiter. Template:Image key

Asteroids except for the largest, Ceres, are classified as small Solar System bodies[lower-alpha 4] and are composed mainly of refractory rocky and metallic minerals, with some ice.[96][97] They range from a few metres to hundreds of kilometres in size. Asteroids smaller than one meter are usually called meteoroids and micrometeoroids (grain-sized), depending on different, somewhat arbitrary definitions.

The asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and 3.3 AU (340 and 490 million km; 210 and 310 million mi) 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.[98] The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometre in diameter.[99] Despite this, the total mass of the asteroid belt is unlikely to be more than a thousandth of that of Earth.[14] The asteroid belt is very sparsely populated; spacecraft routinely pass through without incident.[100]

Ceres

Main page: Astronomy:Ceres (dwarf planet)
Ceres – map of gravity fields: red is high; blue, low.

Ceres (2.77 AU (414 million km; 257 million mi)) is the largest asteroid, a protoplanet, and a dwarf planet.[lower-alpha 4] It has a diameter of slightly under 1,000 km (620 mi), 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.[101] It was then reclassified again as a dwarf planet in 2006 when the IAU definition of planet was established.[102](p218)

Pallas and Vesta

Pallas (2.77 AU) and Vesta (2.36 AU) are the largest asteroids in the asteroid belt, after Ceres. They are the other two protoplanets that survive more or less intact. At about 520 km (320 mi) 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.[103][104][105] Fragments from impacts upon these two bodies survive elsewhere in the asteroid belt, as the Pallas family and Vesta family. Both were considered planets upon their discoveries in 1802 and 1807 respectively, and then like Ceres generally considered as 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.[106]

Asteroid groups

Asteroids in the asteroid belt are divided into asteroid groups and families based on their orbital characteristics. 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 90 Antiope). The asteroid belt also contains main-belt comets, which may have been the source of Earth's water.[107]

Jupiter trojans are located in either of Jupiter's L4 or L5 points (gravitationally stable regions leading and trailing a planet in its orbit); the term trojan is also used for small bodies in any other planetary or satellite Lagrange point. Hilda asteroids are in a 2:3 resonance with Jupiter; that is, they go around the Sun three times for every two Jupiter orbits.[108]

The inner Solar System also contains near-Earth asteroids, many of which cross the orbits of the inner planets.[109] Some of them are potentially hazardous objects.[110]

Outer Solar System

The outer region of the Solar System is home to the giant planets and their large moons. The 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.[51]

Outer planets

Main page: Astronomy:Giant planet
The outer planets (in the background) Jupiter, Saturn, Uranus and Neptune, compared to the inner planets Earth, Venus, Mars and Mercury (in the foreground)
Orrery showing the motions of the outer four planets. The small spheres represent the position of each planet on every 200 Julian days, beginning November 18, 1877 and ending September 3, 2042 (Neptune at perihelion).

The four outer planets, also called giant planets or Jovian planets, collectively make up 99% of the mass known to orbit the Sun.[lower-alpha 5] 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 gas giants.[111] Uranus and Neptune are far less massive—less than 20 Earth masses (M) each—and are composed primarily of ices. For these reasons, some astronomers suggest they belong in their own category, ice giants.[112] 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.

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.[113]

Jupiter

Main page: Astronomy:Jupiter

Jupiter (5.2 AU (780 million km; 480 million mi)), at 318 M, is 2.5 times the mass of all the other planets put together. It is composed largely of hydrogen and helium. Jupiter's strong internal heat creates semi-permanent features in its atmosphere, such as cloud bands and the Great Red Spot. Jupiter has 80 known satellites. The four largest, Ganymede, Callisto, Io, and Europa, are called the Galilean moons: they show similarities to the terrestrial planets, such as volcanism and internal heating.[114] Ganymede, the largest satellite in the Solar System, is larger than Mercury; Callisto is almost as large.

Saturn

Main page: Astronomy:Saturn

Saturn (9.5 AU (1.42 billion km; 880 million mi)), distinguished by its extensive ring system, has several similarities to Jupiter, such as its atmospheric composition and magnetosphere. Although Saturn has 60% of Jupiter's volume, it is less than a third as massive, at 95 M. 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.[115] Saturn has 83 confirmed satellites composed largely of ice. Two of these, Titan and Enceladus, show signs of geological activity;[116] they, as well as five other Saturnian moons (Iapetus, 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.[117][118]

Uranus

Main page: Astronomy:Uranus

Uranus (19.2 AU (2.87 billion km; 1.78 billion mi)), at 14 M, is the lightest of the outer planets. Uniquely among the planets, it orbits the Sun on its side; its axial tilt is over ninety degrees to the ecliptic. It has a much colder core than the other giant planets and radiates very little heat into space.[119] Uranus has 27 known satellites, the largest ones being Titania, Oberon, Umbriel, Ariel, and Miranda.[120]

Neptune

Main page: Astronomy:Neptune

Neptune (30.1 AU (4.50 billion km; 2.80 billion mi)), though slightly smaller than Uranus, is more massive (17 M) and hence more dense. It radiates more internal heat than Uranus, but not as much as Jupiter or Saturn.[121] Neptune has 14 known satellites. The largest, Triton, is geologically active, with geysers of liquid nitrogen.[122] 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.[123] Neptune is accompanied in its orbit by several minor planets, termed Neptune trojans, that either lead or trail the planet by about one-sixth of the way around the Sun, positions known as Lagrange points.[124]

Centaurs

Main page: Astronomy:Centaur (small Solar System body)

The centaurs are icy comet-like bodies whose orbits have semi-major axes greater than Jupiter's (5.5 AU (820 million km; 510 million mi)) and less than Neptune's (30 AU (4.5 billion km; 2.8 billion mi)). The largest known centaur, 10199 Chariklo, has a diameter of about 250 km (160 mi).[125] The first centaur discovered, 2060 Chiron, has also been classified as a comet (95P) because it develops a coma just as comets do when they approach the Sun.[126]

Comets

Hale–Bopp seen in 1997
Main page: Astronomy:Comet

Comets are small Solar System bodies,[lower-alpha 4] 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 ionise, creating a coma: a long tail of gas and dust often visible to the naked eye.[127]

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 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.[128] Some comets with hyperbolic orbits may originate outside the Solar System, but determining their precise orbits is difficult.[129] Old comets whose volatiles have mostly been driven out by solar warming are often categorised as asteroids.[130]

Trans-Neptunian region

Inside the orbit of Neptune is the planetary region of the Solar System. Beyond the orbit of Neptune lies the area of the "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 tilted toward the plane of the Solar System and reaches much further out than the Kuiper belt. The entire region is still 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.[131]

Kuiper belt

Main page: Astronomy:Kuiper belt
Known objects in the Kuiper belt Template:Image key
Size comparison of some large TNOs with Earth: Pluto and its moons, Eris, Makemake, Haumea, Sedna, Gonggong, Quaoar, Orcus, Salacia, and 2002 MS4.

The Kuiper belt is a great ring of debris similar to the asteroid belt, but consisting mainly of objects composed primarily of ice.[132] It extends between 30 and 50 AU (4.5 and 7.5 billion km; 2.8 and 4.6 billion mi) from the Sun. It is composed mainly of small Solar System bodies, although the largest few are probably large enough to be dwarf planets.[9] There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than 50 km (30 mi), but the total mass of the Kuiper belt is thought to be only a tenth or even a hundredth the mass of Earth.[13] Many Kuiper belt objects have multiple satellites,[133] and most have orbits that take them outside the plane of the ecliptic.[134]

The Kuiper belt can be roughly divided into the "classical" belt and the resonances.[132] Resonances are orbits linked to that of Neptune (e.g. twice for every three Neptune orbits, or once for every two). The first resonance begins within the orbit of Neptune itself. The classical belt consists of objects having no resonance with Neptune, and extends from roughly 39.4 to 47.7 AU (5.89 to 7.14 billion km; 3.66 to 4.43 billion mi).[135] Members of the classical Kuiper belt are classified as cubewanos, after the first of their kind to be discovered, 15760 Albion (which previously had the provisional designation 1992 QB1), and are still in near primordial, low-eccentricity orbits.[136]

Pluto and Charon

Main pages: Astronomy:Pluto and Astronomy:Charon (moon)

The dwarf planet Pluto (with an average orbit of 39 AU (5.8 billion km; 3.6 billion mi)) 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 definition of planet. Pluto has a relatively eccentric orbit inclined 17 degrees to the ecliptic plane and ranging from 29.7 AU (4.44 billion km; 2.76 billion mi) from the Sun at perihelion (within the orbit of Neptune) to 49.5 AU (7.41 billion km; 4.60 billion mi) 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 plutinos.[137]

Charon, the largest of Pluto's moons, is sometimes described as part of a binary system with Pluto, as the two bodies orbit a barycentre 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 within the system.[138]

Makemake, Haumea, Quaoar, and Orcus

Besides Pluto, astronomers generally agree that at least four other Kuiper belt objects are dwarf planets.[9] Additional bodies have also been proposed.[139]

Makemake (45.79 AU average), 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. It was assigned a naming committee under the expectation that it would prove to be a dwarf planet in 2008.[6] Its orbit is far more inclined than Pluto's, at 29°.Cite error: Closing </ref> missing for <ref> tag It was named under the same expectation that it would prove to be a dwarf planet.[6] It has two known moons, Hiʻiaka and Namaka.[140]

Quaoar (43.69 AU average) 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.[141] It has one known moon, Weywot.[142]

Orcus (39.40 AU average) is in the same 2:3 orbital resonance with Neptune that Pluto is in, and is the largest such object after Pluto itself.[141] Its eccentricity and inclination are similar to Pluto's, but its perihelion lies about 120° from that of Pluto. Thus, the 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.[143] For this reason, it has been called the anti-Pluto.[144] It has one known moon, Vanth.[145]

Scattered disc

Main page: Astronomy:Scattered disc

The scattered disc, which overlaps the Kuiper belt but extends out to about 200 AU, is thought to be the source of short-period comets. Scattered-disc objects are thought to have been ejected into erratic orbits by the gravitational influence of 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 AU from the Sun). SDOs' orbits are also highly inclined to the ecliptic plane and are often almost perpendicular to it. 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".[146] Some astronomers also classify centaurs as inward-scattered Kuiper belt objects along with the outward-scattered residents of the scattered disc.[147]

Eris and Gonggong

Eris (67.78 AU average) is the largest known scattered disc object, and caused a debate about what constitutes a planet, because it is 25% more massive than Pluto[148] and about the same diameter. It is the most massive of the known dwarf planets. It has one known 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.

Gonggong (67.38 AU average) is in an orbit similar to Eris, except that it is in a 3:10 resonance with Neptune.[149] It has one known moon, Xiangliu.[150]

Farthest regions

The bubble-like heliosphere with its various transitional regions moving through the interstellar medium

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 heliosphere, is considered the beginning of the interstellar medium.[69] The Sun's Hill sphere, the effective range of its gravitational dominance, is thought to extend up to a thousand times farther and encompasses the hypothetical Oort cloud.[151]

Heliosphere

Main page: Astronomy:Heliosphere

The heliosphere is a stellar-wind bubble, a region of space dominated by the Sun whose boundaries occur where the solar wind collides with the interstellar medium.[152] This collision occurs at the termination shock, which is roughly 80–100 AU from the Sun upwind of the interstellar medium and roughly 200 AU from the Sun downwind.[153] Here the wind slows dramatically, condenses and becomes more turbulent,[153] forming a great oval structure known as the heliosheath. This structure is thought to look and behave very much like a comet's tail, extending outward for a further 40 AU on the upwind side but tailing many times that distance downwind; evidence from the Cassini and Interstellar Boundary Explorer spacecraft has suggested that it is forced into a bubble shape by the constraining action of the interstellar magnetic field.[154]

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.[69] Voyager 1 and Voyager 2 are reported to have passed the termination shock and entered the heliosheath, at 94 and 84 AU from the Sun, respectively.[155][156] Voyager 1 is reported to have crossed the heliopause in August 2012.[157]

The shape and form of the outer edge of the heliosphere is likely affected by the 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 AU farther than the southern hemisphere.[153] Beyond the heliopause, at around 230 AU, lies the bow shock, a plasma "wake" left by the Sun as it travels through the Milky Way.[158]

Zooming out the Solar System:
• inner Solar System and Jupiter
• outer Solar System and Pluto
• orbit of Sedna (detached object)
• inner part of the Oort Cloud

Detached objects

Main pages: Astronomy:Detached object and Astronomy:Sednoid

Sedna (with an average orbit of 520 AU) is a large, reddish object with a gigantic, highly elliptical orbit that takes it from about 76 AU at perihelion to 940 AU at aphelion and takes 11,400 years to complete. Mike Brown, who discovered the object in 2003, asserts that it cannot be part of the scattered disc or the Kuiper belt because its perihelion is too distant to have been affected by Neptune's migration. He and other astronomers consider it to be the first in an entirely new population, sometimes termed "distant detached objects" (DDOs), which also may include the object 2000 CR105, which has a perihelion of 45 AU, an aphelion of 415 AU, and an orbital period of 3,420 years.[159] Brown terms this population the "inner Oort cloud" because it may have formed through a similar process, although it is far closer to the Sun.[160] Sedna is very likely a dwarf planet, though its shape has yet to be determined. The second unequivocally detached object, with a perihelion farther than Sedna's at roughly 81 AU, is 2012 VP113, discovered in 2012. Its aphelion is only about half that of Sedna's, at 458 AU.[161][162]

Oort cloud

Main page: Astronomy:Oort cloud
Schematic of the hypothetical Oort cloud, with a spherical outer cloud and a disc-shaped inner 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 AU (around 1 light-year (ly)), and possibly to as far as 100,000 AU (1.87 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 galactic tide, the tidal force exerted by the Milky Way.[163][164]

Boundaries

Much of the Solar System is still unknown. The Sun's gravitational field is estimated to dominate the gravitational forces of 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.[165] 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. There are also ongoing studies of the region between Mercury and the Sun.[166] Objects may yet be discovered in the Solar System's uncharted regions.

Currently, the furthest known objects, such as Comet West, have aphelia around 70,000 AU from the Sun, but as the Oort cloud becomes better known, this may change.

Galactic context

Diagram of the Milky Way with the position of the Solar System marked by a yellow arrow
Close up on the Orion Arm, with major stellar associations (yellow), nebulae (red) and dark nebulae (grey) around the Local Bubble.

The Solar System is located in the Milky Way, a barred spiral galaxy with a diameter of about 100,000 light-years containing more than 100 billion stars.[167] The Sun resides in one of the Milky Way's outer spiral arms, known as the Orion–Cygnus Arm or Local Spur.[168] The Sun lies about 26,660 light-years from the Galactic Centre,[169] and its speed around the center of the Milky Way is about 247 km/s, so that it completes one revolution every 210 million years. This revolution is known as the Solar System's galactic year.[170] The solar apex, the direction of the Sun's path through interstellar space, is near the constellation Hercules in the direction of the current location of the bright star Vega.[171] The plane of the ecliptic lies at an angle of about 60° to the galactic plane.[lower-alpha 7]

The Solar System's location in the Milky Way is a factor in the evolutionary history of life on Earth. Its orbit is close to circular, and orbits near the Sun are at roughly the same speed as that of the spiral arms.[173][174] Therefore, the Sun passes through arms only rarely. Because spiral arms are home to a far larger concentration of supernovae, gravitational instabilities, and radiation that could disrupt the Solar System, this has given Earth long periods of stability for life to evolve.[173] 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 Shiva hypothesis or related theories. The Solar System lies well outside the star-crowded environs of the galactic centre. Near the centre, 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 centre could also interfere with the development of complex life.[173]

Logarithmic depiction of the Solar System's location

Celestial neighbourhood

Beyond the heliosphere is the interstellar medium, consisting of various clouds of gases. The Solar System currently moves through the Local Interstellar Cloud.

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.[175][176] Multiple other interstellar clouds also exist in the region within 300 light-years of the Sun, known as the Local Bubble.[176]

Within ten light-years of the Sun there are relatively few stars, the closest being the triple star system Alpha Centauri, which is about 4.4 light-years away and in the Local Bubble's G-Cloud. Alpha Centauri A and B are a closely tied pair of Sun-like stars, whereas the closest to Earth, the small red dwarf Proxima Centauri, orbits the pair closer at a distance of 0.2 light-year. In 2016, a potentially habitable exoplanet was confirmed to be orbiting Proxima Centauri, called Proxima Centauri b, the closest confirmed exoplanet to the Sun.[177] The next closest known fusors and rogue planets to the Sun are the red dwarf Barnard's Star (at 5.9 ly), the nearest brown dwarfs of the binary Luhman 16 system (6.6 ly), the closest known rogue or free-floating planetary-mass object at less than 10 Jupiter masses the sub-brown dwarf WISE 0855−0714,[178] (7 ly), as well as the red dwarfs Wolf 359 (7.8 ly) and Lalande 21185 (8.3 ly).

The next closest at 8.6 ly is Sirius, the brightest star in Earth's night sky, with roughly twice the Sun's mass, orbited by the closest white dwarf to Earth, Sirius B. Other systems within ten light-years are the binary red-dwarf system Luyten 726-8 (8.7 ly) and the solitary red dwarf Ross 154 (9.7 ly).[179] The closest solitary Sun-like star to the Solar System is Tau Ceti at 11.9 light-years. It has roughly 80% of the Sun's mass but only 60% of its luminosity.[180]

Distance and angle conformal map of the nearest stars and (sub-) brown dwarfs within 12 light years of the Solar System (Sol).

The nearest and unaided-visible group of stars beyond the immediate celestial neighbourhood is the Ursa Major Moving Group at roughly 80 light-years, which is within the Local Bubble, like the nearest as well as unaided-visible star cluster the Hyades, which lie at its edge. The Local Bubble is an hourglass-shaped cavity or 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.[181] The Local Bubble is a small superbubble compared to the neighbouring wider Gould Belt and Radcliffe wave, each of which are some thousands of light-years in length. 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 closest star-forming regions are the Corona Australis Molecular Cloud, the Rho Ophiuchi cloud complex and the Taurus Molecular Cloud; the latter lies just beyond the Local Bubble and is part of the Radcliffe wave.

Comparison with extrasolar systems

Compared to many other planetary systems, the Solar System stands out in lacking planets interior to the orbit of Mercury.[182][183] The known Solar System also lacks super-Earths, planets between one and ten times as massive as the Earth,[182] although the hypothetical Planet Nine, if it does exist, could be a super-Earth beyond the Solar System as we understand it today.[184] 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). Also, these super-Earths have closer orbits than Mercury.[182] This led to the hypothesis 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.[185]

The orbits of Solar System planets are nearly circular. Compared to other systems, they have smaller orbital eccentricity.[182] Although there are attempts to explain it partly with a bias in the radial-velocity detection method and partly with long interactions of a quite high number of planets, the exact causes remain undetermined.[182][186]

Visual summary

This section is a sampling of Solar System bodies, selected for size and quality of imagery, and sorted by volume. Some large objects are omitted here (notably the seven large TNOs Eris, Haumea, Makemake, Gonggong, Quaoar, Sedna, and Orcus) because they have not been imaged in high quality.

History of the Solar System concept

Main page: Astronomy:Discovery and exploration of the Solar System
Andreas Cellarius's illustration of the Copernican system, from the Harmonia Macrocosmica (1660)

Humanity's knowledge of the Solar System has grown incrementally over the centuries. Most people up to the Late Middle Ages–Renaissance believed Earth to be stationary at the centre of the universe and categorically different from the divine or ethereal objects that moved through the sky. Although the Ancient Greece philosopher Aristarchus of Samos had speculated on a heliocentric reordering of the cosmos, Nicolaus Copernicus was the first person known to have developed a mathematically predictive heliocentric system.[187][188] Heliocentrism did not triumph immediately over geocentrism, but the work of Copernicus had its champions, notably Johannes Kepler. Using a heliocentric model that improved upon Copernicus by allowing elliptical orbits, and the precise observational data of Tycho Brahe, Kepler produced the Rudolphine Tables, which enabled accurate computations of the positions of the then-known planets. Pierre Gassendi used them to predict a transit of Mercury in 1631, and Jeremiah Horrocks did the same for a transit of Venus in 1639. This provided a strong vindication of heliocentrism and Kepler's elliptical orbits.[189][190]

In the 17th century, Galileo publicized the use of the telescope in astronomy; he and Simon Marius independently discovered that Jupiter had four satellites in orbit around it.[191] Christiaan Huygens followed on from these observations by discovering Saturn's moon Titan and the shape of the rings of Saturn.[192] Around 1677, Edmond Halley observed a transit of Mercury across the Sun, leading him to realise that observations of the solar parallax of a planet (more ideally using the transit of Venus) could be used to trigonometrically determine the distances between Earth, Venus, and the Sun.[193] The term "Solar System" entered the English language by 1704, when John Locke used it to refer to the Sun, planets, and comets.[194] In 1705, Halley realised that repeated sightings of 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,[195] though this had been theorized about comets in the 1st century by Seneca.[196] Uranus, having occasionally been observed since antiquity, was recognized to be a planet beyond Saturn by 1783. In 1838, Friedrich Bessel successfully measured a 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.[197] 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.

Improvements in observational astronomy and the use of uncrewed spacecraft have since enabled the detailed investigation of other bodies orbiting the Sun.

Notes

1. As of August 27, 2019.
2. Capitalization of the name varies. The 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 naming guidelines document. The name is commonly rendered in lower case ("solar system"), as, for example, in the Oxford English Dictionary and Merriam-Webster's 11th Collegiate Dictionary.
3. The two moons larger than Mercury are Ganymede, which orbits Jupiter, and Titan, which orbits Saturn. Although bigger than Mercury, both moons have less than half its mass. In addition, the radius of Jupiter's moon Callisto is over 98% that of Mercury.
4. According to IAU definitions, objects orbiting the Sun are classified dynamically and physically into three categories: planets, dwarf planets, and small Solar System bodies.
• A planet is any body orbiting the Sun whose mass is sufficient for gravity to have pulled it into a (near-)spherical shape and that has cleared its immediate neighbourhood 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 neighbourhood of other Kuiper belt objects, Pluto does not fit this definition.[5]
• A 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 planetesimals from its neighbourhood and is also not a satellite.[5] Dwarf planets are considered planets by planetologists but not by the IAU. Pluto is a dwarf planet and the IAU has recognized or named four other bodies in the Solar System under the expectation that they will turn out to be dwarf planets: Ceres, Haumea, Makemake, and Eris.[6] Other objects commonly accepted as dwarf planets include Gonggong, Sedna, Orcus, and Quaoar.[7] In a reference to Pluto, other dwarf planets orbiting in the trans-Neptunian region are sometimes called "plutoids",[8] though this term is seldom used.
• The remaining objects orbiting the Sun are known as small Solar System bodies.[5]
5. 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),[12] the Kuiper belt (estimated at roughly 0.1 Earth mass)[13] and the asteroid belt (estimated to be 0.0005 Earth mass)[14] 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.
6. The date is based on the oldest inclusions found to date in meteorites, 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.[45]
7. If $\displaystyle{ \psi }$ is the angle between the north pole of the ecliptic and the north galactic pole then:
$\displaystyle{ \cos\psi=\cos(\beta_g)\cos(\beta_e)\cos(\alpha_g-\alpha_e)+\sin(\beta_g)\sin(\beta_e) }$
where $\displaystyle{ \beta_g }$ = 27° 07′ 42.01″ and $\displaystyle{ \alpha_g }$ = 12h 51m 26.282s are the declination and right ascension of the north galactic pole,[172] whereas $\displaystyle{ \beta_e }$ = 66° 33′ 38.6″ and $\displaystyle{ \alpha_e }$ = 18h 0m 00s are those for the north pole of the ecliptic. (Both pairs of coordinates are for J2000 epoch.) The result of the calculation is 60.19°.