View source for Astronomy:Exoplanetology

{{short description|study of planets outside the Solar System}}
'''Exoplanetology''', or '''exoplanetary science''', is an integrated field of astronomical science dedicated to the search for and study of [[Astronomy:Exoplanet|exoplanet]]s (extrasolar planets). It employs an interdisciplinary approach which includes [[Astronomy:Astrobiology|astrobiology]], [[Physics:Astrophysics|astrophysics]], [[Astronomy:Astronomy|astronomy]], [[Chemistry:Astrochemistry|astrochemistry]], astrogeology, [[Chemistry:Geochemistry|geochemistry]], and [[Astronomy:Planetary science|planetary science]].

== Nomenclature ==
{{Main|Astronomy:Exoplanet naming convention}}
The exoplanet naming convention is an extension of the system used for naming multiple-star systems as adopted by the [[Organization:International Astronomical Union|International Astronomical Union]] (IAU). For an exoplanet orbiting a single star, the name is normally formed by taking the name of its parent star and adding a lowercase letter. The first planet discovered in a system is given the designation "b" (the parent star is considered to be "a") and later planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbit size. A provisional IAU-sanctioned standard exists to accommodate the naming of [[Astronomy:Circumbinary planet|circumbinary planet]]s. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist.

== Detection methods ==

=== Direct imaging ===

[[File:Beta Pictoris.jpg|thumb|alt=Two directly imaged exoplanets around star Beta Pictoris, star-subtracted and artificially embellished with an outline of the orbit of one of the planets. The white dot in the center is the other exoplanet in the same system.|Directly imaged planet Beta Pictoris b]]
Planets are extremely faint compared with their parent stars. For example, a Sun-like star is about a billion times brighter than the reflected light from any exoplanet orbiting it. It is difficult to detect such a faint light source, and furthermore the parent star causes a glare that tends to wash it out. It is necessary to block the light from the parent star in order to reduce the glare while leaving the light from the planet detectable; doing so is a major technical challenge which requires extreme [[Physics:Optothermal stability|optothermal stability]].<ref name="Perryman_book11">{{Cite book|last = Perryman|first = Michael|title = The Exoplanet Handbook|url = https://archive.org/details/exoplanethandboo00perr|url-access = limited|publisher = Cambridge University Press|date = 2011|page = [https://archive.org/details/exoplanethandboo00perr/page/n162 149]|isbn = 978-0-521-76559-6}}</ref> All exoplanets that have been directly imaged are both large (more massive than [[Astronomy:Jupiter|Jupiter]]) and widely separated from their parent star.

Specially designed direct-imaging instruments such as [[Astronomy:Gemini Planet Imager|Gemini Planet Imager]], VLT-SPHERE, and [[Astronomy:Subaru Telescope#Subaru Coronagraphic Extreme Adaptive Optics system|SCExAO]] will image dozens of gas giants, but the vast majority of known extrasolar planets have only been detected through indirect methods. The following are the indirect methods that have proven useful:

=== Indirect methods ===
* Transit method
:[[File:Dopspec-inline.gif|thumb|alt=Edge-on animation of a star-planet system, showing the geometry considered for the transit method of exoplanet detection|When the star is behind a planet, its brightness will seem to dim]]If a planet crosses (or transits) in front of its parent star's disk, then the observed brightness of the star drops by a small amount. The amount by which the star dims depends on its size and on the size of the planet, among other factors. Because the transit method requires that the planet's orbit intersect a line-of-sight between the host star and Earth, the probability that an exoplanet in a randomly oriented orbit will be observed to transit the star is somewhat small. The [[Engineering:Kepler (spacecraft)|''Kepler'' telescope]] used this method.
[[File:Exoplanet Discovery Methods Bar.png|thumb|300px|alt=Histogram showing of the number of exoplanets discovered per year and per detection method, as of September 2014. The sum of exoplanets discovered from January to September 2014 is at least 4.5 times more than in any previous year.|Discovered extrasolar planets per year and by [[Methods of detecting extrasolar planets|detection method]] (as of September 2014):
{| style="width: 100%; margin-bottom: 4px;"
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|  style="vertical-align:top; width:40%;"|
{{legend2|#ab0000|border=1px solid #610000|direct imaging}}<br />
{{legend2|#ab5500|border=1px solid #613000|[[Physics:Gravitational microlensing|microlensing]]}}
|  style="vertical-align:top; width:28%;"|
{{legend2|#00ab00|border=1px solid #006100|transit}}<br />
{{legend2|#abab00|border=1px solid #616100|[[Astronomy:Transit-timing variation|timing]]}}
| valign=top |
{{legend2|#0000ab|border=1px solid #000047|radial velocity}}
|}
]]

* [[Astronomy:Methods of detecting exoplanets#Radial velocity|Radial velocity or Doppler method]]
:As a planet orbits a star, the star also moves in its own small orbit around the system's center of mass. Variations in the star's radial velocity—that is, the speed with which it moves towards or away from Earth—can be detected from displacements in the star's [[Physics:Spectral line|spectral line]]s due to the [[Physics:Doppler effect|Doppler effect]]. Extremely small radial-velocity variations can be observed, of 1&nbsp;m/s or even somewhat less.<ref name="Pepe_HarpsEarthlike11">{{Cite journal | doi = 10.1051/0004-6361/201117055| title = The HARPS search for Earth-like planets in the habitable zone| journal = Astronomy & Astrophysics| volume = 534| pages = A58| year = 2011| last1 = Pepe | first1 = F.| last2 = Lovis | first2 = C.| last3 = Ségransan | first3 = D.| last4 = Benz | first4 = W.| last5 = Bouchy | first5 = F.| last6 = Dumusque | first6 = X.| last7 = Mayor | first7 = M.| last8 = Queloz | first8 = D.| last9 = Santos | first9 = N. C.| last10 = Udry | first10 = S.| bibcode = 2011A&A...534A..58P|arxiv = 1108.3447 | s2cid = 15088852}}</ref> 
* [[Astronomy:Methods of detecting exoplanets#Transit timing|Transit timing variation]] (TTV) 
:When multiple planets are present, each one slightly perturbs the others' orbits. Small variations in the times of transit for one planet can thus indicate the presence of another planet, which itself may or may not transit. For example, variations in the transits of the planet Kepler-19b suggest the existence of a second planet in the system, the non-transiting Kepler-19c.<ref name=ttv1>[http://www.scientificcomputing.com/news-DS-Planet-Hunting-Finding-Earth-like-Planets-071910.aspx Planet Hunting: Finding Earth-like Planets] {{Webarchive|url=https://web.archive.org/web/20100728093120/http://www.scientificcomputing.com/news-DS-Planet-Hunting-Finding-Earth-like-Planets-071910.aspx |date=2010-07-28 }}. Scientific Computing. 19 July 2010</ref><ref>{{Cite journal | doi = 10.1088/0004-637X/743/2/200|arxiv=1109.1561|bibcode=2011ApJ...743..200B| title = The Kepler-19 System: A Transiting 2.2 R<sub>⊕</sub> Planet and a Second Planet Detected Via Transit Timing Variations| journal = The Astrophysical Journal| volume = 743| issue = 2| page = 200| year = 2011| last1 = Ballard | first1 = S. | last2 = Fabrycky | first2 = D. | last3 = Fressin | first3 = F. | last4 = Charbonneau | first4 = D. | last5 = Desert | first5 = J. M. | last6 = Torres | first6 = G. | last7 = Marcy | first7 = G. | last8 = Burke | first8 = C. J. | last9 = Isaacson | first9 = H. | last10 = Henze | first10 = C. | last11 = Steffen | first11 = J. H. | last12 = Ciardi | first12 = D. R. | last13 = Howell | first13 = S. B. | last14 = Cochran | first14 = W. D. | last15 = Endl | first15 = M. | last16 = Bryson | first16 = S. T. | last17 = Rowe | first17 = J. F. | last18 = Holman | first18 = M. J. | last19 = Lissauer | first19 = J. J. | last20 = Jenkins | first20 = J. M. | last21 = Still | first21 = M. | last22 = Ford | first22 = E. B. | last23 = Christiansen | first23 = J. L. | last24 = Middour | first24 = C. K. | last25 = Haas | first25 = M. R. | last26 = Li | first26 = J. | last27 = Hall | first27 = J. R. | last28 = McCauliff | first28 = S. | last29 = Batalha | first29 = N. M. | last30 = Koch | first30 = D. G. |s2cid=42698813| display-authors = etal}}</ref> 
* Transit duration variation (TDV)
[[File:201008-2a PlanetOrbits 16x9- Transit timing of 1-planet vs 2-planet systems.ogv|thumb|300px|alt=Animation showing the difference between planet transit timing of one-planet and two-planet systems|Animation showing difference between planet transit timing of one-planet and two-planet systems]]
:When a planet orbits multiple stars or if the planet has moons, its transit time can significantly vary per transit. Although no new planets or moons have been discovered with this method, it is used to successfully confirm many transiting circumbinary planets.<ref>{{cite journal |last1=Pál |first1=A. |last2=Kocsis |first2=B. |title=Periastron Precession Measurements in Transiting Extrasolar Planetary Systems at the Level of General Relativity |date=2008 |doi=10.1111/j.1365-2966.2008.13512.x |journal=Monthly Notices of the Royal Astronomical Society |volume=389 |issue=1 |pages=191–198 |arxiv=0806.0629|bibcode = 2008MNRAS.389..191P |s2cid=15282437 }}</ref>
* Gravitational microlensing
:Microlensing occurs when the gravitational field of a star acts like a lens, magnifying the light of a distant background star. Planets orbiting the lensing star can cause detectable anomalies in the magnification as it varies over time. Unlike most other methods which have detection bias towards planets with small (or for resolved imaging, large) orbits, microlensing method is most sensitive to detecting planets around 1–10&nbsp;[[Astronomy:Astronomical unit|AU]] away from Sun-like stars.
* Astrometry
:Astrometry consists of precisely measuring a star's position in the sky and observing the changes in that position over time. The motion of a star due to the gravitational influence of a planet may be observable. Because the motion is so small, however, this method has not yet been very productive. It has produced only a few disputed detections, though it has been successfully used to investigate the properties of planets found in other ways.
* Pulsar timing
:A [[Astronomy:Pulsar|pulsar]] (the small, ultradense remnant of a star that has exploded as a [[Astronomy:Supernova|supernova]]) emits radio waves extremely regularly as it rotates. If planets orbit the pulsar, they will cause slight anomalies in the timing of its observed radio pulses. [[Astronomy:PSR B1257+12|The first confirmed discovery of an extrasolar planet]] was made using this method. But as of 2011, it has not been very productive; five planets have been detected in this way, around three different pulsars.
* Variable star timing (pulsation frequency)
:Like pulsars, there are some other types of stars which exhibit periodic activity. Deviations from the periodicity can sometimes be caused by a planet orbiting it. As of 2013, a few planets have been discovered with this method.<ref>{{Cite journal | doi = 10.1038/nature06143|pmid=17851517|bibcode = 2007Natur.449..189S |url=http://www.physics.udel.edu/gp/darc/wet/pubs/silvotti.pdf| title = A giant planet orbiting the 'extreme horizontal branch' star V 391 Pegasi| journal = Nature| volume = 449| issue = 7159| pages = 189–91| year = 2007| last1 = Silvotti | first1 = R.| last2 = Schuh | first2 = S.| last3 = Janulis | first3 = R.| last4 = Solheim | first4 = J. -E. | last5 = Bernabei | first5 = S.| last6 = Østensen | first6 = R.| last7 = Oswalt | first7 = T. D.| last8 = Bruni | first8 = I.| last9 = Gualandi | first9 = R.| last10 = Bonanno | first10 = A.| last11 = Vauclair | first11 = G.| last12 = Reed | first12 = M.| last13 = Chen | first13 = C. -W. | last14 = Leibowitz | first14 = E.| last15 = Paparo | first15 = M.| last16 = Baran | first16 = A.| last17 = Charpinet | first17 = S.| last18 = Dolez | first18 = N.| last19 = Kawaler | first19 = S.| last20 = Kurtz | first20 = D.| last21 = Moskalik | first21 = P.| last22 = Riddle | first22 = R.| last23 = Zola | first23 = S.|s2cid=4342338}}</ref>
* Reflection/emission modulations
:When a planet orbits very close to the star, it catches a considerable amount of starlight. As the planet orbits around the star, the amount of light changes due to planets having phases from Earth's viewpoint or planet glowing more from one side than the other due to temperature differences.<ref>{{cite journal|last=Jenkins|first=J.M.|author2=Laurance R. Doyle|date=20 September 2003|title=Detecting reflected light from close-in giant planets using space-based photometers|journal=Astrophysical Journal|volume=1|issue=595|pages=429–445|doi=10.1086/377165|bibcode=2003ApJ...595..429J|arxiv = astro-ph/0305473 |s2cid=17773111}}</ref>
* Relativistic beaming
:Relativistic beaming measures the observed flux from the star due to its motion. The brightness of the star changes as the planet moves closer or further away from its host star.<ref>{{cite journal
 |arxiv=astro-ph/0303212
 |bibcode=2003ApJ...588L.117L
 |doi=10.1086/375551
 |title=Periodic Flux Variability of Stars due to the Reflex Doppler Effect Induced by Planetary Companions
 |date=2003
 |last1=Loeb |first1=A.
 |last2=Gaudi |first2=B. S.
 |journal=The Astrophysical Journal Letters
 |volume=588 |issue=2 |pages=L117
|s2cid=10066891
 }}</ref>
* Ellipsoidal variations
:Massive planets close to their host stars can slightly deform the shape of the star. This causes the brightness of the star to slightly deviate depending how it is rotated relative to Earth.<ref>Atkinson, Nancy (13 May 2013) [http://www.universetoday.com/102112/using-the-theory-of-relativity-and-beer-to-find-exoplanets/ Using the Theory of Relativity and BEER to Find Exoplanets]. ''Universe Today''.</ref>
* Polarimetry
:With polarimetry method, a polarized light reflected off the planet is separated from unpolarized light emitted from the star. No new planets have been discovered with this method although a few already discovered planets have been detected with this method.<ref>{{Cite journal | doi = 10.1017/S1743921306009252| title = Search and investigation of extra-solar planets with polarimetry| journal = Proceedings of the International Astronomical Union| volume = 1| page = 165| year = 2006| last1 = Schmid | first1 = H. M.| last2 = Beuzit | first2 = J. -L. | last3 = Feldt | first3 = M.| last4 = Gisler | first4 = D.| last5 = Gratton | first5 = R.| last6 = Henning | first6 = T. | last7 = Joos | first7 = F.| last8 = Kasper | first8 = M.| last9 = Lenzen | first9 = R.| last10 = Mouillet | first10 = D.| last11 = Moutou | first11 = C.| last12 = Quirrenbach | first12 = A.| last13 = Stam | first13 = D. M.| last14 = Thalmann | first14 = C.| last15 = Tinbergen | first15 = J.| last16 = Verinaud | first16 = C.| last17 = Waters | first17 = R.| last18 = Wolstencroft | first18 = R.| bibcode = 2006dies.conf..165S| doi-access = free}}</ref><ref>{{Cite journal | doi = 10.1086/527320| title = First Detection of Polarized Scattered Light from an Exoplanetary Atmosphere| journal = The Astrophysical Journal| volume = 673| issue = 1| pages = L83| year = 2008| last1 = Berdyugina | first1 = S. V.| last2 = Berdyugin | first2 = A. V.| last3 = Fluri | first3 = D. M.| last4 = Piirola | first4 = V. | bibcode=2008ApJ...673L..83B|arxiv = 0712.0193 | s2cid = 14366978}}</ref>
* Circumstellar disks
:Disks of space dust surround many stars, thought to originate from collisions among asteroids and comets. The dust can be detected because it absorbs starlight and re-emits it as [[Physics:Infrared|infrared]] radiation. Features in the disks may suggest the presence of planets, though this is not considered a definitive detection method.

== Orbital parameters ==
Most known extrasolar planet candidates have been discovered using indirect methods and therefore only some of their physical and orbital parameters can be determined. For example, out of the six independent parameters that define an orbit, the radial-velocity method can determine four: semi-major axis, [[Astronomy:Orbital eccentricity|eccentricity]], longitude of periastron, and time of periastron. Two parameters remain unknown: inclination and [[Longitude of the ascending node|longitude of the ascending node]].

=== Distance from star and orbital period ===
[[File:Exoplanet Period-Mass Scatter.png|thumb|300px|alt=Log-log scatterplot showing masses, orbital radii, and period of all extrasolar planets discovered through September 2014, with colors indicating method of detection|Log-log scatterplot showing masses, orbital radii, and period of all extrasolar planets discovered through September 2014, with colors indicating method of detection:
{| style="width: 100%; margin-bottom: 3px;"
|-
|  style="vertical-align:top; width:27%;"|
{{legend|#00ab00|border=1px solid#004700|transit}}
{{legend|#550055|border=1px solid #0A000A|[[Astronomy:Transit-timing variation|timing]]}}
|  style="vertical-align:top; width:38%;"|
{{legend|#0000ab|border=1px solid #000061|radial velocity}}
{{legend|#ab0000|border=1px solid #610000|direct imaging}}
| valign=top |
{{legend|#ab5500|border=1px solid #613000|[[Physics:Gravitational microlensing|microlensing]]}}
|}
For reference, Solar System planets are marked as gray circles. The horizontal axis plots the [[Logarithm|logarithm]] of the semi-major axis, and the vertical axis plots the logarithm of the mass.
]]

There are exoplanets that are much closer to their parent star than any planet in the Solar System is to the Sun, and there are also exoplanets that are much further from their star. [[Astronomy:Mercury (planet)|Mercury]], the closest planet to the Sun at 0.4&nbsp;[[Astronomy:Astronomical unit|astronomical unit]]s (AU), takes 88 days for an orbit, but the smallest known orbits of exoplanets have orbital periods of only a few hours, see [[Astronomy:Ultra-short period planet|Ultra-short period planet]]. The [[Astronomy:Kepler-11|Kepler-11]] system has five of its planets in smaller orbits than Mercury's. [[Astronomy:Neptune|Neptune]] is 30&nbsp;AU from the Sun and takes 165 years to orbit it, but there are exoplanets that are thousands of AU from their star and take tens of thousands of years to orbit, e.g. [[Astronomy:GU Piscium b|GU Piscium b]].<ref>{{cite web |url=http://www.technology.org/2014/05/14/enlightening-pisces-star-signs-lead-scientists-discovery/ |title=Enlightening Pisces star signs lead scientists to discovery |publisher=technology.org |date=14 May 2014 |access-date=July 23, 2016}}</ref>

The radial-velocity and transit methods are most sensitive to planets with small orbits. The earliest discoveries such as 51 Peg b were [[Astronomy:Gas giant|gas giant]]s with orbits of a few days.<ref name="cumming08">{{Cite journal| year=2008| title=The Keck Planet Search: Detectability and the Minimum Mass and Orbital Period Distribution of Extrasolar Planets| journal=Publications of the Astronomical Society of the Pacific| volume=120| issue=867|pages=531–554| arxiv=0803.3357|doi=10.1086/588487| bibcode=2008PASP..120..531C| last1=Cumming| first1=Andrew|last2=Butler|first2=R. Paul|last3=Marcy|first3=Geoffrey W.| last4=Vogt| first4=Steven S.|last5=Wright|first5=Jason T.|last6=Fischer|first6=Debra A.| s2cid=10979195}}</ref> These "[[Astronomy:Hot Jupiter|hot Jupiter]]s" likely formed further out and migrated inwards.

The direct imaging method is most sensitive to planets with large orbits, and has discovered some planets that have planet–star separations of hundreds of AU. However, protoplanetary disks are usually only around 100&nbsp;AU in radius, and [[Astronomy:Nebular hypothesis#Giant planets|core accretion models]] predict giant planet formation to be within 10&nbsp;AU, where the planets can coalesce quickly enough before the disk [[Astronomy:Photoevaporation|evaporates]].
Very-long-period giant planets may have been [[Astronomy:Rogue planet|rogue planet]]s that were captured,<ref name="WideRecapture">{{Cite journal | doi = 10.1088/0004-637X/750/1/83| title = On the Origin of Planets at Very Wide Orbits from the Recapture of Free Floating Planets| journal = The Astrophysical Journal| volume = 750| issue = 1| page = 83| year = 2012| last1 = Perets | first1 = H. B. | last2 = Kouwenhoven | first2 = M. B. N.| bibcode = 2012ApJ...750...83P| arxiv = 1202.2362 | s2cid = 119195442}}</ref> or formed close-in and gravitationally scattered outwards, or the planet and star could be a mass-imbalanced wide [[Astronomy:Binary star|binary system]] with the planet being the primary object of its own separate protoplanetary disk. [[Astronomy:Nebular hypothesis#Giant planets|Gravitational instability models]] might produce planets at multi-hundred AU separations but this would require unusually large disks.<ref>{{cite journal |doi=10.1088/0004-637X/693/2/L113 |title=Long-Period Exoplanets from Dynamical Relaxation |date=2009 |last1=Scharf |first1=Caleb |last2=Menou |first2=Kristen |journal=The Astrophysical Journal |volume=693 |issue=2 |pages=L113|arxiv = 0811.1981 |bibcode = 2009ApJ...693L.113S |s2cid=7879291 }}</ref><ref name=ddl2011>{{cite book|last=D'Angelo|first=G.|author2=Durisen, R. H. |author3=Lissauer, J. J.|chapter=Giant Planet Formation |bibcode=2010exop.book..319D| title=Exoplanets |publisher=University of Arizona Press, Tucson, AZ| editor=Seager, S. |pages=319–346| date=2011| chapter-url=http://www.uapress.arizona.edu/Books/bid2263.htm|arxiv=1006.5486 }}</ref> For planets with very wide orbits up to several hundred thousand AU it may be difficult to observationally determine whether the planet is gravitationally bound to the star.

Most planets that have been discovered are within a couple of AU from their host star because the most used methods (radial-velocity and transit) require observation of several orbits to confirm that the planet exists and there has only been enough time since these methods were first used to cover small separations. Some planets with larger orbits have been discovered by direct imaging but there is a middle range of distances, roughly equivalent to the Solar System's gas giant region, which is largely unexplored. Direct imaging equipment for exploring that region was installed on two large telescopes that began operation in 2014, e.g. [[Astronomy:Gemini Planet Imager|Gemini Planet Imager]] and VLT-SPHERE. The microlensing method has detected a few planets in the 1–10&nbsp;AU range.<ref>[http://exoplanet.eu/catalog/?f=%22microlensing%22+IN+detection Catalog Listing]. Extrasolar Planets Encyclopaedia</ref>
It appears plausible that in most exoplanetary systems, there are one or two giant planets with orbits comparable in size to those of Jupiter and Saturn in the Solar System. Giant planets with substantially larger orbits are now known to be rare, at least around Sun-like stars.<ref>{{Cite journal | doi = 10.1088/0004-637X/717/2/878| title = A Uniform Analysis of 118 Stars with High-Contrast Imaging: Long-Period Extrasolar Giant Planets Are Rare Around Sun-Like Stars| journal = The Astrophysical Journal| volume = 717| issue = 2| pages = 878–896| year = 2010| last1 = Nielsen | first1 = E. L. | last2 = Close | first2 = L. M. |bibcode=2010ApJ...717..878N|arxiv = 0909.4531 | s2cid = 119272089}}</ref>

The distance of the habitable zone from a star depends on the type of star and this distance changes during the star's lifetime as the size and temperature of the star changes.

=== Eccentricity ===
The [[Astronomy:Orbital eccentricity|eccentricity]] of an orbit is a measure of how elliptical (elongated) it is. All the planets of the Solar System except for [[Astronomy:Mercury (planet)|Mercury]] have near-circular orbits (e<0.1).<ref name="marcyprogth05">{{Cite journal
 |date        = 2005
 |title       = Observed Properties of Exoplanets: Masses, Orbits and Metallicities
 |url         = http://ptp.ipap.jp/link?PTPS/158/24
 |journal     = Progress of Theoretical Physics Supplement
 |volume      = 158
 |pages       = 24–42
 |doi         = 10.1143/PTPS.158.24
 |arxiv       = astro-ph/0505003
 |bibcode     = 2005PThPS.158...24M
 |last1       = Marcy
 |first1      = Geoffrey
 |last2       = Butler
 |first2      = R. Paul
 |last3       = Fischer
 |first3      = Debra
 |last4       = Vogt
 |first4      = Steven
 |last5       = Wright
 |first5      = Jason T.
 |last6       = Tinney
 |first6      = Chris G.
 |last7       = Jones
 |first7      = Hugh R. A.
 |s2cid = 16349463
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20081002085400/http://ptp.ipap.jp/link?PTPS%2F158%2F24
 |archive-date = 2008-10-02
}}</ref> Most exoplanets with orbital periods of 20 days or less have near-circular orbits, i.e. very low eccentricity. That is thought to be due to [[Tidal circularization|tidal circularization]]: reduction of eccentricity over time due to gravitational interaction between two bodies. The mostly sub-Neptune-sized planets found by the ''Kepler'' spacecraft with short orbital periods have very circular orbits.<ref name="kepler1700">{{cite web |last1=Johnson |first1=Michele |last2=Harrington |first2=J.D. |title=NASA's Kepler Mission Announces a Planet Bonanza, 715 New Worlds |url=http://www.nasa.gov/ames/kepler/nasas-kepler-mission-announces-a-planet-bonanza/ |date=26 February 2014 |work=[[Organization:NASA|NASA]] |access-date=26 February 2014 }}</ref> By contrast, the giant planets with longer orbital periods discovered by radial-velocity methods have quite eccentric orbits. (As of July 2010, 55% of such exoplanets have eccentricities greater than 0.2, whereas 17% have eccentricities greater than 0.5.<ref name="Encyclopaedia">{{cite encyclopedia|last1=Schneider |first1=J. |title=Interactive Extra-solar Planets Catalog |url=http://exoplanet.eu/catalog.php|encyclopedia=The Extrasolar Planets Encyclopedia}}</ref>) Moderate to high eccentricities (e>0.2) of giant planets are ''not'' an observational selection effect, because a planet can be detected about equally well regardless of the eccentricity of its orbit. The statistical significance of elliptical orbits in the ensemble of observed giant planets is somewhat surprising, because current theories of [[Astronomy:Nebular hypothesis#Formation of planets|planetary formation]] suggest that low-mass planets should have their [[Astronomy:Orbital eccentricity|orbital eccentricity]] circularized by gravitational interactions with the surrounding [[Astronomy:Protoplanetary disk|protoplanetary disk]].<ref>{{Cite journal| last1=Tanaka |first1=Hidekazu |title=Three-dimensional Interaction between a Planet and an Isothermal Gaseous Disk. II. Eccentricity Waves and Bending Waves |journal=The Astrophysical Journal |volume=602 |issue=2 |pages=388–395 |last2= Ward |first2=William R. |year=2004 |doi=10.1086/380992 |bibcode = 2004ApJ...602..388T |doi-access=free }}</ref><ref name="boss_book p26">{{Cite book
|last = Boss
|first = Alan
|title = The Crowded Universe: The Search for Living Planets
|url = https://archive.org/details/crowdeduniverses00boss
|url-access = limited
|publisher = Basic Books
|date = 2009
|page = [https://archive.org/details/crowdeduniverses00boss/page/n36 26]
|isbn = 978-0-465-00936-7}}</ref>
However, as a planet grows more massive and its interaction with the disk becomes nonlinear, it may induce eccentric motion of the surrounding disk's gas, which in turn may excite the planet's orbital eccentricity.<ref>{{Cite journal |arxiv=astro-ph/0608355 |last1=D'Angelo |first1=Gennaro |title=Evolution of Giant Planets in Eccentric Disks |journal=The Astrophysical Journal |volume=652 |issue=2 |pages=1698–1714 |last2= Lubow |first2=Stephen H. |last3= Bate |first3=Matthew R. |year=2006 |doi=10.1086/508451 |bibcode = 2006ApJ...652.1698D |s2cid=53135965 }}</ref><ref>{{Cite journal|arxiv=1603.00653 |last1=Teyssandier |first1=Jean |title=Growth of eccentric modes in disc-planet interactions |journal=Monthly Notices of the Royal Astronomical Society |volume=458 |issue=3 |pages=3221–3247 |last2= Ogilvie |first2=Gordon I. |year=2016 |doi=10.1093/mnras/stw521 |bibcode = 2016MNRAS.458.3221T }}</ref><ref>{{Cite journal|arxiv=1603.02544 |last1=Barker |first1=Adrian J. |title=Nonlinear hydrodynamical evolution of eccentric Keplerian discs in two dimensions: validation of secular theory |journal=Monthly Notices of the Royal Astronomical Society |volume=458 |issue=4 |pages=3739–3751 |last2= Ogilvie |first2=Gordon I. |year=2016 |doi=10.1093/mnras/stw580 |bibcode = 2016MNRAS.458.3739B }}</ref>  Low eccentricities are correlated with high multiplicity (number of planets in the system).<ref>{{Cite journal | doi =  10.1073/pnas.1406545111| title = The Exoplanet Orbital Eccentricity – Multiplicity Relation and the Solar System| journal = Proceedings of the National Academy of Sciences| volume = 112| issue = 1| pages = 20–24| year = 2015| last1 = Limbach | first1 = M. A.| last2 = Turner | first2 = E. L. | bibcode = 2015PNAS..112...20L| arxiv = 1404.2552 | pmid=25512527 | pmc=4291657| doi-access = free}}</ref> Low eccentricity is needed for habitability, especially advanced life.<ref>{{cite book|ref=Ward|author= Ward, Peter |author2=Brownlee, Donald |title=Rare Earth: Why Complex Life is Uncommon in the Universe|publisher= Springer|year= 2000|isbn=978-0-387-98701-9| pages=122–123}}</ref>

For weak [[Astronomy:Doppler spectroscopy|Doppler]] signals near the limits of the current detection ability, the eccentricity becomes poorly constrained and biased towards higher values. It is suggested that some of the high eccentricities reported for low-mass exoplanets may be overestimates, because simulations show that many observations are also consistent with two planets on circular orbits. Reported observations of single planets in moderately eccentric orbits have about a 15% chance of being a pair of planets.<ref>{{Cite journal | doi = 10.1088/0004-637X/702/1/716| title = Which Radial Velocity Exoplanets Have Undetected Outer Companions?| journal = The Astrophysical Journal| volume = 702| issue = 1| pages = 716–723| year = 2009| last1 = Rodigas | first1 = T. J. | last2 = Hinz | first2 = P. M. | bibcode = 2009ApJ...702..716R|arxiv = 0907.0020 | s2cid = 14817457}}</ref> This misinterpretation is especially likely if the two planets orbit with a 2:1 resonance. With the exoplanet sample known in 2009, a group of astronomers estimated that "(1) around 35% of the published eccentric one-planet solutions are statistically indistinguishable from planetary systems in 2:1 orbital resonance, (2) another 40% cannot be statistically distinguished from a circular orbital solution" and "(3) planets with masses comparable to Earth could be hidden in known orbital solutions of eccentric super-Earths and Neptune mass planets".<ref>{{Cite journal | doi = 10.1088/0004-637X/709/1/168| title = How Eccentric Orbital Solutions Can Hide Planetary Systems in 2:1 Resonant Orbits| journal = The Astrophysical Journal| volume = 709| issue = 1| pages = 168–178| year = 2010| last1 = Anglada-Escudé | first1 = G. | last2 = López-Morales | first2 = M. | last3 = Chambers | first3 = J. E. | bibcode=2010ApJ...709..168A|arxiv = 0809.1275 | s2cid = 2756148}}</ref>

Radial velocity surveys found exoplanet orbits beyond 0.1 AU to be eccentric, particularly for large planets. Transit data obtained by the ''Kepler'' spacecraft, is consistent with the RV surveys and also revealed that smaller planets tend to have less eccentric orbits.<ref>{{cite journal |arxiv=1203.1631|bibcode = 2012MNRAS.425..757K |doi = 10.1111/j.1365-2966.2012.21627.x | volume=425 |issue = 1 |title=The exoplanet eccentricity distribution from Kepler planet candidates |journal=Monthly Notices of the Royal Astronomical Society |pages=757–762|year = 2012 |last1 = Kane |first1 = Stephen R. |last2 = Ciardi |first2 = David R. |last3 = Gelino |first3 = Dawn M. |last4 = von Braun |first4 = Kaspar |s2cid = 5496129 }}</ref>

=== Inclination vs. spin–orbit angle ===
[[Astronomy:Orbital inclination|Orbital inclination]] is the angle between a planet's [[Astronomy:Orbital plane|orbital plane]] and another plane of reference. For exoplanets, the inclination is usually stated with respect to an observer on Earth: the angle used is that between the [[Normal (geometry)|normal]] to the planet's orbital plane and the line of sight from Earth to the star. Therefore, most planets observed by the transit method are close to 90 degrees.<ref>Mason, John (2008) ''Exoplanets: Detection, Formation, Properties, Habitability''.  Springer. {{ISBN|3-540-74007-4}}. p. 2</ref> Because the word 'inclination' is used in exoplanet studies for this line-of-sight inclination then the angle between the planet's orbit and the star's rotation must use a different word and is termed the spin–orbit angle or spin–orbit alignment. In most cases the orientation of the star's rotational axis is unknown. The ''Kepler'' spacecraft has found a few hundred multi-planet systems and in most of these systems the planets all orbit in nearly the same plane, much like the Solar System.<ref name="kepler1700" /> However, a combination of astrometric and radial-velocity measurements has shown that some planetary systems contain planets whose orbital planes are significantly tilted relative to each other.<ref>[http://www.scientificamerican.com/article.cfm?id=exoplanet-tilt Out of Flatland: Orbits Are Askew in a Nearby Planetary System]. ''Scientific American''. 24 May 2010.</ref> More than half of [[Astronomy:Hot Jupiter|hot Jupiter]]s have orbital planes substantially misaligned with their parent star's rotation. A substantial fraction of hot-Jupiters even have retrograde orbits, meaning that they orbit in the opposite direction from the star's rotation.<ref name="2010question">{{cite web| url=http://www.astro.gla.ac.uk/nam2010/pr10.php |title=Turning planetary theory upside down |publisher=Astro.gla.ac.uk |date=13 April 2010}}</ref> Rather than a planet's orbit having been disturbed, it may be that the star itself flipped early in their system's formation due to interactions between the star's magnetic field and the planet-forming disk.<ref>[https://www.newscientist.com/article/mg20727765.200-tilting-stars-may-explain-backwards-planets.html "Tilting stars may explain backwards planets"], ''New Scientist'', 1 September 2010, Vol. 2776.</ref>

=== Periastron precession ===
[[Astronomy:Apsidal precession|Periastron precession]] is the rotation of a planet's orbit within the orbital plane, i.e. the axes of the ellipse change direction. In the Solar System, perturbations from other planets are the main cause, but for close-in exoplanets the largest factor can be tidal forces between the star and planet. For close-in exoplanets, the [[Physics:General relativity|general relativistic]] contribution to the precession is also significant and can be orders of magnitude larger than the same effect for Mercury. Some exoplanets have significantly eccentric orbits, which makes it easier to detect the precession. The effect of general relativity can be detectable in timescales of about 10 years or less.<ref>{{cite journal| arxiv=0806.0630| bibcode = 2008ApJ...685..543J |doi = 10.1086/590549 | volume=685| issue = 1 |title=Observability of the General Relativistic Precession of Periastra in Exoplanets|journal=The Astrophysical Journal| pages=543–552| year = 2008 |last1 = Jordán |first1 = Andrés |last2 = Bakos |first2 = Gáspár Á.| s2cid = 13939328 }}</ref>

=== Nodal precession ===
[[Astronomy:Nodal precession|Nodal precession]] is rotation of a planet's orbital plane. Nodal precession is more easily seen as distinct from periastron precession when the orbital plane is inclined to the star's rotation, the extreme case being a polar orbit.

[[Astronomy:WASP-33|WASP-33]] is a fast-rotating star that hosts a [[Astronomy:Hot Jupiter|hot Jupiter]] in an almost polar orbit. The [[Physics:Quadrupole#Gravitational quadrupole|quadrupole mass moment]] and the proper [[Physics:Angular momentum|angular momentum]] of the star are 1900 and 400 times, respectively, larger than those of the Sun. This causes significant [[Physics:Newton's law of universal gravitation|classical]] and [[Physics:General relativity|relativistic]] deviations from Kepler's laws. In particular, the fast rotation causes large nodal precession because of the star's oblateness and the [[Physics:Lense–Thirring precession|Lense–Thirring effect]].<ref>{{cite journal| arxiv=1006.2707|doi = 10.1007/s10509-010-0468-x |bibcode = 2011Ap&SS.331..485I | volume=331|issue = 2 |title=Classical and relativistic node precessional effects in WASP-33b and perspectives for detecting them| journal=Astrophysics and Space Science| pages=485–496| year = 2010 |last1 = Iorio |first1 = Lorenzo |s2cid = 119253639 }}</ref>

== Rotation and axial tilt ==
[[File:Beta Pictoris b - The universal relation between mass and rotation speed of planets.jpg|thumb|400px|alt=Log-linear plot of planet mass (in Jupiter masses) vs. spin velocity (in km/s), comparing exoplanet Beta Pictoris b to the Solar System planets|Plot of equatorial spin velocity vs. mass for planets comparing Beta Pictoris b to the Solar System planets.]]

In April 2014, the first measurement of a planet's [[Rotation period|rotation period]] was announced: the length of day for the [[Astronomy:Super-Jupiter|super-Jupiter]] gas giant [[Astronomy:Beta Pictoris b|Beta Pictoris b]] is 8 hours (based on the assumption that the [[Astronomy:Axial tilt|axial tilt]] of the planet is small.)<ref name="spinpress">[http://www.eso.org/public/news/eso1414/ Length of Exoplanet Day Measured for First Time]. Eso.org. 30 April 2014</ref><ref name="snellenspin">{{Cite journal | doi = 10.1038/nature13253| arxiv=1404.7506| pmid = 24784216| title = Fast spin of the young extrasolar planet β Pictoris b| journal = Nature| volume = 509| issue = 7498| pages = 63–65| date = 2014| last1 = Snellen | first1 = I. A. G. | last2 = Brandl | first2 = B. R. | last3 = De Kok | first3 = R. J. | last4 = Brogi | first4 = M. | last5 = Birkby | first5 = J. | last6 = Schwarz | first6 = H. |bibcode=2014Natur.509...63S| s2cid=119248149}}</ref><ref name="discoveryspin">Klotz, Irene (30 April 2014) [http://news.discovery.com/space/alien-life-exoplanets/newly-clocked-exoplanet-spins-a-whole-day-in-eight-hours-140430.htm Newly Clocked Exoplanet Spins a Whole Day in 8 Hours]. Discovery.com.</ref> With an equatorial rotational velocity of 25&nbsp;km per second, this is faster than for the giant planets of the Solar System, in line with the expectation that the more massive a giant planet, the faster it spins. Beta Pictoris b's distance from its star is 9 AU. At such distances the rotation of Jovian planets is not slowed by tidal effects.<ref name="TidalEvoExop">{{cite book |arxiv=1009.1352 |isbn=978-0-8165-2945-2| last1= Correia| first1=Alexandre C. M. |title=Tidal Evolution of Exoplanets| last2=Laskar|first2=Jacques| year=2010|bibcode=2010exop.book..239C}}</ref> Beta Pictoris b is still warm and young and over the next hundreds of millions of years, it will cool down and shrink to about the size of Jupiter, and if its [[Physics:Angular momentum|angular momentum]] is preserved, then as it shrinks, the length of its day will decrease to about 3 hours and its equatorial rotation velocity will speed up to about 40&nbsp;km/s.<ref name="snellenspin" /> The images of Beta Pictoris b do not have high enough resolution to directly see details but [[Astronomy:Doppler spectroscopy|doppler spectroscopy]] techniques were used to show that different parts of the planet were moving at different speeds and in opposite directions from which it was inferred that the planet is rotating.<ref name="spinpress" /> With the next generation of [[Astronomy:Extremely large telescope|large ground-based telescopes]] it will be possible to use doppler imaging techniques to make a global map of the planet, like the mapping of the brown dwarf [[Astronomy:Luhman 16#Atmosphere|Luhman 16B]] in 2014.<ref>Cowen, Ron (30 April 2014)  [http://www.scientificamerican.com/article/exoplanets-rotation-detected-for-the-first-time/ Exoplanet Rotation Detected for the First Time]. Scientific American</ref><ref>{{Cite journal | doi = 10.1051/0004-6361/201423750|arxiv=1404.7853| title = Doppler imaging of exoplanets and brown dwarfs| journal = Astronomy & Astrophysics| volume = 566| pages = A130| year = 2014| last1 = Crossfield | first1 = I. J. M. |bibcode=2014A&A...566A.130C|s2cid=119164450}}</ref>
A 2017 study of the rotation of several gas giants found no correlation between rotation rate and mass of the planet.<ref>[https://arxiv.org/abs/1712.00457 Constraints on the Spin Evolution of Young Planetary-Mass Companions], Marta L. Bryan, Bjorn Benneke, Heather A. Knutson, Konstantin Batygin, Brendan P. Bowler, 1 Dec 2017</ref>

=== Origin of spin and tilt of terrestrial planets ===
[[Astronomy:Impact event|Giant impacts]] have a large effect on the spin of terrestrial planets. The last few giant impacts during [[Astronomy:Nebular hypothesis#Formation of planets|planetary formation]] tend to be the main determiner of a terrestrial planet's rotation rate. On average the spin [[Physics:Angular velocity|angular velocity]] will be about 70% of the velocity that would cause the planet to break up and fly apart; the natural outcome of planetary embryo impacts at speeds slightly larger than [[Astronomy:Escape velocity|escape velocity]]. In later stages terrestrial planet spin is also affected by impacts with [[Astronomy:Planetesimal|planetesimal]]s. During the giant impact stage, the thickness of a [[Astronomy:Protoplanetary disk|protoplanetary disk]] is far larger than the size of planetary embryos so collisions are equally likely to come from any direction in three-dimensions. This results in the [[Astronomy:Axial tilt|axial tilt]] of accreted planets ranging from 0 to 180 degrees with any direction as likely as any other with both [[Astronomy:Retrograde and prograde motion|prograde and retrograde]] spins equally probable. Therefore, prograde spin with a small axial tilt, common for the Solar System's terrestrial planets except Venus, is not common in general for terrestrial planets built by giant impacts. The initial axial tilt of a planet determined by giant impacts can be substantially changed by stellar tides if the planet is close to its star and by satellite tides if the planet has a large satellite.<ref>{{Cite book |arxiv=1312.1689|bibcode = 2014prpl.conf..595R |doi = 10.2458/azu_uapress_9780816531240-ch026 | title=Terrestrial Planet Formation at Home and Abroad |journal=Protostars and Planets VI|page = 595 |year = 2014 |last1 = Raymond |first1 = S. N. |last2 = Kokubo |first2 = E. |last3 = Morbidelli |first3 = A |last4 = Morishima |first4 = R. |last5 = Walsh |first5 = K. J. |isbn = 978-0-8165-3124-0 |s2cid = 55207905 }}</ref>

=== Tidal effects ===
For most planets, the rotation period and [[Astronomy:Axial tilt|axial tilt]] (also called obliquity) are not known, but a large number of planets have been detected with very short orbits (where tidal effects are greater) that will probably have reached an [[Physics:Mechanical equilibrium|equilibrium]] rotation that can be predicted (''i.e.'' tidal lock, [[Astronomy:Mercury (planet)#Spin-orbit resonance|spin–orbit resonances]], and [[Astronomy:Retrograde and prograde motion#Planets|non-resonant equilibria such as retrograde rotation]]).<ref name="TidalEvoExop"/>

Gravitational tides tend to reduce the axial tilt to zero but over a longer timescale than the rotation rate reaches equilibrium. However, the presence of multiple planets in a system can cause axial tilt to be captured in a resonance called a Cassini state. There are small oscillations around this state and in the case of [[Astronomy:Mars|Mars]] these axial tilt variations are chaotic.<ref name="TidalEvoExop"/>

[[Astronomy:Hot Jupiter|Hot Jupiter]]s' close proximity to their host star means that their spin–orbit evolution is mostly due to the star's gravity and not the other effects. Hot Jupiters' rotation rate is not thought to be captured into spin–orbit resonance because of the way in which such a fluid-body reacts to tides; a planet like this therefore slows down into synchronous rotation if its orbit is circular, or, alternatively, it slows down into a non-synchronous rotation if its orbit is eccentric. Hot Jupiters are likely to evolve towards zero axial tilt even if they had been in a Cassini state during planetary migration when they were further from their star. Hot Jupiters' orbits will become more circular over time, however the presence of other planets in the system on eccentric orbits, even ones as small as Earth and as far away as the habitable zone, can continue to maintain the eccentricity of the Hot Jupiter so that the length of time for [[Tidal circularization|tidal circularization]] can be billions instead of millions of years.<ref name="TidalEvoExop"/>

The rotation rate of planet [[Astronomy:HD 80606 b|HD 80606 b]] is predicted to be about 1.9 days.<ref name="TidalEvoExop"/> HD 80606 b avoids spin–orbit resonance because it is a gas giant. The eccentricity of its orbit means that it avoids becoming tidally locked.

== Physical parameters ==

=== Mass ===
When a planet is found by the radial-velocity method, its orbital inclination ''i'' is unknown and can range from 0 to 90 degrees. The method is unable to determine the [[Astronomy:True mass|true mass]] (''M'') of the planet, but rather gives a [[Astronomy:Minimum mass|lower limit for its mass]], ''M''&nbsp;sin''i''. In a few cases an apparent exoplanet may be a more massive object such as a brown dwarf or red dwarf. However, the probability of a small value of i (say less than 30 degrees, which would give a true mass at least double the observed lower limit) is relatively low (1−{{radic|3}}/2 ≈ 13%) and hence most planets will have true masses fairly close to the observed lower limit.<ref name="cumming08" />

If a planet's orbit is nearly perpendicular to the line of vision (i.e. ''i'' close to 90°), a planet can be detected through the transit method. The inclination will then be known, and the inclination combined with ''M''&nbsp;sin''i'' from radial-velocity observations will give the planet's true mass.

Also, [[Astronomy:Astrometry|astrometric]] observations and dynamical considerations in multiple-planet systems can sometimes provide an upper limit to the planet's true mass.

In 2013 it was proposed that the mass of a transiting exoplanet can also be determined from the transmission spectrum of its atmosphere, as it can be used to constrain independently the atmospheric composition, temperature, pressure, and [[Scale height|scale height]],<ref name="JulienSeager2013">{{cite journal| last=de Wit| first=Julien|author2=Seager, S.| title=Constraining Exoplanet Mass from Transmission Spectroscopy| journal=Science| date=19 December 2013| volume=342| issue=6165|pages=1473–1477| arxiv=1401.6181| doi=10.1126/science.1245450|pmid=24357312| bibcode = 2013Sci...342.1473D | s2cid=206552152}}</ref> however a 2017 study found that the transmission spectrum cannot unambiguously determine the mass.<ref>[https://iopscience.iop.org/article/10.3847/2041-8213/aa5c7d/pdf Challenges to Constraining Exoplanet Masses via Transmission Spectroscopy], Natasha E. Batalha1, Eliza M.-R. Kempton, Rostom Mbarek, 2017</ref>

[[Astronomy:Transit-timing variation|Transit-timing variation]] can also be used to find a planet's mass.<ref>{{Cite journal | doi = 10.1086/592230| title = Mass and Orbit Determination from Transit Timing Variations of Exoplanets| journal = The Astrophysical Journal| volume = 688| issue = 1| pages = 636–646| year = 2008| last1 = Nesvorný | first1 = D. | last2 = Morbidelli | first2 = A. | bibcode=2008ApJ...688..636N| doi-access = free}}</ref>

=== Radius, density, and bulk composition ===
Prior to recent results from the ''Kepler'' space observatory, most confirmed planets were [[Astronomy:Gas giant|gas giant]]s comparable in size to Jupiter or larger because they are most easily detected. However, the planets detected by ''Kepler'' are mostly between the size of Neptune and the size of Earth.<ref name="kepler1700" />

If a planet is detectable by both the radial-velocity and the transit methods, then both its true mass and its radius can be determined, as well as its [[Physics:Density|density]]. Planets with low density are inferred to be composed mainly of [[Chemistry:Hydrogen|hydrogen]] and [[Chemistry:Helium|helium]], whereas planets of intermediate density are inferred to have water as a major constituent. A planet of high density is inferred to be rocky, like Earth and the other terrestrial planets of the Solar System.

{{multiple image
| align     = center
| direction = horizontal
| image1    = Size of Kepler Planet Candidates.jpg
| width1    = 320
| alt1      = Histogram showing the radius-comparison of ''B4D
'' exoplanet candidates to radii of Earth, a super-Earth, Neptune, Jupiter, and a super-Jupiter. Neptune and super-Jupiter are the most and least populated size-ranges, respectively.
| caption1  = Sizes of ''Kepler'' Planet Candidates – based on 2,740 candidates orbiting 2,036 stars {{As of|2013|11|04|lc=on}} (NASA).

| image2    = Planet sizes.svg
| width2    = 342
| alt2      = Size-comparison of planets with different compositions to a Sun-like star, and to Earth
| caption2  = Comparison of sizes of planets with [http://www.nasa.gov/centers/goddard/news/topstory/2007/earthsized_planets.html different compositions.]
}}

==== Gas giants, puffy planets, and super-Jupiters ====
[[File:Exoplanet Comparison WASP-17 b.png|300px|thumb|alt=Size comparison of Jupiter and exoplanet WASP-17b|Size comparison of [[Astronomy:WASP-17b|WASP-17b]] (right) with Jupiter (left).]]
Gaseous planets that are hot are caused by extreme proximity to their host star, or because they are still hot from their formation and are expanded by the heat. For colder gas planets, there is a maximum radius which is slightly larger than Jupiter which occurs when the mass reaches a few Jupiter-masses. Adding mass beyond this point causes the radius to shrink.<ref name="whatbasribrown">{{cite journal |doi=10.1146/annurev.earth.34.031405.125058 |journal=Annu. Rev. Earth Planet. Sci. |volume=34 |title=Planetesimals To Brown Dwarfs: What is a Planet? |pages=193–216 |date=2006 |arxiv=astro-ph/0608417 |bibcode=2006AREPS..34..193B|last1=Basri |first1=Gibor |last2=Brown |first2=Michael E. |s2cid=119338327 |url=https://authors.library.caltech.edu/5028/1/BASareps06.pdf |type=Submitted manuscript }}</ref><ref>Seager, S. and Lissauer, J. J. (2010) "Introduction to Exoplanets", pp. 3–13 in ''Exoplanets'', Sara Seager (ed.), University of Arizona Press. {{ISBN|0-8165-2945-0}}</ref><ref>Lissauer, J. J. and de Pater, I. (2013) ''Fundamental Planetary Science: Physics, Chemistry and Habitability''. Cambridge University Press. {{ISBN|0-521-61855-X}}. p. 74</ref>

Even when taking heat from the star into account, many transiting exoplanets are much larger than expected given their mass, meaning that they have surprisingly low density.<ref name="baraffe10">{{Cite journal | doi = 10.1088/0034-4885/73/1/016901| title = The physical properties of extra-solar planets| journal = Reports on Progress in Physics| volume = 73| issue = 1| page = 016901| year = 2010| last1 = Baraffe | first1 = I. | last2 = Chabrier | first2 = G. | last3 = Barman | first3 = T. |arxiv = 1001.3577 |bibcode = 2010RPPh...73a6901B | citeseerx = 10.1.1.754.8799| s2cid = 118698516}}</ref>
See the [[Astronomy:Exoplanet#Magnetic field|magnetic field section]] for one possible explanation.
[[File:Exoplanet density vs radius and diffusity vs radius.jpg|thumb|alt=Two plots of exoplanet density vs. radius (in Jupiter radii). One shows density in g/cm<sup>3</sup>. The other shows diffusivity, or 1/density, or cm<sup>3</sup>/g.|Plots of exoplanet density and [[Radius|radius]].<ref group=lower-alpha name=density-radius-diagrams>Data from [http://exoplanetarchive.ipac.caltech.edu NASA catalog July 2014], excluding objects described as having unphysically high density</ref> Top: Density vs. Radius. Bottom: Diffusity=1/Density vs. Radius. Units: Radius in Jupiter radii ({{Jupiter radius|jup=yes}}). Density in g/cm<sup>3</sup>. Diffusity in cm<sup>3</sup>/g. These plots show that there are a wide range of densities for planets between Earth and Neptune size, then the planets of {{Jupiter radius|jup=yes|0.6}} size are very low-density and there are very few of them, then the gas giants have a large range of densities.]]
Besides the inflated [[Astronomy:Hot Jupiter|hot Jupiter]]s, there is another type of low-density planet: [[Astronomy:Super-puff|super-puff]]s with masses only a few times Earth's but with radii larger than Neptune. The planets around [[Astronomy:Kepler-51|Kepler-51]]<ref name="kepler-51">{{Cite journal | doi = 10.1088/0004-637X/783/1/53| title = Very Low Density Planets Around Kepler-51 Revealed with Transit Timing Variations and an Anomaly Similar to a Planet-Planet Eclipse Event| journal = The Astrophysical Journal| volume = 783| issue = 1| page = 53| year = 2014| last1 = Masuda | first1 = K. | bibcode = 2014ApJ...783...53M|arxiv = 1401.2885 | s2cid = 119106865}}</ref> are far less dense (far more diffuse) than the inflated hot Jupiters as can be seen in the plots on the right where the three Kepler-51 planets stand out in the diffusity vs. radius plot.

==== Ice giants and super-Neptunes ====
{{Main|Astronomy:Ice giant}}

Kepler-101b was the first [[Astronomy:Super-Neptune|super-Neptune]] discovered. It has three times Neptune's mass but its density suggests that heavy elements make up more than 60% of its total mass, unlike hydrogen–helium-dominated gas giants.<ref>{{Cite journal | doi = 10.1051/0004-6361/201424617| title = Characterization of the planetary system Kepler-101 with HARPS-N| journal = Astronomy & Astrophysics| volume = 572| pages = A2| year = 2014| last1 = Bonomo | first1 = A. S.| last2 = Sozzetti | first2 = A.| last3 = Lovis | first3 = C.| last4 = Malavolta | first4 = L.| last5 = Rice | first5 = K.| last6 = Buchhave | first6 = L. A.| last7 = Sasselov | first7 = D.| last8 = Cameron | first8 = A. C.| last9 = Latham | first9 = D. W.| last10 = Molinari | first10 = E.| last11 = Pepe | first11 = F.| last12 = Udry | first12 = S.| last13 = Affer | first13 = L.| last14 = Charbonneau | first14 = D.| last15 = Cosentino | first15 = R.| last16 = Dressing | first16 = C. D.| last17 = Dumusque | first17 = X.| last18 = Figueira | first18 = P.| last19 = Fiorenzano | first19 = A. F. M.| last20 = Gettel | first20 = S.| last21 = Harutyunyan | first21 = A.| last22 = Haywood | first22 = R. D.| last23 = Horne | first23 = K.| last24 = Lopez-Morales | first24 = M.| last25 = Mayor | first25 = M.| last26 = Micela | first26 = G.| last27 = Motalebi | first27 = F.| last28 = Nascimbeni | first28 = V.| last29 = Phillips | first29 = D. F.| last30 = Piotto | first30 = G.| display-authors = etal | bibcode=2014A&A...572A...2B|arxiv = 1409.4592 | s2cid = 204937746}}</ref>

==== Super-Earths, mini-Neptunes, and gas dwarfs ====
{{Main|Astronomy:Super-Earth|Astronomy:Mini-Neptune|Astronomy:Helium planet|Astronomy:Gas dwarf}}
If a planet has a radius and/or mass between that of Earth and Neptune, then there is a question about whether the planet is rocky like Earth, a mixture of volatiles and gas like Neptune, a small planet with a hydrogen/helium envelope (mini-Jupiter), or of some other composition.

Some of the Kepler transiting planets with radii in the range of 1–4 Earth radii have had their masses measured by radial-velocity or transit-timing methods. The calculated densities show that up to 1.5 Earth radii, these planets are rocky and that density increases with increasing radius due to gravitational compression. However, between 1.5 and 4 Earth radii the density decreases with increasing radius. This indicates that above 1.5 Earth radii, planets tend to have increasing amounts of volatiles and gas. Despite this general trend, there is a wide range of masses at a given radius, which could be because gas planets can have rocky cores of different masses and compositions,<ref>{{Cite journal | doi = 10.1088/2041-8205/783/1/L6| title = The Mass-Radius Relation for 65 Exoplanets Smaller Than 4 Earth Radii| journal = The Astrophysical Journal| volume = 783| issue = 1| pages = L6| year = 2014| last1 = Weiss | first1 = L. M. | last2 = Marcy | first2 = G. W. |arxiv = 1312.0936 |bibcode = 2014ApJ...783L...6W | s2cid = 31561392}}</ref> and could also be due to [[Astronomy:Photoevaporation|photoevaporation]] of volatiles.<ref>{{Cite journal | doi = 10.1073/pnas.1304197111|arxiv=1404.2960| title = Occurrence and core-envelope structure of 1–4× Earth-size planets around Sun-like stars| journal = Proceedings of the National Academy of Sciences| volume = 111| issue = 35| pages = 12655–12660| year = 2014| last1 = Marcy | first1 = G. W.| last2 = Weiss | first2 = L. M.| last3 = Petigura | first3 = E. A.| last4 = Isaacson | first4 = H.| last5 = Howard | first5 = A. W.| last6 = Buchhave | first6 = L. A.|bibcode=2014PNAS..11112655M | pmid=24912169 | pmc=4156743|doi-access=free}}</ref>
Thermal evolutionary atmosphere models suggest a radius of 1.75 times that of Earth as a dividing line between rocky and gaseous planets.<ref>{{Cite journal | doi = 10.1088/0004-637X/792/1/1| title = Understanding the Mass-Radius Relation for Sub-Neptunes: Radius As a Proxy for Composition| journal = The Astrophysical Journal| volume = 792| issue = 1| page = 1| year = 2014| last1 = Lopez | first1 = E. D. | last2 = Fortney | first2 = J. J. | bibcode=2014ApJ...792....1L|arxiv = 1311.0329 | s2cid = 118516362}}</ref>
Excluding close-in planets that have lost their gas envelope due to stellar irradiation, studies of the [[Astronomy:Metallicity|metallicity]] of stars suggest a dividing line of 1.7 Earth radii between rocky planets and gas dwarfs, then another dividing line at 3.9 Earth radii between gas dwarfs and gas giants. These dividing lines are statistical trends and do not apply universally, because there are many other factors besides metallicity that affect planet formation, including distance from star – there may be larger rocky planets that formed at larger distances.<ref>{{Cite journal | doi = 10.1038/nature13254| pmid = 24870544| title = Three regimes of extrasolar planet radius inferred from host star metallicities| journal = Nature| volume = 509| issue = 7502| pages = 593–595| year = 2014| last1 = Buchhave | first1 = L. A.| last2 = Bizzarro | first2 = M.| last3 = Latham | first3 = D. W.| last4 = Sasselov | first4 = D.| last5 = Cochran | first5 = W. D.| last6 = Endl | first6 = M.| last7 = Isaacson | first7 = H.| last8 = Juncher | first8 = D.| last9 = Marcy | first9 = G. W.|arxiv = 1405.7695 |bibcode = 2014Natur.509..593B | pmc=4048851}}</ref>
An independent reanalysis of the data suggests that there are no such dividing lines and that there is a continuum of planet formation between 1 and 4 Earth radii and no reason to suspect that the amount of solid material in a protoplanetary disk determines whether super-Earths or mini-Neptunes form.<ref>{{cite journal |arxiv=1501.05953|bibcode = 2015ApJ...799L..26S |doi = 10.1088/2041-8205/799/2/L26 | volume=799 |issue = 2 |title=A Continuum of Planet Formation between 1 and 4 Earth Radii |journal=The Astrophysical Journal |pages=L26|year = 2015 |last1 = Schlaufman |first1 = Kevin C. |s2cid = 53412036 }}</ref> Studies done in 2016 based on over 300 planets suggest that most objects over approximately two Earth masses collect significant hydrogen–helium envelopes, meaning rocky super-Earths may be rare.<ref>{{cite journal| arxiv=1603.08614| title=Probabilistic Forecasting of the Masses and Radii of Other Worlds| author=Jingjing Chen| author2=David M. Kipping| date=29 March 2016| doi=10.3847/1538-4357/834/1/17| volume=834| issue=1| journal=The Astrophysical Journal| page=17|bibcode = 2017ApJ...834...17C | s2cid=119114880}}</ref>

The discovery of the low-density Earth-mass planet [[Astronomy:Kepler-138|Kepler-138]]d shows that there is an overlapping range of ''masses'' in which both rocky planets and low-density planets occur.<ref>{{Cite journal| url=http://www.nature.com/news/earth-mass-exoplanet-is-no-earth-twin-1.14477| doi=10.1038/nature.2014.14477 |title=Earth-mass exoplanet is no Earth twin| journal=Nature News|date=6 January 2014|access-date=7 January 2014| last1=Cowen|first1=Ron| s2cid=124963676 }}</ref> A low-mass low-density planets could be an [[Astronomy:Ocean planet|ocean planet]] or [[Astronomy:Super-Earth|super-Earth]] with a remnant hydrogen atmosphere, or a hot planet with a steam atmosphere, or a mini-Neptune with a hydrogen–helium atmosphere.<ref>Cabrera, Juan; Grenfell, John Lee; Nettelmann, Nadine (2014)  [http://meetingorganizer.copernicus.org/EGU2014/session/15518 PS6.3. Observations and Modeling of Low Mass Low Density (LMLD) Exoplanets]. ''European Geosciences Union General Assembly 2014''</ref> Another possibility for a low-mass low-density planet is that it has a large atmosphere made up chiefly of [[Chemistry:Carbon monoxide|carbon monoxide]], [[Chemistry:Carbon dioxide|carbon dioxide]], [[Chemistry:Methane|methane]], or [[Chemistry:Nitrogen|nitrogen]].<ref>{{Cite journal| arxiv=1306.6325|last1=Benneke|first1=Bjorn |title=How to Distinguish between Cloudy Mini-Neptunes and Water/Volatile-Dominated Super-Earths| journal=The Astrophysical Journal |volume=778| issue=2| page=153| last2=Seager| first2=Sara| year=2013| doi=10.1088/0004-637X/778/2/153| bibcode = 2013ApJ...778..153B |s2cid=40723710}}</ref>

==== Massive solid planets {{Anchor|Massive solid planets}} ====
[[File:Exoplanet Comparison Kepler-10 c.png|300px|thumb|alt=Size comparison of Kepler-10c with Earth and Neptune|Size comparison of Kepler-10c with Earth and Neptune]]
In 2014, new measurements of [[Astronomy:Kepler-10c|Kepler-10c]] found it to be a Neptune-mass planet (17 Earth masses) with a density higher than Earth's, indicating that Kepler-10c is composed mostly of rock with possibly up to 20% high-pressure water ice but without a hydrogen-dominated envelope. Because this is well above the 10-Earth-mass upper limit that is commonly used for the term 'super-Earth', the term '''[[Astronomy:Mega-Earth|mega-Earth]]''' has been coined.<ref>{{cite video|author=Sasselov, Dimitar|date=2 June 2014|title=Exoplanets: From Exhilarating to Exasperating &mdash; Kepler-10c: The "Mega-Earth"|minutes=23|url=https://www.youtube.com/watch?v=t6x29ntnQb0}}<!-- http://aas.org/media-press/archived-aas-press-conference-webcasts --></ref><ref>{{cite web| url=https://www.cfa.harvard.edu/news/2014-14| title=Astronomers Find a New Type of Planet: The "Mega-Earth"| date=2 June 2014| author=Aguilar, D. A.| author2=Pulliam, C.|publisher=Harvard-Smithsonian Center for Astrophysics| website=www.cfa.harvard.edu}}</ref> A similarly massive and dense planet could be Kepler-131b, although its density is not as well measured as that of Kepler 10c. The next most massive known solid planets are half this mass: [[Astronomy:55 Cancri e|55 Cancri e]] and [[Astronomy:Kepler-20b|Kepler-20b]].<ref>{{Cite journal | doi = 10.1088/0004-637X/789/2/154| title = The Kepler-10 Planetary System Revisited by HARPS-N: A Hot Rocky World and a Solid Neptune-Mass Planet| journal = The Astrophysical Journal| volume = 789| issue = 2| page = 154| year = 2014| last1 = Dumusque | first1 = X. | last2 = Bonomo | first2 = A. S. | last3 = Haywood | first3 = R. L. D. | last4 = Malavolta | first4 = L. | last5 = Ségransan | first5 = D. | last6 = Buchhave | first6 = L. A. | last7 = Cameron | first7 = A. C. | last8 = Latham | first8 = D. W. | last9 = Molinari | first9 = E. | last10 = Pepe | first10 = F. | last11 = Udry | first11 = S. P. | last12 = Charbonneau | first12 = D. | last13 = Cosentino | first13 = R. | last14 = Dressing | first14 = C. D. | last15 = Figueira | first15 = P. | last16 = Fiorenzano | first16 = A. F. M. | last17 = Gettel | first17 = S. | last18 = Harutyunyan | first18 = A. | last19 = Horne | first19 = K. | last20 = Lopez-Morales | first20 = M. | last21 = Lovis | first21 = C. | last22 = Mayor | first22 = M. | last23 = Micela | first23 = G. | last24 = Motalebi | first24 = F. | last25 = Nascimbeni | first25 = V. | last26 = Phillips | first26 = D. F. | last27 = Piotto | first27 = G. | last28 = Pollacco | first28 = D. | last29 = Queloz | first29 = D. | last30 = Rice | first30 = K. | display-authors = etal | bibcode=2014ApJ...789..154D|arxiv = 1405.7881 | s2cid = 53475787}}</ref>

Gas planets can have large solid cores. The Saturn-mass planet [[Astronomy:HD 149026 b|HD 149026 b]] has only two-thirds of Saturn's radius, so it may have a rock–ice core of 60 Earth masses or more.<ref name="whatbasribrown" /> Corot-20b has 4.24 times Jupiter's mass but a radius of only 0.84 that of Jupiter; it may have a metal core of 800 Earth masses if the heavy elements are concentrated in the core, or a core of 300 Earth masses if the heavy elements are more distributed throughout the planet.<ref>{{Cite journal |arxiv=1510.01630|last1=Nayakshin|first1=Sergei| title=Tidal Downsizing Model. IV. Destructive feedback in planets|journal=Monthly Notices of the Royal Astronomical Society|volume=461|issue=3|pages=3194–3211|year=2015|doi=10.1093/mnras/stw1404}}</ref><ref>{{cite journal |arxiv=1109.3203|doi = 10.1051/0004-6361/201117681 |bibcode = 2012A&A...538A.145D | volume=538 |title=Transiting exoplanets from the CoRoT space mission |journal=Astronomy and Astrophysics |pages=A145|year = 2012 |last1 = Deleuil |first1 = M. |last2 = Bonomo |first2 = A. S. |last3 = Ferraz-Mello |first3 = S. |last4 = Erikson |first4 = A. |last5 = Bouchy |first5 = F. |last6 = Havel |first6 = M. |last7 = Aigrain |first7 = S. |last8 = Almenara |first8 = J.-M. |last9 = Alonso |first9 = R. |last10 = Auvergne |first10 = M. |last11 = Baglin |first11 = A. |last12 = Barge |first12 = P. |last13 = Bordé |first13 = P. |last14 = Bruntt |first14 = H. |last15 = Cabrera |first15 = J. |last16 = Carpano |first16 = S. |last17 = Cavarroc |first17 = C. |last18 = Csizmadia |first18 = Sz. |last19 = Damiani |first19 = C. |last20 = Deeg |first20 = H. J. |last21 = Dvorak |first21 = R. |last22 = Fridlund |first22 = M. |last23 = Hébrard |first23 = G. |last24 = Gandolfi |first24 = D. |last25 = Gillon |first25 = M. |last26 = Guenther |first26 = E. |last27 = Guillot |first27 = T. |last28 = Hatzes |first28 = A. |last29 = Jorda |first29 = L. |last30 = Léger |first30 = A. |s2cid = 118339296 |display-authors = etal }}</ref>

[[Astronomy:Transit-timing variation|Transit-timing variation]] measurements indicate that Kepler-52b, Kepler-52c and Kepler-57b have maximum masses between 30 and 100 times that of Earth, although the actual masses could be much lower. With radii about 2 Earth radii<ref>[https://arxiv.org/abs/1208.3499 Transit Timing Observations from Kepler: VII. Confirmation of 27 planets in 13 multiplanet systems via Transit Timing Variations and orbital stability], Jason H. Steffen et al, 16 Aug 2012</ref> in size, they might have densities larger than that of an [[Astronomy:Iron planet|iron planet]] of the same size. They orbit very close to their stars, so they could each be the remnant core ([[Astronomy:Chthonian planet|chthonian planet]]) of an evaporated [[Astronomy:Gas giant|gas giant]] or [[Astronomy:Brown dwarf|brown dwarf]]. If a remnant core is massive enough it could remain in such a state for billions of years despite having lost the atmospheric mass.<ref>Mocquet, A.; Grasset, O.; Sotin, C. (2013) [http://meetingorganizer.copernicus.org/EPSC2013/EPSC2013-986-1.pdf Super-dense remnants of gas giant exoplanets], EPSC Abstracts, Vol. 8, EPSC2013-986-1, European Planetary Science Congress 2013</ref><ref>{{cite journal | last1 = Mocquet | first1 = A. | last2 = Grasset | first2 = O. | last3 = Sotin | first3 = C. | year = 2014 | title = Very high-density planets: a possible remnant of gas giants |journal = Phil. Trans. R. Soc. A | volume = 372 | issue = 2014| doi = 10.1098/rsta.2013.0164 | pmid = 24664925 |bibcode = 2014RSPTA.37230164M | page=20130164| doi-access = free }}</ref>

Solid planets up to thousands of Earth masses may be able to form around massive stars ([[Astronomy:B-type main-sequence star|B-type]] and [[Astronomy:O-type main-sequence star|O-type]] stars; 5–120 solar masses), where the [[Astronomy:Protoplanetary disk|protoplanetary disk]] would contain enough heavy elements. Also, these stars have high UV radiation and [[Astronomy:Stellar wind|winds]] that could [[Astronomy:Photoevaporation|photoevaporate]] the gas in the disk, leaving just the heavy elements.<ref name="mass/radius sold">{{Cite journal | doi = 10.1086/521346| title = Mass‐Radius Relationships for Solid Exoplanets| journal = The Astrophysical Journal| volume = 669| issue = 2| pages = 1279–1297| year = 2007| last1 = Seager | first1 = S.| last2 = Kuchner | first2 = M.| last3 = Hier‐Majumder | first3 = C. A.| last4 = Militzer | first4 = B. | bibcode=2007ApJ...669.1279S|arxiv = 0707.2895 | s2cid = 8369390}}</ref>
For comparison, Neptune's mass equals 17 Earth masses, Jupiter has 318 Earth masses, and the 13-Jupiter-mass limit used in the IAU's working definition of an exoplanet equals approximately 4000 Earth masses.<ref name="mass/radius sold" />

Cold planets have a maximum radius because adding more mass at that point causes the planet to compress under the weight instead of increasing the radius. The maximum radius for solid planets is lower than the maximum radius for gas planets.<ref name="mass/radius sold"/>

=== Shape ===
When the size of a planet is described using its radius, this is approximating the shape by a sphere. However, the rotation of a planet causes it to be flattened at the poles; so the equatorial radius is larger than the polar radius, making it closer to an oblate spheroid. The oblateness of transiting exoplanets will affect the transit light curves. At the limits of current technology it has been possible to show that HD 189733b is less oblate than [[Astronomy:Saturn|Saturn]].<ref name="CarterWinn">{{Cite journal | doi = 10.1088/0004-637X/709/2/1219| title = Empirical Constraints on the Oblateness of an Exoplanet| journal = The Astrophysical Journal| volume = 709| issue = 2| pages = 1219–1229| year = 2010| last1 = Carter | first1 = J. A. | last2 = Winn | first2 = J. N. | bibcode=2010ApJ...709.1219C|arxiv = 0912.1594 | s2cid = 73583633}}</ref>
If the planet is close to its star, then gravitational tides will elongate the planet in the direction of the star, making the planet closer to a triaxial ellipsoid.<ref>{{Cite journal | doi = 10.1051/0004-6361/201015811| title = Distorted, nonspherical transiting planets: Impact on the transit depth and on the radius determination| journal = Astronomy & Astrophysics| volume = 528| pages = A41| year = 2011| last1 = Leconte | first1 = J.| last2 = Lai | first2 = D.| last3 = Chabrier | first3 = G. | bibcode=2011A&A...528A..41L|arxiv = 1101.2813 | s2cid = 56543291}}</ref> Because tidal deformation is along a line between the planet and the star, it is difficult to detect from transit photometry; it will have an effect on the transit light curves an order of magnitude less than that caused by rotational deformation even in cases where tidal deformation is larger than rotational deformation (as is the case for tidally locked [[Astronomy:Hot Jupiter|hot Jupiter]]s).<ref name="CarterWinn" /> Material rigidity of rocky planets and rocky cores of gas planets will cause further deviations from the aforementioned shapes.<ref name="CarterWinn" /> Thermal tides caused by unevenly irradiated surfaces are another factor.<ref>{{cite arXiv |eprint=0901.0735 |last1=Arras |first1=Phil |title=Thermal Tides in Short Period Exoplanets |last2=Socrates |first2=Aristotle |class=astro-ph.EP |year=2009 }}</ref>

== Atmosphere ==
[[File:PIA18410-TitanSunsetStudies-CassiniSpacecraft-20140527.jpg|thumb|alt=Artist's concept of the ''Cassini'' spacecraft in front of a sunset on Saturn's moon Titan|Sunset studies on [[Astronomy:Titan (moon)|Titan]] by ''Cassini'' help understand exoplanet atmospheres (artist's concept).]]
As of February 2014, more than fifty transiting and five directly imaged exoplanet atmospheres have been observed,<ref>{{cite book |last1=Madhusudhan|first1=Nikku|journal=Protostars and Planets Vi|pages=739|last2=Knutson|first2=Heather|last3=Fortney|first3=Jonathan|last4=Barman|first4=Travis|title=Protostars and Planets VI| year=2014|  doi=10.2458/azu_uapress_9780816531240-ch032|chapter=Exoplanetary Atmospheres|isbn=978-0-8165-3124-0|arxiv = 1402.1169 |bibcode = 2014prpl.conf..739M |s2cid=118337613}}</ref> resulting in detection of molecular spectral features; observation of day–night temperature gradients; and constraints on vertical atmospheric structure.<ref name="SeagerDeming2010">{{cite journal |arxiv=1005.4037 |last1=Seager |first1=S. |last2=Deming |first2=D. |title=Exoplanet Atmospheres |date=2010|doi = 10.1146/annurev-astro-081309-130837 |bibcode = 2010ARA&A..48..631S |volume=48 |journal=Annual Review of Astronomy and Astrophysics |pages=631–672|s2cid=119269678 }}</ref> Also, an atmosphere has been detected on the non-transiting hot Jupiter [[Astronomy:Tau Boötis b|Tau Boötis b]].<ref name="Rodler2012">{{cite journal | title=Weighing the Non-transiting Hot Jupiter τ Boo b | last1=Rodler | first1=F. | last2=Lopez-Morales | first2=M. | last3=Ribas | first3=I. | journal=The Astrophysical Journal Letters | volume=753 | issue=1 | pages=L25 | id=L25| date=July 2012 | arxiv=1206.6197 | bibcode=2012ApJ...753L..25R | doi=10.1088/2041-8205/753/1/L25 | s2cid=119177983 }}</ref><ref>{{Cite journal | doi = 10.1038/nature11161| pmid = 22739313| title = The signature of orbital motion from the dayside of the planet τ Boötis b| journal = Nature| volume = 486| issue = 7404| pages = 502–504| year = 2012| last1 = Brogi | first1 = M. | last2 = Snellen | first2 = I. A. G. | last3 = De Kok | first3 = R. J. | last4 = Albrecht | first4 = S. | last5 = Birkby | first5 = J. | last6 = De Mooij | first6 = E. J. W. |arxiv = 1206.6109 |bibcode = 2012Natur.486..502B | s2cid = 4368217}}</ref>

== Surface ==

=== Surface composition ===
Surface features can be distinguished from atmospheric features by comparing emission and reflection spectroscopy with transmission spectroscopy. Mid-infrared spectroscopy of exoplanets may detect rocky surfaces, and near-infrared may identify magma oceans or high-temperature lavas, hydrated silicate surfaces and water ice, giving an unambiguous method to distinguish between rocky and gaseous exoplanets.<ref>{{cite journal| arxiv=1204.1544|bibcode = 2012ApJ...752....7H |doi = 10.1088/0004-637X/752/1/7 | volume=752|issue = 1 | title=Theoretical Spectra of Terrestrial Exoplanet Surfaces| journal=The Astrophysical Journal| page=7|year = 2012 |last1 = Hu |first1 = Renyu |last2 = Ehlmann |first2 = Bethany L. |last3 = Seager |first3 = Sara |s2cid = 15219541 }}</ref>

=== Surface temperature ===
[[File:Artist’s illustration of temperature inversion in exoplanet’s atmosphere.jpg|thumb|alt=Artist's illustration of temperature inversion in an exoplanet's atmosphere, with and without a stratosphere|Artist's illustration of temperature inversion in exoplanet's atmosphere.<ref>{{cite web |title=NASA, ESA, and K. Haynes and A. Mandell (Goddard Space Flight Center)|url=http://www.spacetelescope.org/images/opo1525a/|access-date=15 June 2015}}</ref>]]
The temperature of an exoplanet can be estimated by measuring the intensity of the light it receives from its parent star. For example, the planet [[Astronomy:OGLE-2005-BLG-390Lb|OGLE-2005-BLG-390Lb]] is estimated to have a surface temperature of roughly −220&nbsp;°C (50 K). However, such estimates may be substantially in error because they depend on the planet's usually unknown [[Earth:Albedo|albedo]], and because factors such as the [[Earth:Greenhouse effect|greenhouse effect]] may introduce unknown complications. A few planets have had their temperature measured by observing the variation in infrared radiation as the planet moves around in its orbit and is eclipsed by its parent star. For example, the planet HD 189733b has been estimated to have an average temperature of 1,205 K (932&nbsp;°C) on its dayside and 973 K (700&nbsp;°C) on its nightside.<ref name="knutson_charbonneau07">{{Cite journal | doi = 10.1038/nature05782 | url=http://www.ucolick.org/~jfortney/papers/Knutson07.pdf|arxiv=0705.0993 | pmid=17495920 | bibcode = 2007Natur.447..183K  | title = A map of the day–night contrast of the extrasolar planet HD 189733b | journal = Nature | volume = 447 | issue = 7141 | pages = 183–6 | year = 2007 | last1 = Knutson | first1 = H. A.  | last2 = Charbonneau | first2 = D.  | last3 = Allen | first3 = L. E.  | last4 = Fortney | first4 = J. J.  | last5 = Agol | first5 = E.  | last6 = Cowan | first6 = N. B.  | last7 = Showman | first7 = A. P.  | last8 = Cooper | first8 = C. S.  | last9 = Megeath | first9 = S. T. | s2cid=4402268}}</ref>

==Habitability==
{{Main|Astronomy:Planetary habitability|Astronomy:Astrobiology|Astronomy:Drake equation}}

=== Habitable zone ===
{{main|Astronomy:Habitable zone}}
The habitable zone around a star is the region where the temperature is just right to allow liquid water to exist on the surface of planet; that is, not too close to the star for the water to evaporate and not too far away from the star for the water to freeze. The heat produced by stars varies depending on the size and age of the star, so that the habitable zone can be at different distances for different stars. Also, the atmospheric conditions on the planet influence the planet's ability to retain heat so that the location of the habitable zone is also specific to each type of planet: [[Astronomy:Desert planet|desert planet]]s (also known as dry planets), with very little water, will have less water vapor in the atmosphere than Earth and so have a reduced greenhouse effect, meaning that a desert planet could maintain oases of water closer to its star than Earth is to the Sun. The lack of water also means there is less ice to reflect heat into space, so the outer edge of desert-planet habitable zones is further out.<ref>Choi, Charles Q. (1 September 2011) [http://www.astrobio.net/exclusive/4188/alien-life-more-likely-on-%E2%80%98dune%E2%80%99-planets Alien Life More Likely on 'Dune' Planets] {{webarchive |url=https://web.archive.org/web/20131202223111/http://www.astrobio.net/exclusive/4188/alien-life-more-likely-on-%E2%80%98dune%E2%80%99-planets |date=2 December 2013 }}. ''Astrobiology Magazine''</ref><ref>{{Cite journal | last1 = Abe | first1 = Y. | last2 = Abe-Ouchi | first2 = A. | last3 = Sleep | first3 = N. H. | last4 = Zahnle | first4 = K. J. | title = Habitable Zone Limits for Dry Planets | doi = 10.1089/ast.2010.0545 | journal = Astrobiology | volume = 11 | issue = 5 | pages = 443–460 | year = 2011 | pmid =  21707386|bibcode = 2011AsBio..11..443A }}</ref> Rocky planets with a thick hydrogen atmosphere could maintain surface water much further out than the Earth–Sun distance.<ref>{{Cite journal | doi = 10.1126/science.1232226|pmid=23641111 | title = Exoplanet Habitability| journal = Science| volume = 340| issue = 6132| pages = 577–81| year = 2013| last1 = Seager | first1 = S.|bibcode=2013Sci...340..577S|citeseerx=10.1.1.402.2983 |s2cid=206546351 }}</ref> Planets with larger mass have wider habitable zones because the gravity reduces the water cloud column depth which reduces the greenhouse effect of water vapor, thus moving the inner edge of the habitable zone closer to the star.<ref>{{cite journal| arxiv=1404.5292| bibcode = 2014ApJ...787L..29K |doi = 10.1088/2041-8205/787/2/L29 | volume=787| issue = 2 |title=Habitable Zones around Main-sequence Stars: Dependence on Planetary Mass| journal=The Astrophysical Journal|pages=L29|year = 2014 |last1 = Kopparapu |first1 = Ravi Kumar |last2 = Ramirez |first2 = Ramses M. |last3 = Schottelkotte |first3 = James |last4 = Kasting |first4 = James F. |last5 = Domagal-Goldman |first5 = Shawn |last6 = Eymet |first6 = Vincent | s2cid = 118588898 }}</ref>

Planetary rotation rate is one of the major factors determining the circulation of the atmosphere and hence the pattern of clouds: slowly rotating planets create thick clouds that [[Earth:Albedo|reflect]] more and so can be habitable much closer to their star. Earth with its current atmosphere would be habitable in Venus's orbit, if it had Venus's slow rotation. If Venus lost its water ocean due to a [[Earth:Runaway greenhouse effect#Venus|runaway greenhouse effect]], it is likely to have had a higher rotation rate in the past. Alternatively, Venus never had an ocean because water vapor was lost to space during its formation <ref>{{Cite journal | doi = 10.1038/nature12163|pmid=23719462| title = Emergence of two types of terrestrial planet on solidification of magma ocean| journal = Nature| volume = 497| issue = 7451| pages = 607–10| year = 2013| last1 = Hamano | first1 = K. | last2 = Abe | first2 = Y. | last3 = Genda | first3 = H. |bibcode=2013Natur.497..607H|s2cid=4416458}}</ref> and could have had its slow rotation throughout its history.<ref>{{Cite journal | doi = 10.1088/2041-8205/787/1/L2 | arxiv = 1404.4992 | url = http://home.uchicago.edu/~junyang28/Papers/Yang-et-al-Rotation_Rate.pdf | title = Strong Dependence of the Inner Edge of the Habitable Zone on Planetary Rotation Rate | journal = The Astrophysical Journal | volume = 787 | issue = 1 | pages = L2 | year = 2014 | last1 = Yang | first1 = J. | last2 = Boué | first2 = G. L. | last3 = Fabrycky | first3 = D. C. | last4 = Abbot | first4 = D. S. | bibcode = 2014ApJ...787L...2Y | s2cid = 56145598 | access-date = 2016-07-28 | archive-url = https://web.archive.org/web/20160412161026/http://home.uchicago.edu/~junyang28/Papers/Yang-et-al-Rotation_Rate.pdf | archive-date = 2016-04-12 | url-status = dead }}</ref>

[[Astronomy:Tidal locking|Tidally locked planets]] (a.k.a. "eyeball" planets<ref>{{cite web| url=http://planetplanet.net/2014/10/07/real-life-sci-fi-world-2-the-hot-eyeball-planet/| title=Real-life Sci-Fi World #2: the Hot Eyeball planet|work=planetplanet| date=2014-10-07}}</ref>) can be habitable closer to their star than previously thought due to the effect of clouds: at high stellar flux, strong convection produces thick water clouds near the substellar point that greatly increase the planetary albedo and reduce surface temperatures.<ref>{{cite journal| arxiv=1307.0515|bibcode = 2013ApJ...771L..45Y |doi = 10.1088/2041-8205/771/2/L45 | volume=771|issue = 2 | journal=The Astrophysical Journal| pages=L45|year = 2013 |last1 = Yang |first1 = Jun |title = Stabilizing Cloud Feedback Dramatically Expands the Habitable Zone of Tidally Locked Planets |last2 = Cowan |first2 = Nicolas B. |last3 = Abbot |first3 = Dorian S. |s2cid = 14119086 }}</ref>

Habitable zones have usually been defined in terms of surface temperature, however over half of Earth's biomass is from subsurface microbes,<ref>{{Cite journal | doi = 10.1016/j.palaeo.2004.10.018| title = Expanding frontiers in deep subsurface microbiology| journal = Palaeogeography, Palaeoclimatology, Palaeoecology| volume = 219| issue = 1–2| pages = 131–155| year = 2005| last1 = Amend | first1 = J. P. | last2 = Teske | first2 = A. | bibcode = 2005PPP...219..131A}}</ref> and the temperature increases with depth, so the subsurface can be conducive for microbial life when the surface is frozen and if this is considered, the habitable zone extends much further from the star,<ref>[https://www.bbc.co.uk/news/uk-scotland-north-east-orkney-shetland-25639306 Further away planets 'can support life' say researchers], BBC, 7 January 2014.</ref> even [[Astronomy:Rogue planet|rogue planet]]s could have liquid water at sufficient depths underground.<ref>{{Cite journal | doi = 10.1088/2041-8205/735/2/L27| arxiv=1102.1108| url=https://www.researchgate.net/publication/48202561| title = The Steppenwolf: A Proposal for a Habitable Planet in Interstellar Space| journal = The Astrophysical Journal| volume = 735| issue = 2| pages = L27| year = 2011| last1 = Abbot | first1 = D. S.| last2 = Switzer | first2 = E. R.|bibcode=2011ApJ...735L..27A| s2cid=73631942}}</ref> In an earlier era of the [[Astronomy:Universe|universe]] the temperature of the [[Astronomy:Cosmic microwave background|cosmic microwave background]] would have allowed any rocky planets that existed to have liquid water on their surface regardless of their distance from a star.<ref>{{Cite journal | doi = 10.1017/S1473550414000196| title = The habitable epoch of the early Universe| journal = International Journal of Astrobiology| volume = 13| issue = 4| pages = 337–339| year = 2014| last1 = Loeb | first1 = A. |arxiv = 1312.0613 |bibcode = 2014IJAsB..13..337L | citeseerx = 10.1.1.748.4820| s2cid = 2777386}}</ref> Jupiter-like planets might not be habitable, but they could have [[Astronomy:Habitability of natural satellites|habitable moons]].<ref>[https://www.newscientist.com/article/mg22730320-300-home-sweet-exomoon-the-new-frontier-in-the-search-for-et/ Home, sweet exomoon: The new frontier in the search for ET], New Scientist, 29 July 2015</ref>

=== Ice ages and snowball states ===
The outer edge of the habitable zone is where planets are completely frozen, but  planets well inside the habitable zone can periodically become frozen. If orbital fluctuations or other causes produce cooling then this creates more ice, but ice reflects sunlight causing even more cooling, creating a feedback loop until the planet is completely or nearly completely frozen. When the surface is frozen, this stops carbon dioxide weathering, resulting in a build-up of carbon dioxide in the atmosphere from volcanic emissions. This creates a [[Earth:Greenhouse effect|greenhouse effect]] which thaws the planet again. Planets with a large [[Astronomy:Axial tilt|axial tilt]]<ref>{{cite journal| arxiv=1401.5323|bibcode = 2015P&SS..105...43L|title = Habitability of Earth-like planets with high obliquity and eccentric orbits: Results from a general circulation model |journal = Planetary and Space Science|last1 = Linsenmeier |first1 = Manuel |last2 = Pascale |first2 = Salvatore |last3 = Lucarini |first3 = Valerio |year = 2014 |doi=10.1016/j.pss.2014.11.003 |volume=105 |pages=43–59|s2cid = 119202437}}</ref> are less likely to enter snowball states and can retain liquid water further from their star. Large fluctuations of axial tilt can have even more of a warming effect than a fixed large tilt.<ref>Kelley, Peter (15 April 2014) [http://www.washington.edu/news/2014/04/15/astronomers-tilt-a-worlds-could-harbor-life/ Astronomers: 'Tilt-a-worlds' could harbor life]. www.washington.edu</ref><ref>{{Cite journal | doi = 10.1089/ast.2013.1129| pmid = 24611714| title = Effects of Extreme Obliquity Variations on the Habitability of Exoplanets| journal = Astrobiology| volume = 14| issue = 4| pages = 277–291| year = 2014| last1 = Armstrong | first1 = J. C. | last2 = Barnes | first2 = R.| last3 = Domagal-Goldman | first3 = S.| last4 = Breiner | first4 = J.| last5 = Quinn | first5 = T. R. | last6 = Meadows | first6 = V. S. | bibcode=2014AsBio..14..277A|arxiv = 1404.3686 | pmc=3995117}}</ref> Paradoxically, planets orbiting cooler stars, such as red dwarfs, are less likely to enter snowball states because the infrared radiation emitted by cooler stars is mostly at wavelengths that are absorbed by ice which heats it up.<ref>Kelley, Peter (18 July 2013) [http://www.washington.edu/news/2013/07/18/a-warmer-planetary-haven-around-cool-stars-as-ice-warms-rather-than-cools/ A warmer planetary haven around cool stars, as ice warms rather than cools]. www.washington.edu</ref><ref>{{Cite journal | doi = 10.1088/2041-8205/785/1/L9| title = Spectrum-Driven Planetary Deglaciation Due to Increases in Stellar Luminosity| journal = The Astrophysical Journal| volume = 785| issue = 1| pages = L9| year = 2014| last1 = Shields | first1 = A. L. | last2 = Bitz | first2 = C. M. | last3 = Meadows | first3 = V. S. | last4 = Joshi | first4 = M. M. | last5 = Robinson | first5 = T. D. |arxiv = 1403.3695 |bibcode = 2014ApJ...785L...9S | s2cid = 118544889}}</ref>

=== Tidal heating ===
If a planet has an eccentric orbit, then [[Astronomy:Tidal heating|tidal heating]] can provide another source of energy besides stellar radiation. This means that eccentric planets in the radiative habitable zone can be too hot for liquid water. Tides also [[Tidal circularization|circularize]] orbits over time so there could be planets in the habitable zone with circular orbits that have no water because they used to have eccentric orbits.<ref>{{Cite journal | doi = 10.1089/ast.2012.0851| pmid = 23537135| title = Tidal Venuses: Triggering a Climate Catastrophe via Tidal Heating| journal = Astrobiology| volume = 13| issue = 3| pages = 225–250| year = 2013| last1 = Barnes | first1 = R. | last2 = Mullins | first2 = K. | last3 = Goldblatt | first3 = C. | last4 = Meadows | first4 = V. S. | last5 = Kasting | first5 = J. F. | last6 = Heller | first6 = R. | bibcode=2013AsBio..13..225B|arxiv = 1203.5104 | pmc=3612283}}</ref> Eccentric planets further out than the habitable zone would still have frozen surfaces but the tidal heating could create a subsurface ocean similar to [[Astronomy:Europa (moon)|Europa]]'s.<ref name="Superhabitable">{{Cite journal | doi = 10.1089/ast.2013.1088| pmid = 24380533| title = Superhabitable Worlds| journal = Astrobiology| volume = 14| issue = 1| pages = 50–66| year = 2014| last1 = Heller | first1 = R. | last2 = Armstrong | first2 = J. | bibcode=2014AsBio..14...50H|arxiv = 1401.2392 | s2cid = 1824897}}</ref> In some planetary systems, such as in the [[Astronomy:Upsilon Andromedae|Upsilon Andromedae]] system, the eccentricity of orbits is maintained or even periodically varied by perturbations from other planets in the system. Tidal heating can cause outgassing from the mantle, contributing to the formation and replenishment of an atmosphere.<ref>{{Cite journal | doi = 10.1111/j.1365-2966.2008.13868.x| title = Tidal heating of terrestrial extrasolar planets and implications for their habitability| journal = Monthly Notices of the Royal Astronomical Society| volume = 391| issue = 1| pages = 237–245| year = 2008| last1 = Jackson | first1 = B. | last2 = Barnes | first2 = R. | last3 = Greenberg | first3 = R. | bibcode = 2008MNRAS.391..237J|arxiv = 0808.2770 | s2cid = 19930771}}</ref>

=== Potentially habitable planets ===

A review in 2015 identified exoplanets [[Astronomy:Kepler-62f|Kepler-62f]], [[Astronomy:Kepler-186f|Kepler-186f]] and [[Astronomy:Kepler-442b|Kepler-442b]] as the best candidates for being potentially habitable.<ref name=centauridreams>{{cite web| url=http://www.centauri-dreams.org/?p=32470| title=A Review of the Best Habitable Planet Candidates |author=Paul Gilster, Andrew LePage| date=2015-01-30| publisher=Centauri Dreams, Tau Zero Foundation| access-date=2015-07-24}}</ref> These are at a distance of 1200, 490 and 1,120 light-years away, respectively. Of these, Kepler-186f is in similar size to Earth with its 1.2-Earth-radius measure, and it is located towards the outer edge of the habitable zone around its [[Astronomy:Red dwarf|red dwarf]] star.

When looking at the nearest terrestrial exoplanet candidates, [[Astronomy:Proxima Centauri b|Proxima Centauri b]] is about 4.2 light-years away. Its equilibrium temperature is estimated to be {{convert|-39|C|K|abbr=}}.<ref>{{cite book| title=The Mystery of the Seven Spheres: How Homo sapiens will Conquer Space| author=Giovanni F. Bignami| publisher=Springer| year=2015| isbn=978-3-319-17004-6 | url = https://books.google.com/books?id=crvpCQAAQBAJ&pg=PA110| page = 110}}</ref>

==== Earth-size planets ====
* In November 2013 it was estimated that 22±8% of Sun-like<ref group="lower-alpha" name="footnoteA">For the purpose of this 1 in 5 statistic, "Sun-like" means G-type star. Data for Sun-like stars was not available so this statistic is an extrapolation from data about [[Astronomy:K-type star|K-type star]]s</ref> stars in the Milky Way galaxy may have an Earth-sized<ref group="lower-alpha" name="footnoteB">For the purpose of this 1 in 5 statistic, Earth-sized means 1–2 Earth radii</ref> planet in the habitable<ref group=lower-alpha name=footnoteC>For the purpose of this 1 in 5 statistic, "habitable zone" means the region with 0.25 to 4 times Earth's stellar flux (corresponding to 0.5–2 AU for the Sun).</ref> zone.<ref name="ucb1in5">{{cite web|last=Sanders |first=R.|date=4 November 2013| title=Astronomers answer key question: How common are habitable planets?|url=http://newscenter.berkeley.edu/2013/11/04/astronomers-answer-key-question-how-common-are-habitable-planets/
|work=newscenter.berkeley.edu}}</ref><ref name="earthsunhzprev">{{cite journal
|last1=Petigura |first1=E. A.|last2=Howard |first2=A. W.|last3=Marcy |first3=G. W.
|date=2013| title=Prevalence of Earth-size planets orbiting Sun-like stars| journal=Proceedings of the National Academy of Sciences| volume= 110|issue= 48| pages=19273–19278
|arxiv= 1311.6806| bibcode= 2013PNAS..11019273P| doi=10.1073/pnas.1319909110 | pmid=24191033 | pmc=3845182|doi-access=free}}</ref> Assuming 200 billion stars in the Milky Way,<ref group="lower-alpha" name="footnoteD">About 1/4 of stars are GK Sun-like stars. The number of stars in the galaxy is not accurately known, but assuming 200 billion stars in total, the [[Astronomy:Milky Way|Milky Way]] would have about 50 billion Sun-like (GK) stars, of which about 1 in 5 (22%) or 11 billion would be Earth-sized in the habitable zone. Including red dwarfs would increase this to 40 billion.</ref> that would be 11 billion potentially habitable Earths, rising to 40 billion if [[Astronomy:Red dwarf|red dwarf]]s are included.<ref name="LATimes-20131104">{{cite news |last=Khan |first=Amina |title=Milky Way may host billions of Earth-size planets |url=http://www.latimes.com/science/la-sci-earth-like-planets-20131105,0,2673237.story |date=4 November 2013 |work={{wipe|Los Angeles Times}} |access-date=5 November 2013 }}</ref>
* [[Astronomy:Kepler-186f|Kepler-186f]], a 1.2-Earth-radius planet in the habitable zone of a [[Astronomy:Red dwarf|red dwarf]], reported in April 2014.
*Proxima Centauri b, a planet in the habitable zone of [[Astronomy:Proxima Centauri|Proxima Centauri]], the nearest known star to the solar system with an estimated minimum mass of 1.27 times the mass of the Earth.
* In February 2013, researchers speculated that up to 6% of small red dwarfs may have Earth-size planets. This suggests that the closest one to the Solar System could be 13 light-years away. The estimated distance increases to 21 light-years when a 95% [[Confidence interval|confidence interval]] is used.<ref name="howell-2013">{{cite news | url=http://www.space.com/19667-closest-alien-earth-exoplanets.html | title=Closest 'Alien Earth' May Be 13 Light-Years Away | work=Space.com | date=6 February 2013 | agency=TechMediaNetwork | access-date=7 February 2013 | author=Howell, Elizabeth}}</ref> In March 2013 a revised estimate gave an occurrence rate of 50% for Earth-size planets in the habitable zone of red dwarfs.<ref name="Habitable Exoplanet Re-estimate">{{cite journal| last=Kopparapu |first=Ravi Kumar |title=A revised estimate of the occurrence rate of terrestrial planets in the habitable zones around Kepler M-dwarfs |journal=The Astrophysical Journal Letters |date=March 2013 |arxiv=1303.2649 |bibcode=2013ApJ...767L...8K|volume=767|issue=1 |pages=L8| doi=10.1088/2041-8205/767/1/L8|s2cid=119103101 }}</ref>
* At 1.63 times Earth's radius [[Astronomy:Kepler-452b|Kepler-452b]] is the first discovered near-Earth-size planet in the [[Astronomy:Circumstellar habitable zone|"habitable zone"]] around a G2-type Sun-like star (July 2015).<ref>{{cite web| title = NASA's Kepler Mission Discovers Bigger, Older Cousin to Earth| url = http://www.nasa.gov/press-release/nasa-kepler-mission-discovers-bigger-older-cousin-to-earth |access-date = 2015-07-23| date = 2015-07-23 }}</ref>

==See also==
{{div col|colwidth=30em}}
* {{annotated link|Astronomy:Circumstellar habitable zone}}
* [[Astronomy:List of exoplanet search projects|List of exoplanet search projects]]
* [[Astronomy:Lists of exoplanets|Lists of exoplanets]]
* {{annotated link|Organization:Nexus for Exoplanet System Science}}
* {{annotated link|Astronomy:Planetary science}}
{{div col end}}

== Notes ==
{{Notelist}}

==References==
{{reflist}}

==External links==
* [http://archive.stsci.edu/kepler/ ''Kepler'' public data archive] by the [[Organization:Space Telescope Science Institute|Space Telescope Science Institute]]
* [http://www.mso.anu.edu.au/saga/ Strömgren Survey for Asteroseismology and Galactic Archaeology]

;[[Astronomy:Exoplanet|Exoplanet]] catalogs and databases
* [http://exoplanet.eu/ Extrasolar Planets Encyclopaedia] by the Paris Observatory
* [http://phl.upr.edu/projects/habitable-exoplanets-catalog The Habitable Exoplanets Catalog] by UPR Arecibo
* [http://planetquest.jpl.nasa.gov/newworldsatlas New Worlds Atlas] by the NASA/JPL PlanetQuest

{{Exoplanet}}
{{Exoplanet search projects}}
{{Astrobiology}}
{{Astronomy subfields}}

[[Category:Exoplanetology| ]]
[[Category:Astrobiology]]
[[Category:Planetary science]]

{{Sourceattribution|Exoplanetology|1}}