View source for Exoplanet

{{Short description|Planet outside of the Solar System}}
{{For|the album by The Contortionist|Exoplanet (album)}}
{{Use dmy dates|date=September 2019}}
[[File:Hr8799 orbit hd.gif|alt=Timelapse of exoplanets orbit motion|thumb|upright=1.5|Four exoplanets of the [[HR 8799]] system imaged by the [[W. M. Keck Observatory]] over the course of seven years. Motion is interpolated from annual observations.]]
[[File:A Moon-size Line Up.jpg|thumb|295x295px|Comparison of the size of exoplanets orbiting [[Kepler-37]] to Mercury, Mars and Earth]]
An '''exoplanet''' or '''extrasolar planet''' is a [[planet]] outside of the [[Solar System]]. The first confirmed detection of an exoplanet was in 1992 around a [[pulsar]], and the first detection around a [[main-sequence star]] was in 1995. A different planet, first detected in 1988, was confirmed in 2003. In 2016, it was recognized that the first possible evidence of an exoplanet had been noted in 1917.  {{Extrasolar planet counts|full|source=NEA}}<ref name="NASA-20220321">{{cite news |last=Brennan |first=Pat |title=Cosmic Milestone: NASA Confirms 5,000 Exoplanets |url=https://exoplanets.nasa.gov/news/1702/cosmic-milestone-nasa-confirms-5000-exoplanets/ |date=21 March 2022 |work=[[NASA]] |access-date=2 April 2022}}</ref>

There are many [[methods of detecting exoplanets]]. [[Astronomical transit|Transit photometry]] and [[Doppler spectroscopy]] have found the most, but these methods suffer from a clear observational bias favoring the detection of planets near the star; thus, 85% of the exoplanets detected are inside the [[tidal locking]] zone.<ref>{{cite journal |author=Ballesteros |first1=F. J. |last2=Fernandez-Soto |first2=A. |last3=Martinez |first3=V. J. |date=2019 |title=Title: Diving into Exoplanets: Are Water Seas the Most Common? |journal=[[Astrobiology]] |volume=19 |issue=5 |pages=642–654 |doi=10.1089/ast.2017.1720 |pmid=30789285 |s2cid=73498809 |hdl-access=free |hdl=10261/213115}}</ref> About 1 in 5 [[Solar analog|Sun-like stars]]<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 [[K-type star]]s.</ref> are estimated to 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 zone]].<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><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="earthsunhz">{{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 there are 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&nbsp;billion stars in total, the [[Milky Way]] would have about 50&nbsp;billion Sun-like (GK) stars, of which about 1 in 5 (22%) or 11&nbsp;billion would have Earth-sized planets in the habitable zone. Including red dwarfs would increase this to 40&nbsp;billion.</ref> it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous [[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=[[Los Angeles Times]] |access-date=5 November 2013}}</ref>

The [[List of exoplanet extremes#Planetary characteristics|least massive exoplanet]] known is [[PSR B1257+12 A|Draugr]], which is about twice the mass of the [[Moon]]. The [[List of exoplanet extremes#Planetary characteristics|most massive exoplanet]] listed on the [[NASA Exoplanet Archive]] is [[HR 2562 b]],<ref name="CT-Exo-2018">{{cite web |title=HR 2562 b |url=https://exoplanetarchive.ipac.caltech.edu/cgi-bin/DisplayOverview/nph-DisplayOverview?objname=HR+2562+b&type=CONFIRMED_PLANET |work=[[Caltech]] |access-date=15 February 2018 }}</ref><ref name="HR-2562">{{Cite journal |author=Konopacky, Quinn M. |author2=Rameau, Julien |author3=Duchêne, Gaspard |author4=Filippazzo, Joseph C. |author5=Giorla Godfrey, Paige A. |author6=Marois, Christian |author7=Nielsen, Eric L. |title=Discovery of a Substellar Companion to the Nearby Debris Disk Host HR 2562 |bibcode=2016ApJ...829L...4K |date=20 September 2016 |arxiv=1608.06660 |journal=The Astrophysical Journal Letters |doi=10.3847/2041-8205/829/1/L4 |volume=829 |issue=1 |page=10|url=http://dro.dur.ac.uk/20763/1/20763.pdf |hdl=10150/621980 |s2cid=44216698 |doi-access=free }}</ref><ref>{{cite journal |last1=Maire |first1=A. |last2=Rodet |first2=L. |last3=Lazzoni |first3=C. |last4=Boccaletti |first4=A. |last5=Brandner |first5=W. |last6=Galicher |first6=R. |last7=Cantalloube |first7=F. |last8=Mesa |first8=D. |last9=Klahr |first9=H. |last10=Beust |first10=H. |last11=Chauvin |first11=G. |last12=Desidera |first12=S. |last13=Janson |first13=M. |last14=Keppler |first14=M. |last15=Olofsson |first15=J. |last16=Augereau |first16=J. |last17=Daemgen |first17=S. |last18=Henning |first18=T. |last19=Thébault |first19=P. |last20=Bonnefoy |first20=M. |last21=Feldt |first21=M. |last22=Gratton |first22=R. |last23=Lagrange |first23=A. |last24=Langlois |first24=M. |last25=Meyer |first25=M. R. |last26=Vigan |first26=A. |last27=D'Orazi |first27=V. |last28=Hagelberg |first28=J. |last29=Le Coroller |first29=H. |last30=Ligi |first30=R. |last31=Rouan |first31=D. |last32=Samland |first32=M. |last33=Schmidt |first33=T. |last34=Udry |first34=S. |last35=Zurlo |first35=A. |last36=Abe |first36=L. |last37=Carle |first37=M. |last38=Delboulbé |first38=A. |last39=Feautrier |first39=P. |last40=Magnard |first40=Y. |last41=Maurel |first41=D. |last42=Moulin |first42=T. |last43=Pavlov |first43=A. |last44=Perret |first44=D. |last45=Petit |first45=C. |last46=Ramos |first46=J. R. |last47=Rigal |first47=F. |last48=Roux |first48=A. |last49=Weber |first49=L. |date=2018 |title=VLT/SPHERE astrometric confirmation and orbital analysis of the brown dwarf companion HR 2562 B |journal=Astronomy & Astrophysics |volume=615 |pages= A177| doi=10.1051/0004-6361/201732476|arxiv=1804.04584 |bibcode=2018A&A...615A.177M | doi-access=free}}</ref> about 30 times the mass of [[Jupiter]]. However, according to some definitions of a planet (based on the nuclear fusion of [[deuterium]]<ref name="bodenheimer2013" />), it is too massive to be a planet and might be a [[brown dwarf]]. Known orbital times for exoplanets vary from [[List of exoplanet extremes#Orbital characteristics|less than an hour]] (for those closest to their star) to thousands of years. Some exoplanets are so far away from the star that it is difficult to tell whether they are gravitationally bound to it.

The [[List of nearest exoplanets|nearest exoplanets]] are located 4.2 [[light-year]]s (1.3 [[parsec]]s) from Earth and orbit [[Proxima Centauri]], the closest star to the Sun.<ref name="proxima b discovery paper">{{cite journal| bibcode         = 2016Natur.536..437A| title = A terrestrial planet candidate in a temperate orbit around Proxima Centauri| journal = Nature| volume = 536| issue = 7617| pages = 437–440| last1 = Anglada-Escudé| first1 = Guillem| last2 = Amado| first2 = Pedro J.| last3 = Barnes| first3 = John| last4 = Berdiñas| first4 = Zaira M.| last5 = Butler| first5 = R. Paul| last6 = Coleman| first6 = Gavin A. L.| last7 = de la Cueva| first7 = Ignacio| last8 = Dreizler| first8 = Stefan| last9 = Endl| first9 = Michael| last10 = Giesers| first10 = Benjamin| last11 = Jeffers| first11 = Sandra V.| last12 = Jenkins| first12 = James S.| last13 = Jones| first13 = Hugh R. A.| last14 = Kiraga| first14 = Marcin| last15 = Kürster| first15 = Martin| last16 = López-González| first16 = María J.| last17 = Marvin| first17 = Christopher J.| last18 = Morales| first18 = Nicolás| last19 = Morin| first19 = Julien| last20 = Nelson| first20 = Richard P.| last21 = Ortiz| first21 = José L.| last22 = Ofir| first22 = Aviv| last23 = Paardekooper| first23 = Sijme-Jan| last24 = Reiners| first24 = Ansgar| last25 = Rodríguez| first25 = Eloy| last26 = Rodríguez-López| first26 = Cristina| last27 = Sarmiento| first27 = Luis F.| last28 = Strachan| first28 = John P.| last29 = Tsapras| first29 = Yiannis| last30 = Tuomi| first30 = Mikko| first31=Mathias|last31=Zechmeister| display-authors  = 3| year = 2016| arxiv = 1609.03449| doi = 10.1038/nature19106| pmid = 27558064| s2cid = 4451513| url=https://www.nature.com/articles/nature19106}}</ref> At the other extreme, there is evidence for [[extragalactic planet]]s{{snd}} exoplanets located in other galaxies.<ref name="NG-20180205">{{cite web |last=Zachos |first=Elaine |title=More Than a Trillion Planets Could Exist Beyond Our Galaxy |url=https://www.nationalgeographic.com/science/article/exoplanets-discovery-milky-way-galaxy-spd|archive-url=https://web.archive.org/web/20210428194238/https://www.nationalgeographic.com/science/article/exoplanets-discovery-milky-way-galaxy-spd|archive-date=28 April 2021|date=5 February 2018 |work=[[National Geographic]] |access-date=5 February 2018 |url-status=live }}</ref><ref name="GZM-20180205">{{cite web |last=Mandelbaum |first=Ryan F. |title=Scientists Find Evidence of Thousands of Planets in Distant Galaxy |url=https://gizmodo.com/scientists-find-evidence-of-thousands-of-planets-in-dis-1822727151 |date=5 February 2018 |work=[[Gizmodo]] |access-date=5 February 2018 }}</ref> 

The discovery of exoplanets has intensified interest in the search for [[extraterrestrial life]]. There is special interest in planets that orbit in a star's [[habitable zone]] (sometimes called "goldilocks zone"), where it is possible for liquid water, a prerequisite for [[life]] as we know it, to exist on the surface. However, the study of [[planetary habitability]] also considers a wide range of other factors in determining the suitability of a planet for hosting life.<ref name="NYT-20150106-DB">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=As Ranks of Goldilocks Planets Grow, Astronomers Consider What's Next |url=https://www.nytimes.com/2015/01/07/science/space/as-ranks-of-goldilocks-planets-grow-astronomers-consider-whats-next.html |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2015/01/07/science/space/as-ranks-of-goldilocks-planets-grow-astronomers-consider-whats-next.html |archive-date=2022-01-01 |url-access=limited |date=6 January 2015 |work=The New York Times}}{{cbignore}}</ref>
In collaboration with ground-based and other space-based observatories, the [[James Webb Space Telescope]] (JWST) is expected to give more insight into exoplanet traits, such as their [[Extraterrestrial atmosphere|composition]], [[Natural environment|environmental conditions]], and habitability.<ref name="SA-20230123">{{cite news |last=O'Callaghan |first=Jonathan |title=JWST Heralds a New Dawn for Exoplanet Science – The James Webb Space Telescope is opening an exciting new chapter in the study of exoplanets and the search for life beyond Earth |url=https://www.scientificamerican.com/article/jwst-heralds-a-new-dawn-for-exoplanet-science/ |date=23 January 2023 |work=[[Scientific American]] |access-date=23 January 2023 }}</ref>

[[Rogue planets]] are those that are not in planetary systems. Such objects are generally considered in a separate category from planets, especially if they are [[gas giant]]s, often counted as [[sub-brown dwarf]]s.<ref>{{cite journal|first1=C. |last1=Beichman|first2=Christopher R. |last2=Gelino|first3=J. Davy|last3=Kirkpatrick|first4=Michael C. |last4=Cushing|first5=Sally |last5=Dodson-Robinson|first6=Mark S.|last6=Marley|first7=Caroline V. |last7=Morley|first8=E. L. |last8=Wright|year=2014|title=WISE Y Dwarfs As Probes of the Brown Dwarf-Exoplanet Connection|journal=[[The Astrophysical Journal]]|volume=783 |issue=2 |page=68|arxiv=1401.1194 |bibcode=2014ApJ...783...68B|doi=10.1088/0004-637X/783/2/68|s2cid=119302072}}</ref> The rogue planets in the Milky Way possibly number in the billions or more.<ref>{{Cite web|date=2014-03-13|title=A Guide to Lonely Planets in the Galaxy|url=https://www.nationalgeographic.com/science/article/a-guide-to-lonely-planets-in-the-galaxy|archive-url=https://web.archive.org/web/20210518220530/https://www.nationalgeographic.com/science/article/a-guide-to-lonely-planets-in-the-galaxy|archive-date=18 May 2021|access-date=2022-01-17|website=National Geographic | last = Drake | first = Nadia | author-link = Nadia Drake | language=en}}</ref><ref>{{cite journal|last1=Strigari |first1=L. E.|last2=Barnabè |first2=M.|last3=Marshall |first3=P. J.|last4=Blandford|first4=R. D.|title=Nomads of the Galaxy|date=2012|volume=423 |issue=2 |pages=1856–1865|journal=[[Monthly Notices of the Royal Astronomical Society]]|arxiv=1201.2687|bibcode=2012MNRAS.423.1856S|doi=10.1111/j.1365-2966.2012.21009.x|doi-access=free |s2cid=119185094}} estimates 700 objects >10<sup>−6</sup> solar masses (roughly the mass of Mars) per main-sequence star between 0.08 and 1 Solar mass, of which there are billions in the Milky Way.</ref>

== Definition ==

=== IAU ===
The official [[Definition of planet|definition of the term ''planet'']] used by the [[International Astronomical Union]] (IAU) only covers the [[Solar System]] and thus does not apply to exoplanets.<ref>{{cite web
|title=IAU 2006 General Assembly: Result of the IAU Resolution votes
|date=2006
|url=https://www.iau.org/news/pressreleases/detail/iau0603/
|archive-url=https://web.archive.org/web/20241225044449/https://www.iau.org/news/pressreleases/detail/iau0603/
|access-date=31 March 2025
|archive-date=25 December 2024
}}</ref><ref>{{cite web
|author=Brit, R. R.
|date=2006
|title=Why Planets Will Never Be Defined
|url=http://www.space.com/3142-planets-defined.html
|work=[[Space.com]]
|access-date=13 February 2008
}}</ref> The IAU Working Group on Extrasolar Planets issued a position statement containing a working definition of "planet" in 2001 and which was modified in 2003.<ref>{{cite web
|date=28 February 2003
|title=Working Group on Extrasolar Planets: Definition of a "Planet"
|url=http://w.astro.berkeley.edu/~basri/defineplanet/IAU-WGExSP.htm
|work=IAU position statement
|archive-url=https://web.archive.org/web/20210419012736/http://w.astro.berkeley.edu/~basri/defineplanet/IAU-WGExSP.htm
|access-date=31 March 2025
|archive-date=19 April 2021
}}</ref> An ''exoplanet'' was defined by the following criteria:

{{blockquote|
* Objects with [[true mass]]es below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar [[metallicity]]) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in the Solar System.
* Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "[[brown dwarfs]]", no matter how they formed or where they are located.
* Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).
}}

This working definition was amended by the IAU's Commission F2: Exoplanets and the Solar System in August 2018.<ref>{{cite web
|title=Official Working Definition of an Exoplanet
|url=https://www.iau.org/science/scientific_bodies/commissions/F2/info/documents/
|work=IAU position statement
|archive-url=https://web.archive.org/web/20250212195344/https://www.iau.org/science/scientific_bodies/commissions/F2/info/documents/
|access-date=31 March 2025
|archive-date=12 February 2025
}}</ref><ref>{{Cite journal |last1=Lecavelier des Etangs |first1=A. |last2=Lissauer |first2=Jack J. |date=June 2022 |title=The IAU working definition of an exoplanet |url=https://linkinghub.elsevier.com/retrieve/pii/S138764732200001X |journal=New Astronomy Reviews |language=en |volume=94 |article-number=101641 |doi=10.1016/j.newar.2022.101641|arxiv=2203.09520 |bibcode=2022NewAR..9401641L |s2cid=247065421 }}</ref> The official working definition of an ''exoplanet'' is now as follows:

{{blockquote|
* Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars, brown dwarfs or stellar remnants and that have a mass ratio with the central object below the [[Lagrange point#Stability|L4/L5 instability]] (M/M<sub>central</sub> < 2/(25+{{math|{{radical|621}}}})) are "planets" (no matter how they formed).
* The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.
}}

=== Alternatives ===
The IAU's working definition is not always used. One alternate suggestion is that planets should be distinguished from [[brown dwarf]]s on the basis of their formation. It is widely thought that giant planets form through core [[Accretion (astrophysics)|accretion]], which may sometimes produce planets with masses above the deuterium fusion threshold;<ref>{{cite journal |arxiv=0710.5667 |title=Giant Planet Formation by Core Accretion |journal=Extreme Solar Systems |volume=398 |page=235 |bibcode=2008ASPC..398..235M |last1=Mordasini| first1=C. |last2=Alibert |first2=Yann |last3=Benz |first3=Willy |last4=Naef |first4=Dominique |year=2008 }}</ref><ref>{{Cite journal |arxiv=0802.1810 |title=Structure and evolution of super-Earth to super-Jupiter exoplanets. I. Heavy element enrichment in the interior |last1=Baraffe |first1=I. |date=2008 |journal=Astronomy and Astrophysics |volume=482 |issue=1 |pages=315–332 |doi=10.1051/0004-6361:20079321 |bibcode=2008A&A...482..315B |last2=Chabrier |first2=G. |last3=Barman |first3=T. |s2cid=16746688 }}</ref><ref name="bodenheimer2013">{{cite journal |title=Deuterium Burning in Massive Giant Planets and Low-mass Brown Dwarfs Formed by Core-nucleated Accretion |journal=The Astrophysical Journal |date=2013 |volume=770 |issue=2 |page=120 |doi=10.1088/0004-637X/770/2/120 |arxiv=1305.0980 |bibcode=2013ApJ...770..120B |last1=Bodenheimer |first1=Peter |last2=D'Angelo |first2=Gennaro |last3=Lissauer |first3=Jack J. |last4=Fortney |first4=Jonathan J. |last5=Saumon |first5=Didier |s2cid=118553341 }}</ref> massive planets of that sort may have already been observed.<ref>{{cite journal |doi=10.1051/0004-6361/200912427 |title=The SOPHIE northern extrasolar planets. I. A companion close to the planet/brown-dwarf transition around HD16760 |last1=Bouchy |first1=François |last2=Hébrard |first2=Guillaume |last3=Udry |first3=Stéphane |last4=Delfosse |first4=Xavier |last5=Boisse |first5=Isabelle |last6=Desort |first6=Morgan |last7=Bonfils |first7=Xavier |last8=Eggenberger |first8=Anne |last9=Ehrenreich |first9=David |last10=Forveille |first10=Thierry |last11=Le Coroller |first11=Hervé |last12=Lagrange |first12=Anne-Marie |last13=Lovis |first13=Christophe |last14=Moutou |first14=Claire |last15=Pepe |first15=Francesco |last16=Perrier |first16=Christian |last17=Pont |first17=Frédéric |last18=Queloz |first18=Didier |last19=Santos |first19=Nuno C. |last20=Ségransan |first20=Damien |last21=Vidal-Madjar |first21=Alfred |date=2009 |journal=Astronomy and Astrophysics |volume=505 |issue=2 |pages=853–858 |bibcode=2009A&A...505..853B |doi-access=free }}</ref> Brown dwarfs form like stars from the direct gravitational collapse of clouds of gas, and this formation mechanism also produces objects that are below the {{Jupiter mass|13|jup=y|link=y}} limit and can be as low as {{Jupiter mass|1|jup=y}}.<ref name=ShivKumar>{{cite journal| bibcode=2003IAUS..211..529B| title=Nomenclature: Brown Dwarfs, Gas Giant Planets, and ?| last1=Kumar| first1=Shiv S.|volume=211| date=2003| page=532| journal=Brown Dwarfs }}</ref> Objects in this mass range that orbit their stars with wide separations of hundreds or thousands of [[astronomical unit]]s (AU) and have large star/object mass ratios likely formed as brown dwarfs; their atmospheres would likely have a composition more similar to their host star than accretion-formed planets, which would contain increased abundances of heavier elements. Most directly imaged planets as of April 2014 are massive and have wide orbits so probably represent the low-mass end of a brown dwarf formation.<ref>{{Cite journal | doi = 10.1088/0004-637X/794/2/159| title = A Statistical Analysis of Seeds and Other High-Contrast Exoplanet Surveys: Massive Planets or Low-Mass Brown Dwarfs?| journal = The Astrophysical Journal| volume = 794| issue = 2| page = 159| year = 2014| last1 = Brandt | first1 = T. D. | last2 = McElwain | first2 = M. W. | last3 = Turner | first3 = E. L. | last4 = Mede | first4 = K. | last5 = Spiegel | first5 = D. S. | last6 = Kuzuhara | first6 = M. | last7 = Schlieder | first7 = J. E. | last8 = Wisniewski | first8 = J. P. | last9 = Abe | first9 = L.| last10 = Biller | first10 = B.| last11 = Brandner | first11 = W.| last12 = Carson | first12 = J.| last13 = Currie | first13 = T.| last14 = Egner | first14 = S.| last15 = Feldt | first15 = M.| last16 = Golota | first16 = T.| last17 = Goto | first17 = M.| last18 = Grady | first18 = C. A.| last19 = Guyon | first19 = O.| last20 = Hashimoto | first20 = J.| last21 = Hayano | first21 = Y.| last22 = Hayashi | first22 = M.| last23 = Hayashi | first23 = S.| last24 = Henning | first24 = T.| last25 = Hodapp | first25 = K. W.| last26 = Inutsuka | first26 = S.| last27 = Ishii | first27 = M.| last28 = Iye | first28 = M.| last29 = Janson | first29 = M.| last30 = Kandori | first30 = R.| display-authors = etal| bibcode = 2014ApJ...794..159B|arxiv = 1404.5335 | s2cid = 119304898}}</ref> One study suggests that objects above {{Jupiter mass|10|jup=y}} formed through gravitational instability and should not be thought of as planets.<ref>{{Cite journal|last=Schlaufman|first=Kevin C.|date=2018-01-22|title=Evidence of an Upper Bound on the Masses of Planets and its Implications for Giant Planet Formation|journal=The Astrophysical Journal|volume=853|issue=1|page=37|doi=10.3847/1538-4357/aa961c|arxiv=1801.06185|bibcode=2018ApJ...853...37S|s2cid=55995400|issn=1538-4357 |doi-access=free }}</ref>

Also, the 13-Jupiter-mass cutoff does not have a precise physical significance. Deuterium fusion can occur in some objects with a mass below that cutoff.<ref name="bodenheimer2013" /> The amount of deuterium fused depends to some extent on the composition of the object.<ref>{{Cite journal | doi = 10.1088/0004-637X/727/1/57| title = The Deuterium-Burning Mass Limit for Brown Dwarfs and Giant Planets| journal = The Astrophysical Journal| volume = 727| issue = 1| page = 57| year = 2011| last1 = Spiegel | first1 = D. S. |last2=Burrows |first2=Adam | last3 = Milsom | first3 = J. A. | bibcode = 2011ApJ...727...57S|arxiv = 1008.5150 | s2cid = 118513110}}</ref> In 2011, the [[Extrasolar Planets Encyclopaedia]] included objects up to 25 Jupiter masses, saying, "The fact that there is no special feature around {{Jupiter mass|13|jup=y}} in the observed mass spectrum reinforces the choice to forget this mass limit".<ref>{{cite journal|last1=Schneider |first1=J. |last2=Dedieu |first2=C. |last3=Le Sidaner |first3=P. |last4=Savalle |first4=R. |last5=Zolotukhin |first5=I. |title=Defining and cataloging exoplanets: The exoplanet.eu database| date=2011| volume=532| issue=79| journal=[[Astronomy & Astrophysics]] |arxiv=1106.0586| doi=10.1051/0004-6361/201116713|pages=A79 |bibcode=2011A&A...532A..79S|s2cid=55994657 }}</ref> As of 2016, this limit was increased to 60 Jupiter masses<ref>{{cite book|last=Schneider|first=Jean|title=Exoplanets versus brown dwarfs: the CoRoT view and the future|chapter=III.8 Exoplanets versus brown dwarfs: The CoRoT view and the future|year=2016|page=157|doi=10.1051/978-2-7598-1876-1.c038|arxiv=1604.00917|isbn=978-2-7598-1876-1|s2cid=118434022}}</ref> based on a study of mass–density relationships.<ref>{{cite journal |arxiv=1506.05097|last1= Hatzes Heike Rauer|first1= Artie P.|title= A Definition for Giant Planets Based on the Mass-Density Relationship|year= 2015|doi=10.1088/2041-8205/810/2/L25|volume=810|issue= 2|journal=The Astrophysical Journal|page=L25|bibcode = 2015ApJ...810L..25H |s2cid= 119111221}}</ref> The [[Exoplanet Data Explorer]] includes objects up to 24 Jupiter masses with the advisory: "The 13 Jupiter-mass distinction by the IAU Working Group is physically unmotivated for planets with rocky cores, and observationally problematic due to the sin i ambiguity."<ref name="eod">{{cite journal| arxiv=1012.5676 |title=The Exoplanet Orbit Database|date=2010| bibcode = 2011PASP..123..412W |doi = 10.1086/659427| volume=123| issue=902|journal=Publications of the Astronomical Society of the Pacific| pages=412–422| last1=Wright|first1=J. T.| last2=Fakhouri|first2=O.|last3=Marcy|first3=G. W.| last4=Han| first4=E.| last5=Feng| first5=Y.| last6=Johnson| first6=John Asher| last7=Howard| first7=A. W.|  last8=Fischer|first8=D. A.|last9=Valenti |first9=J. A.| last10=Anderson| first10=J.| last11=Piskunov|first11=N.|s2cid=51769219}}</ref> The [[NASA Exoplanet Archive]] includes objects with a mass (or minimum mass) equal to or less than 30 Jupiter masses.<ref>{{Cite web|title=Exoplanet Criteria for Inclusion in the Exoplanet Archive|url=https://exoplanetarchive.ipac.caltech.edu/docs/exoplanet_criteria.html|access-date=2022-01-17|website=exoplanetarchive.ipac.caltech.edu}}</ref> Another criterion for separating planets and brown dwarfs, rather than deuterium fusion, formation process or location, is whether the core [[pressure]] is dominated by [[Coulomb barrier|Coulomb pressure]] or [[electron degeneracy pressure]] with the dividing line at around 5 Jupiter masses.<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 name=JamesLiebert>{{cite journal|bibcode=2003IAUS..211..529B|title=Nomenclature: Brown Dwarfs, Gas Giant Planets, and ?|last1=Liebert|first1=James|volume=211|date=2003|page=533|journal=Brown Dwarfs }}</ref>

=== Confirmation ===
An exoplanet is confirmed for NASA's Exoplanet Archive either when "different observation techniques reveal features that can only be explained by a planet"<ref>{{cite web | url=https://www.nasa.gov/missions/kepler/new-deep-learning-method-adds-301-planets-to-keplers-total-count/ | title=New Deep Learning Method Adds 301 Planets to Kepler's Total Count - NASA | date=23 November 2021 }}</ref> or by analytical techniques.<ref name=NASA-20220321/> For the Extrasolar Planets Encyclopedia, "A planet is considered as Confirmed if it is claimed unambiguously in an accepted paper or a professional conference."<ref>{{cite web | url=https://exoplanet.eu/catalog/ | title=Catalogue of Exoplanets | date=1995 }}</ref>

== Nomenclature ==
[[File:The unusual exoplanet HIP 65426b — SPHERE's firs.jpg|thumb|Exoplanet [[HIP 65426b]] is the first discovered planet around star [[HIP 65426]].<ref>{{cite web|title=ESO's SPHERE Unveils its First Exoplanet|url=https://www.eso.org/public/announcements/ann17041/|website=www.eso.org|access-date=7 July 2017}}</ref>]]
<section begin=nomenclature />{{Main|Exoplanet naming convention}}
The convention for naming exoplanets is an extension of the system used for designating multiple-star systems as adopted by the [[International Astronomical Union]] (IAU). For exoplanets orbiting a single star, the IAU designation is formed by taking the designated or proper name of its parent star, and adding a lower case letter.<ref>{{Cite web|url=http://www.iau.org/public/themes/naming_exoplanets/|title=International Astronomical Union {{!}} IAU|website=www.iau.org|access-date=29 January 2017}}</ref> Letters are given in order of each planet's discovery around the parent star, so that the first planet discovered in a system is designated "b" (the parent star is considered "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 orbital size. A provisional IAU-sanctioned standard exists to accommodate the designation of [[circumbinary planet]]s. A limited number of exoplanets have [[List of proper names of exoplanets|IAU-sanctioned proper names]]. Other naming systems exist.<section end=nomenclature />{{cn|date=July 2025}}

== History of detection ==
[[File:Exoplanet-Missions-graphic-final-Dec2022.png|thumb|upright=1.3|NASA graphic of present and future exoplanet missions as of 2022.]]

For centuries scientists, philosophers, and science fiction writers suspected that extrasolar planets existed, but there was no way of knowing whether they were real in fact, how common they were, or how similar they might be to the planets of the [[Solar System]]. Various detection claims made in the nineteenth century were rejected by astronomers.{{cn|date=July 2025}}

The first evidence of a possible exoplanet, orbiting [[Van Maanen 2]], was recorded in 1917, but was not recognized as such until 2016.<ref>{{Cite journal |last=Farihi |first=J. |date=2016-04-01 |title=Circumstellar debris and pollution at white dwarf stars |url=https://linkinghub.elsevier.com/retrieve/pii/S1387647315300075 |journal=New Astronomy Reviews |volume=71 |pages=9–34 |doi=10.1016/j.newar.2016.03.001 |issn=1387-6473 |arxiv=1604.03092|bibcode=2016NewAR..71....9F }}</ref> The astronomer [[Walter Sydney Adams]] produced a spectrum of the star using [[Mount Wilson Observatory#60-inch telescope|Mount Wilson's 60-inch telescope]] which he interpreted the spectrum to be of an [[F-type main-sequence star]]. This spectrum was reexamined during studies of [[white dwarf]] stars with unpredicted compositions. It is now thought that such a spectrum could be caused by the residue of a nearby exoplanet that had been pulverized by the gravity of the star, the resulting dust then falling onto the star.<ref name="NASA-20171101">{{cite web |last=Landau |first=Elizabeth |title=Overlooked Treasure: The First Evidence of Exoplanets |url=https://exoplanets.nasa.gov/news/1467/overlooked-treasure-the-first-evidence-of-exoplanets |date=1 November 2017 |work=[[NASA]] |access-date=1 November 2017 }}</ref>

Numerous other claims of discovery took place in the mid 20th century, involving 61 Cygnus, Lalande 21185, and Barnard's Star, which were not discredited until the mid to late 1970s (see Discredited claims below). Another [[Discoveries of exoplanets#1988.E2.80.931994|suspected scientific detection]] of an exoplanet occurred in 1988. Shortly afterwards, the first detection<ref>Encyclopedia of the Solar System, third edition, 2014, page 963, Tilman Spohn, Doris Breuer, Torrence Johnson</ref> that is currently accepted came in 1992 when [[Aleksander Wolszczan]] and [[Dale Frail]] announced the discovery of two terrestrial-mass planets orbiting the [[millisecond pulsar]] [[PSR B1257+12]].<ref name="Wolszczan" /> The first confirmation of an exoplanet orbiting a [[main-sequence]] star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star [[51 Pegasi]]. Some exoplanets have been [[Direct imaging|imaged directly]] by telescopes, but the vast majority have been detected through indirect methods, such as the [[transit method]] and the [[Doppler spectroscopy|radial-velocity method]].

[[File:Exoplanet detections per year.png|thumb|upright=1.4|Exoplanet detections per year as of September 2024<ref>{{cite web |title=Pre-generated Exoplanet Plots |url=https://exoplanetarchive.ipac.caltech.edu/exoplanetplots/ |website=exoplanetarchive.ipac.caltech.edu |publisher=[[NASA Exoplanet Archive]] |access-date=10 July 2023}}</ref>]]
In February 2018, researchers using the [[Chandra X-ray Observatory]], combined with a planet detection technique called [[microlensing]], found evidence of planets in a distant galaxy, stating, "Some of these exoplanets are as (relatively) small as the moon, while others are as massive as Jupiter. Unlike Earth, most of the exoplanets are not tightly bound to stars, so they're actually wandering through space or loosely orbiting between stars. We can estimate that the number of planets in this [faraway] galaxy is more than a trillion."<ref>{{cite magazine |last=Zachos |first=Elaina |title=More Than a Trillion Planets Could Exist Beyond Our Galaxy – A new study gives the first evidence that exoplanets exist beyond the Milky Way. |url=https://www.nationalgeographic.com/science/article/exoplanets-discovery-milky-way-galaxy-spd |archive-url=https://web.archive.org/web/20210428194238/https://www.nationalgeographic.com/science/article/exoplanets-discovery-milky-way-galaxy-spd |archive-date=28 April 2021 |magazine=[[National Geographic Society]] |date=5 February 2018 |access-date=31 July 2022}}</ref>

=== Early speculations ===
{{Rquote |right |This space we declare to be infinite... In it are an infinity of worlds of the same kind as our own.|Giordano Bruno (1584)<ref>{{cite book |title=To Infinity and Beyond: A Cultural History of the Infinite |author=Eli Maor |chapter=Chapter 24: The New Cosmology |date=1987 |isbn=978-1-4612-5396-9 |publisher=Birkhäuser |location=Boston, MA |page=[https://archive.org/details/toinfinitybeyond0000maor/page/198 198] |chapter-url=https://books.google.com/books?id=v0btBwAAQBAJ&pg=PA198 |others=Originally in ''De l'infinito universo et mondi'' [''On the Infinite Universe and Worlds''] by Giordano Bruno (1584). |url=https://archive.org/details/toinfinitybeyond0000maor/page/198 }}</ref>}}

In the sixteenth century, the Italian philosopher [[Giordano Bruno]], an early supporter of the [[Nicolaus Copernicus|Copernican]] theory that Earth and other planets orbit the Sun ([[heliocentrism]]), put forward the view that fixed stars are similar to the Sun and are likewise accompanied by planets.<ref>{{Cite web |date=2025-07-05 |title=Giordano Bruno {{!}} Biography, Death, & Facts {{!}} Britannica |url=https://www.britannica.com/biography/Giordano-Bruno |access-date=2025-08-10 |website=www.britannica.com |language=en}}</ref>

In the eighteenth century, the same possibility was mentioned by [[Isaac Newton]] in the "[[General Scholium]]" that concludes his ''[[Philosophiae Naturalis Principia Mathematica|Principia]]''. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of ''One''."<ref>{{Cite book |last = Newton|first = Isaac|author2 = I. Bernard Cohen |author3= Anne Whitman|title = The Principia: A New Translation and Guide|publisher = University of California Press|date=1999|orig-date=1713|page = 940|isbn = 978-0-520-08816-0}}</ref>

In 1938, D.Belorizky demonstrated that it was realistic to search for exo-Jupiters by using [[transit method|transit photometry]].<ref>[https://ui.adsabs.harvard.edu/abs/1938LAstr..52..359B/abstract Le Soleil, Etoile Variable],  D.Belorizky, 1938</ref>

In 1952, more than 40 years before the first [[hot Jupiter]] was discovered, [[Otto Struve]] wrote that there is no compelling reason that planets could not be much closer to their parent star than is the case in the Solar System, and proposed that [[Doppler spectroscopy]] and the [[transit method]] could detect [[super-Jupiter]]s in short orbits.<ref>{{cite journal|title= Proposal for a project of high-precision stellar radial velocity work|last=Struve|first= Otto |journal= The Observatory|volume=72|pages=199–200 |year=1952|bibcode = 1952Obs....72..199S }}</ref>

=== Discredited claims ===
Claims of exoplanet detections have been made since the nineteenth century. Some of the earliest involve the [[binary star]] [[70 Ophiuchi]]. In 1855, [[William Stephen Jacob]] at the [[East India Company]]'s [[Madras Observatory]] reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system.<ref>{{Cite journal|author=Jacob, W. S.|date=1855|title=On Certain Anomalies presented by the Binary Star 70 Ophiuchi|url=https://books.google.com/books?id=pQsAAAAAMAAJ&pg=PA228|journal=[[Monthly Notices of the Royal Astronomical Society]]|volume=15 |issue=9|pages=228–230|bibcode=1855MNRAS..15..228J |doi=10.1093/mnras/15.9.228|doi-access=free}}</ref> In the 1890s, [[Thomas Jefferson Jackson See|Thomas J. J. See]] of the [[University of Chicago]] and the [[United States Naval Observatory]] stated that the orbital anomalies proved the existence of a dark body in the 70&nbsp;Ophiuchi system with a 36-year [[orbital period|period]] around one of the stars.<ref>{{cite journal |last=See |first=T. J. J. |author-link=Thomas Jefferson Jackson See |year=1896 |title=Researches on the orbit of 70 Ophiuchi, and on a periodic perturbation in the motion of the system arising from the action of an unseen body |journal=The Astronomical Journal |volume=16 |pages=17–23 |doi=10.1086/102368 |bibcode=1896AJ.....16...17S}}</ref> However, [[Forest Ray Moulton]] published a paper proving that a three-body system with those orbital parameters would be highly unstable.<ref>{{Cite journal|author=Sherrill, T. J.|date=1999|journal=[[Journal for the History of Astronomy]]|title=A Career of Controversy: The Anomaly of T. J. J. See|url=http://www.shpltd.co.uk/jha.pdf|volume=30 |issue=98 |pages=25–50|bibcode=1999JHA....30...25S |doi=10.1177/002182869903000102|s2cid=117727302}}</ref>

Multiple claims have been made that [[61 Cygni]] might have a planetary system. [[Kaj Aage Gunnar Strand|Kaj Strand]] of the Sproul Observatory in 1942 observed tiny but systematic variations in the orbital motions of 61 Cygni A and B, suggesting that a third body of about 16 Jupiter masses must be orbiting 61 Cygni A.<ref>{{cite journal
 | last=Strand | first=K. Aa.
 | title=61 Cygni as a Triple System
 | journal=Publications of the Astronomical Society of the Pacific
 | date=1943 | volume=55 | issue=322 | pages=29–32
 | bibcode=1943PASP...55...29S
 | doi=10.1086/125484 | doi-access=free}}</ref> Multiple further claims were made, but more recent observations have yet to find confirmation. More information at [[61 Cygni#Claims of a planetary system|61 Cygni:Claims of a planetary system]].

Around the same time that 61 Cygni was being investigated, similar claims about the presence of exoplanets were made about Lalande 21185: [[Lalande 21185#Past claims of planets]].

During the 1950s and 1960s, [[Peter van de Kamp]] of [[Swarthmore College]] made another prominent series of detection claims, this time for planets orbiting [[Barnard's Star]].<ref>{{Cite journal|author=van de Kamp, P. |date=1969|title=Alternate dynamical analysis of Barnard's star|journal=[[Astronomical Journal]]|volume=74 |pages=757–759|doi=10.1086/110852|bibcode=1969AJ.....74..757V}}</ref> Astronomers now generally regard all early reports of detection as erroneous.<ref name="boss_book p31">{{Cite book|last = Boss|first = Alan|title = The Crowded Universe: The Search for Living Planets|publisher = Basic Books|date = 2009|pages = 31–32|isbn = 978-0-465-00936-7}}</ref>

In 1991, [[Andrew Lyne]], [[Matthew Bailes|M. Bailes]] and S. L. Shemar claimed to have discovered a [[pulsar planet]] in orbit around [[PSR 1829-10]], using [[pulsar timing]] variations.<ref name="LyneBailes">{{Cite journal | last1 = Bailes | first1 = M. | last2 = Lyne | first2 = A. G. | author-link2 = Andrew Lyne| last3 = Shemar | first3 = S. L. | doi = 10.1038/352311a0 |bibcode=1991Natur.352..311B| title = A planet orbiting the neutron star PSR1829–10 | journal = Nature | volume = 352 | issue = 6333 | pages = 311–313 | year = 1991 | s2cid = 4339517 }}</ref> The claim briefly received intense attention, but Lyne and his team soon retracted it.<ref name="LyneRetraction">{{Cite journal | doi = 10.1038/355213b0| title = No planet orbiting PS R1829–10| journal = Nature| volume = 355| issue = 6357| page = 213| year = 1992| last1 = Lyne | first1 = A. G.| last2 = Bailes | first2 = M. | bibcode = 1992Natur.355..213L| s2cid = 40526307| doi-access = free}}</ref>

=== Confirmed discoveries ===
{{Main|Discoveries of exoplanets}}
{{See also|List of exoplanet firsts}}
[[File:The Star AB Pictoris and its Companion - Phot-14d-05-normal.jpg|thumb|300px|[[Coronagraph]]ic image of [[AB Pictoris]] showing a companion (bottom left), which is either a brown dwarf or a massive planet. The data were obtained on 16 March 2003 with [[List of instruments at the Very Large Telescope|NACO]] on the [[Very Large Telescope|VLT]], using a 1.4 arcsec occulting mask on top of AB Pictoris.]]
{{multiple image
| align             = right
| direction         = vertical
| width             = 300
| image1            = 444226main exoplanet20100414-a-full.jpg
| alt1              = False-color, star-subtracted, direct image using a vortex coronagraph of 3 exoplanets around star HR8799
| caption1          = The three known planets of the star [[HR 8799]], as imaged by the [[Hale Telescope]]. The light from the central star was blanked out by a [[vector vortex coronagraph]].
| image2            = Brown dwarf 2M J044144 and planet.jpg
| alt2              = Hubble image of brown dwarf 2MASS J044144 and its 5–10 Jupiter-mass companion, before and after star-subtraction
| caption2          = [[2MASS J044144]] is a [[brown dwarf]] with a companion about 5–10 times the mass of Jupiter. It is not clear whether this companion object is a [[sub-brown dwarf]] or a planet.
}}

As of {{Extrasolar planet counts|asof}}, a total of {{Extrasolar planet counts|planet_count|source=NEA}} confirmed exoplanets are listed in the [[NASA Exoplanet Archive]], including a few that were confirmations of controversial claims from the late 1980s.{{Extrasolar planet counts|ref}} The first published discovery to receive subsequent confirmation was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang of the [[University of Victoria]] and the [[University of British Columbia]].<ref name="Campbell">{{Cite journal | last1 = Campbell | first1 = B. | last2 = Walker | first2 = G. A. H. | last3 = Yang | first3 = S. | title = A search for substellar companions to solar-type stars | doi = 10.1086/166608 | journal = The Astrophysical Journal | volume = 331 | page = 902 | year = 1988 | bibcode=1988ApJ...331..902C| doi-access = free }}</ref> Although they were cautious about claiming a planetary detection, their radial-velocity observations suggested that a planet orbits the star [[Gamma Cephei]]. Partly because the observations were at the very limits of instrumental capabilities at the time, astronomers remained skeptical for several years about this and other similar observations. It was thought some of the apparent planets might instead have been [[brown dwarf]]s, objects intermediate in mass between planets and stars. In 1990, additional observations were published that supported the existence of the planet orbiting Gamma Cephei,<ref>{{Cite journal|last1=Lawton |first1=A. T.|last2=Wright |first2=P.|date=1989|title=A planetary system for Gamma Cephei?|journal=[[Journal of the British Interplanetary Society]]|volume=42|pages=335–336|bibcode=1989JBIS...42..335L}}</ref> but subsequent work in 1992 again raised serious doubts.<ref name="Walker">{{Cite journal|last1=Walker |first1=G. A. H|date=1992|title=Gamma Cephei&nbsp;– Rotation or planetary companion?|journal=[[Astrophysical Journal Letters]]|volume=396|issue=2|pages=L91–L94|doi=10.1086/186524|bibcode=1992ApJ...396L..91W|last2=Bohlender |first2=D. A.|last3=Walker |first3=A. R.|last4=Irwin |first4=A. W.|last5=Yang |first5=S. L. S.|last6=Larson |first6=A.|doi-access=free}}</ref> Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.<ref>{{Cite journal|last1=Hatzes |first1=A. P.|last2=Cochran |first2=William D.|title=A Planetary Companion to Gamma Cephei A|journal=[[Astrophysical Journal]]|date=2003|volume=599|issue=2|pages=1383–1394|doi =10.1086/379281|bibcode=2003ApJ...599.1383H|arxiv = astro-ph/0305110|last3=Endl|first3=Michael|last4=McArthur|first4=Barbara|last5=Paulson|first5=Diane B.|last6=Walker|first6=Gordon A. H.|last7=Campbell|first7=Bruce|last8=Yang|first8=Stephenson|s2cid=11506537}}</ref>

On 9 January 1992, radio astronomers [[Aleksander Wolszczan]] and [[Dale Frail]] announced the discovery of two planets orbiting the [[millisecond pulsar]] [[PSR B1257+12|PSR 1257+12]] based on the variability of timing of the pulses.<ref name="Wolszczan">{{Cite journal | last1 = Wolszczan | first1 = A. |bibcode=1992Natur.355..145W| last2 = Frail | first2 = D. A. | doi = 10.1038/355145a0 | title = A planetary system around the millisecond pulsar PSR1257 + 12 | journal = Nature | volume = 355 | issue = 6356 | pages = 145–147 | year = 1992 | s2cid = 4260368 }}</ref> This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. Follow-up observations solidified these results, and confirmation of a third planet in 1994 revived the topic in the popular press.<ref>{{cite news | url=http://tech.mit.edu/V114/N22/psr.22w.html | title=Scientists Uncover Evidence of New Planets Orbiting Star | newspaper=Los Angeles Times via [[The Tech (newspaper)|The Tech Online]] | first=Robert | last=Holtz | date=22 April 1994 | access-date=20 April 2012 | archive-date=17 May 2013 | archive-url=https://web.archive.org/web/20130517225034/http://tech.mit.edu/V114/N22/psr.22w.html }}</ref> These pulsar planets are thought to have formed from the unusual remnants of the [[supernova]] that produced the pulsar, in a second round of planet formation, or else to be the [[Chthonian planet|remaining rocky cores]] of [[gas giant]]s that somehow survived the supernova and then decayed into their current orbits. As pulsars are aggressive stars, it was considered unlikely at the time that a planet could form in their orbit.<ref>{{cite book |last= Rodriguez Baquero|first= Oscar Augusto|date= 2017|title= La presencia humana más allá del sistema solar|trans-title= Human presence beyond the solar system|url= |language= Spanish|page=29|location= |publisher= RBA|isbn=978-84-473-9090-8}}</ref>

In the early 1990s, a group of astronomers led by [[Donald Backer]], who were studying what they thought was a binary pulsar ([[PSR B1620−26 b]]), determined that a third object was needed to explain the observed [[Doppler shift]]s. Within a few years, the gravitational effects of the planet on the orbit of the pulsar and [[white dwarf]] had been measured, giving an estimate of the mass of the third object that was too small to be a star. The conclusion that the third object was a planet was announced by [[Stephen Thorsett]] and his collaborators in 1993.<ref name="STScI">{{cite web | title=Oldest Known Planet Identified | work=[[HubbleSite]] | url=http://hubblesite.org/newscenter/newsdesk/archive/releases/2003/19/ | access-date=2006-05-07}}</ref>

On 6 October 1995, [[Michel Mayor]] and [[Didier Queloz]] of the [[University of Geneva]] announced the first definitive detection of [[51 Pegasi b|an exoplanet]] orbiting a [[main sequence|main-sequence]] star, nearby [[G-type star]] [[51 Pegasi]].<ref name="Wenz"/><ref name="Mayor">{{Cite journal | doi = 10.1038/378355a0| title = A Jupiter-mass companion to a solar-type star| journal = Nature| volume = 378| issue = 6555| pages = 355–359| year = 1995| last1 = Mayor | first1 = M. | last2 = Queloz | first2 = D. | bibcode = 1995Natur.378..355M| s2cid = 4339201}}</ref><ref name=gibn>{{cite journal|last1=Gibney|first1=Elizabeth|title=In search of sister earths|journal=Nature|date=18 December 2013|volume=504|issue=7480|pages=357–365|doi=10.1038/504357a|pmid=24352276|bibcode = 2013Natur.504..357. |doi-access=free}}</ref> This discovery, made at the [[Observatoire de Haute-Provence]], ushered in the modern era of exoplanetary discovery, and was recognized by a share of the 2019 [[Nobel Prize in Physics]]. Technological advances, most notably in high-resolution [[spectroscopy]], led to the rapid detection of many new exoplanets: astronomers could detect exoplanets indirectly by measuring their [[gravity|gravitational]] influence on the motion of their host stars. More extrasolar planets were later detected by observing the variation in a star's apparent luminosity as an orbiting planet transited in front of it.<ref name="Wenz"/>

Initially, the most known exoplanets were massive planets that orbited very close to their parent stars. Astronomers were surprised by these "[[hot Jupiter]]s", because theories of [[Nebular hypothesis#Formation of planets|planetary formation]] had indicated that giant planets should only form at large distances from stars. But eventually more planets of other sorts were found, and it is now clear that hot Jupiters make up the minority of exoplanets.<ref name="Wenz">{{cite journal |last1=Wenz |first1=John |title=Lessons from scorching hot weirdo-planets |journal=Knowable Magazine |publisher= Annual Reviews |date=10 October 2019 |doi=10.1146/knowable-101019-2|doi-access=free |url=https://knowablemagazine.org/article/physical-world/2019/hot-jupiter-formation-theories |access-date=4 April 2022 |language=en}}</ref> In 1999, [[Upsilon Andromedae]] became the first main-sequence star known to have multiple planets.<ref name="Lissauer_multiple99">{{Cite journal | doi = 10.1038/19409| year = 1999| last1 = Lissauer | first1 = J. J. | title = Three planets for Upsilon Andromedae| journal = Nature| volume = 398| issue = 6729| page = 659| bibcode = 1999Natur.398..659L| s2cid = 204992574| doi-access = free}}</ref> [[Kepler-16]] contains the first discovered planet that orbits a binary main-sequence star system.<ref name=Doyle>{{Cite journal | doi = 10.1126/science.1210923| pmid = 21921192| title = Kepler-16: A Transiting Circumbinary Planet| journal = Science| volume = 333| issue = 6049| pages = 1602–1606| year = 2011| last1 = Doyle | first1 = L. R.| last2 = Carter | first2 = J. A.| last3 = Fabrycky | first3 = D. C.| last4 = Slawson | first4 = R. W.| last5 = Howell | first5 = S. B.| last6 = Winn | first6 = J. N.| last7 = Orosz | first7 = J. A.| last8 = Prša | first8 = A.| last9 = Welsh | first9 = W. F.| last10 = Quinn | first10 = S. N.| last11 = Latham | first11 = D.| last12 = Torres | first12 = G.| last13 = Buchhave | first13 = L. A.| last14 = Marcy | first14 = G. W.| last15 = Fortney | first15 = J. J.| last16 = Shporer | first16 = A.| last17 = Ford | first17 = E. B.| last18 = Lissauer | first18 = J. J.| last19 = Ragozzine | first19 = D.| last20 = Rucker | first20 = M.| last21 = Batalha | first21 = N.| last22 = Jenkins | first22 = J. M.| last23 = Borucki | first23 = W. J.| last24 = Koch | first24 = D.| last25 = Middour | first25 = C. K.| last26 = Hall | first26 = J. R.| last27 = McCauliff | first27 = S.| last28 = Fanelli | first28 = M. N.| last29 = Quintana | first29 = E. V.| last30 = Holman | first30 = M. J.| display-authors = etal| bibcode = 2011Sci...333.1602D|arxiv = 1109.3432 | s2cid = 206536332}}</ref>

On 26 February 2014, NASA announced the discovery of 715 newly verified exoplanets around 305 stars by the [[Kepler (spacecraft)|''Kepler'' Space Telescope]]. These exoplanets were checked using a statistical technique called "verification by multiplicity".<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=[[NASA]] |access-date=26 February 2014 |archive-date=26 February 2014 |archive-url=https://web.archive.org/web/20140226202703/http://www.nasa.gov/ames/kepler/nasas-kepler-mission-announces-a-planet-bonanza/ }}</ref><ref name="space.com">{{cite web|last=Wall|first=Mike|title=Population of Known Alien Planets Nearly Doubles as NASA Discovers 715 New Worlds|url=http://www.space.com/24824-alien-planets-population-doubles-nasa-kepler.html|date=26 February 2014|access-date=27 February 2014|publisher=space.com}}</ref><ref name="bbcnews">{{cite news |author=Amos |first=Jonathan |date=26 February 2014 |title=Kepler telescope bags huge haul of planets |url=https://www.bbc.co.uk/news/science-environment-26362433 |access-date=27 February 2014 |work=BBC News}}</ref> Before these results, most confirmed planets were gas giants comparable in size to Jupiter or larger because they were more easily detected, but the ''Kepler'' planets are mostly between the size of Neptune and the size of Earth.<ref name="kepler1700" />

On 23 July 2015, NASA announced [[Kepler-452b]], a near-Earth-size planet orbiting the habitable zone of a G2-type star.<ref name="NASA-20150723">{{cite web |last1=Johnson |first1=Michelle |last2=Chou |first2=Felicia |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 |date=23 July 2015 |work=[[NASA]]}}</ref>

On 6 September 2018, NASA discovered an exoplanet about 145 light years away from Earth in the constellation Virgo.<ref name="Oddball">{{Cite news |last=NASA |title=Discovery alert! Oddball planet could surrender its secrets |url=https://exoplanets.nasa.gov/news/1521/discovery-alert-oddball-planet-could-surrender-its-secrets/ |access-date=28 November 2018 |work=Exoplanet Exploration: Planets Beyond our Solar System}}</ref> This exoplanet, Wolf 503b, is twice the size of Earth and was discovered orbiting a type of star known as an "Orange Dwarf". Wolf 503b completes one orbit in as few as six days because it is very close to the star. Wolf 503b is the only exoplanet that large that can be found near the so-called [[small planet radius gap]]. The gap, sometimes called the Fulton gap,<ref name="Oddball"/><ref>{{Cite journal |last1=Fulton |first1=Benjamin J. |last2=Petigura |first2=Erik A. |last3=Howard |first3=Andrew W. |last4=Isaacson |first4=Howard |last5=Marcy |first5=Geoffrey W. |last6=Cargile |first6=Phillip A. |last7=Hebb |first7=Leslie |last8=Weiss |first8=Lauren M. |last9=Johnson |first9=John Asher |last10=Morton |first10=Timothy D. |last11=Sinukoff |first11=Evan |last12=Crossfield |first12=Ian J. M. |last13=Hirsch |first13=Lea A. |date=2017-09-01 |title=The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets* |journal=The Astronomical Journal |volume=154 |issue=3 |page=109 |doi=10.3847/1538-3881/aa80eb |doi-access=free |arxiv=1703.10375 |bibcode=2017AJ....154..109F |issn=0004-6256}}</ref> is the observation that it is unusual to find exoplanets with sizes between 1.5 and 2 times the radius of the Earth.<ref>{{Cite web |title=Radius Gap |url=https://sites.astro.caltech.edu/~fdai/radius_gap.html |access-date=2024-04-03 |website=sites.astro.caltech.edu}}</ref>

In January 2020, scientists announced the discovery of [[TOI 700 d]], the first Earth-sized planet in the habitable zone detected by TESS.<ref>{{Cite web|url=https://www.midilibre.fr/2020/01/07/toi-700d-une-planete-de-la-taille-de-la-terre-decouverte-dans-une-zone-habitable,8645004.php|title=[VIDEO] TOI 700d: une planète de la taille de la Terre découverte dans une "zone habitable"|website=midilibre.fr|language=fr|access-date=2020-04-17}}</ref>

=== Candidate discoveries ===
As of January 2020, NASA's [[Kepler (spacecraft)|''Kepler'']] and [[Transiting Exoplanet Survey Satellite|TESS]] missions had identified 4374 planetary candidates yet to be confirmed,<ref>{{Cite web|url=https://exoplanetarchive.ipac.caltech.edu/docs/counts_detail.html|title=Exoplanet and Candidate Statistics|publisher=NASA Exoplanet Archive, California Institute of Technology|access-date=2020-01-17}}</ref> several of them being nearly Earth-sized and located in the habitable zone, some around Sun-like stars.<ref name="keplersite">{{cite web |author=Colen |first=Jerry |date=4 November 2013 |title=Kepler |url=http://www.nasa.gov/mission_pages/kepler/main/index.html |archive-url=https://web.archive.org/web/20131105082102/http://www.nasa.gov/mission_pages/kepler/main/index.html |archive-date=5 November 2013 |access-date=4 November 2013 |website=nasa.gov |publisher=NASA}}</ref><ref name="usher">{{cite web |last1=Harrington |first1=J. D. |last2=Johnson |first2=M. |date=4 November 2013 |title=NASA Kepler Results Usher in a New Era of Astronomy |url=http://www.nasa.gov/press/2013/november/nasa-kepler-results-usher-in-a-new-era-of-astronomy/}}</ref><ref name="NASA-ExoplanetArch">{{cite web|title=NASA's Exoplanet Archive KOI table|url=http://exoplanetarchive.ipac.caltech.edu/cgi-bin/ExoTables/nph-exotbls?dataset=cumulative|publisher=NASA|access-date=28 February 2014|archive-url=https://archive.today/20140226203336/http://exoplanetarchive.ipac.caltech.edu/cgi-bin/ExoTables/nph-exotbls?dataset=cumulative|archive-date=26 February 2014}}</ref>
{{multiple image
| header            = Exoplanet populations – June 2017<!--
  --><ref name="SP-20170619">{{cite web |last=Lewin |first=Sarah |title=NASA's Kepler Space Telescope Finds Hundreds of New Exoplanets, Boosts Total to 4,034 |url=https://www.space.com/37242-nasa-kepler-alien-planets-habitable-worlds-catalog.html |date=19 June 2017 |work=[[NASA]] |access-date=19 June 2017}}</ref><!--
  --><ref name="NYT-20170619">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=Earth-Size Planets Among Final Tally of NASA's Kepler Telescope |url=https://www.nytimes.com/2017/06/19/science/kepler-planets-earth-like-census.html |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2017/06/19/science/kepler-planets-earth-like-census.html |archive-date=2022-01-01 |url-access=limited |date=19 June 2017 |work=The New York Times}}{{cbignore}}</ref>
| align             = center
| caption_align     = center
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| width             = 333
| image1            = ExoplanetPopulations-20170616.png
| alt1              =
| caption1          = Exoplanet populations; colors indicate detection method
| image2            = SmallPlanetsComeInTwoSizes-20170619.png
| alt2              =
| caption2          = Small planets come in two sizes
| image3            = KeplerHabitableZonePlanets-20170616.png
| alt3              =
| caption3          = Kepler habitable zone planets
}}

In September 2020, astronomers reported evidence, for the first time, of an [[extragalactic planet]], [[M51-ULS-1b]], detected by eclipsing a bright [[Astrophysical X-ray source|X-ray source]] (XRS), in the [[Whirlpool Galaxy]] (M51a).<ref name="NS-20200923">{{cite news |last=Crane |first=Leah |title=Astronomers may have found the first planet in another galaxy |url=https://www.newscientist.com/article/2255431-astronomers-may-have-found-the-first-planet-in-another-galaxy/ |date=23 September 2020 |work=[[New Scientist]] |access-date=25 September 2020 }}</ref><ref name="ARX-20200918">{{cite arXiv |author=Di Stafano, R. |display-authors=et al. |title=M51-ULS-1b: The First Candidate for a Planet in an External Galaxy |date=18 September 2020 |class=astro-ph.HE |eprint=2009.08987 }}</ref>

== Detection methods ==
{{Main|Methods of detecting exoplanets}}{{#chart:Exoplanets_by_method.chart|data=Exoplanets_by_method.tab}}

=== Direct imaging ===
[[File:Betapic orbit v2 reduced.gif|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.|thumb|Directly imaged planet [[Beta Pictoris b]], with an edge-on orbit as seen from Earth]]
Planets are extremely faint compared to 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 to reduce the glare while leaving the light from the planet detectable; doing so is a major technical challenge which requires extreme [[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 [[Jupiter]]) and widely separated from their parent stars.

Specially designed direct-imaging instruments such as [[Gemini Planet Imager]], [[VLT-SPHERE]], and [[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.

=== 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 [[Astronomical transit|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 [[Kepler (spacecraft)|Kepler telescope]] used this method.
; [[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 [[spectral line]]s due to the [[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>
; [[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>
; [[Methods of detecting extrasolar planets#Transit duration variation|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 |doi-access=free |arxiv=0806.0629|bibcode = 2008MNRAS.389..191P |s2cid=15282437 }}</ref>
; [[Methods of detecting extrasolar planets#Gravitational microlensing|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 magnification as it varies over time. Unlike most other methods which have a detection bias towards planets with small (or for resolved imaging, large) orbits, the microlensing method is most sensitive to detecting planets around 1–10&nbsp;[[astronomical unit|AU]] away from Sun-like stars.
; [[Methods of detecting extrasolar planets#Astrometry|Astrometry]]
:[[File:Planet reflex sm.gif|alt=|thumb|A planet is able to gravitationally pull its host star]] 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 was not very productive until the 2020s. It has produced only a few confirmed discoveries,<ref name="Curiel2022">{{cite journal |arxiv=2208.14553 |last1=Curiel |first1=Salvador |last2=Ortiz-León |first2=Gisela N. |last3=Mioduszewski |first3=Amy J. |last4=Sanchez-Bermudez |first4=Joel |date=September 2022 |title=3D Orbital Architecture of a Dwarf Binary System and Its Planetary Companion |journal=[[The Astronomical Journal]] |volume=164 |issue=3 |page=93 |doi=10.3847/1538-3881/ac7c66 |bibcode=2022AJ....164...93C|s2cid=251953478 |doi-access=free }}</ref><ref name="Sozzetti2023">{{cite journal |last1=Sozzetti |first1=A. |last2=Pinamonti |first2=M. |display-authors=etal |date=September 2023 |title=The GAPS Programme at TNG. XLVII. A conundrum resolved: HIP 66074b/Gaia-3b characterised as a massive giant planet on a quasi-face-on and extremely elongated orbit |journal=[[Astronomy & Astrophysics]] |volume=677 |issue= |pages=L15 |doi=10.1051/0004-6361/202347329 |arxiv=2307.08653 |doi-access=free |bibcode=2023A&A...677L..15S|hdl=2108/347124 |hdl-access=free }}</ref> though it has been successfully used to investigate the properties of planets found in other ways.
; [[Pulsar timing]]
:A [[pulsar]], a small, dense remnant of a star that has exploded as a [[supernova]], emits radio waves regularly as it rotates. If planets orbit the pulsar, the motion of the pulsar around the system's center of mass alters the pulsar's distance to Earth over time. As a result, the radio pulses from the pulsar arrive on Earth at a later or earlier time. This light travel delay due to the pulsar being physically closer or farther from Earth is known as a Roemer time delay.<ref>{{Cite journal |last=Damour |first=Thibault |date=1992 |title=Strong-field tests of relativistic gravity and binary pulsars |url=https://journals.aps.org/prd/abstract/10.1103/PhysRevD.45.1840 |journal=Physical Review D |volume=45 |issue=6 |pages=1840–1868 |doi=10.1103/PhysRevD.45.1840|pmid=10014561 |bibcode=1992PhRvD..45.1840D }}</ref> [[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.
; [[Methods of detecting extrasolar planets#Variable star timing|Variable star timing (pulsation frequency)]]
:Like pulsars, there are some other types of stars which exhibit periodic activity. Deviations from 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 = 189–191| 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>
; [[Methods of detecting extrasolar planets#Reflection and emission modulations|Reflection/emission modulations]]
:When a planet orbits very close to a star, it catches a considerable amount of starlight. As the planet orbits the star, the amount of light changes due to planets having phases from Earth's viewpoint or planets glowing more from one side than the other due to temperature differences.<ref>{{cite journal |last1=Jenkins |first1=J. M. |last2=Doyle |first2=Laurance R. |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 |arxiv=astro-ph/0305473 |bibcode=2003ApJ...595..429J |doi=10.1086/377165 |s2cid=17773111}}</ref>
; [[Methods of detecting extrasolar planets#Relativistic beaming|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>
; [[Methods of detecting extrasolar planets#Ellipsoidal variations|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 on how it is rotated relative to Earth.<ref>{{Cite web|last=Atkinson|first=Nancy|date=2013-05-13|title=Using the Theory of Relativity and BEER to Find Exoplanets|url=https://www.universetoday.com/102112/using-the-theory-of-relativity-and-beer-to-find-exoplanets/|access-date=2023-02-12|website=Universe Today|language=en-US}}</ref>
; [[Methods of detecting extrasolar planets#Polarimetry|Polarimetry]]
:With the 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>
; [[Methods of detecting extrasolar planets#Circumstellar disks|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 [[infrared]] radiation. Features on the disks may suggest the presence of planets, though this is not considered a definitive detection method.

== Formation and evolution ==
{{See also|Accretion (astrophysics)|Nebular hypothesis|Planetary migration}}

Planets may form within a few to tens (or more) of millions of years of their star forming.<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=S. Seager. |pages=319–346|date=2011|chapter-url=http://www.uapress.arizona.edu/Books/bid2263.htm|arxiv=1006.5486 }}</ref><ref name=dl2018>{{cite book|last=D'Angelo|first=G.|author2=Lissauer, J. J.|chapter=Formation of Giant Planets |bibcode=2018haex.bookE.140D| title=Handbook of Exoplanets |publisher=Springer International Publishing AG, part of Springer Nature| editor=Deeg H., Belmonte J. |pages= 2319–2343|date=2018|arxiv=1806.05649|doi=10.1007/978-3-319-55333-7_140|isbn=978-3-319-55332-0|s2cid=116913980}}</ref>
The planets of the [[Solar System]] can only be observed in their current state, but observations of different planetary systems of varying ages allows us to observe planets at different stages of evolution. Available observations range from young [[protoplanetary disk]]s where planets are still forming<ref>{{cite journal|last1=Calvet|first1=Nuria|author1-link=Nuria Calvet|last2=D'Alessio|first2=Paola|last3=Hartmann|first3=Lee|last4=Wilner|first4=David|last5=Walsh|first5=Andrew|last6=Sitko|first6=Michael|title=Evidence for a developing gap in a 10 Myr old protoplanetary disk|journal=The Astrophysical Journal|date=2001|volume=568|issue=2|pages=1008–1016|doi=10.1086/339061|bibcode=2002ApJ...568.1008C|arxiv=astro-ph/0201425|s2cid=8706944}}</ref> to planetary systems of over 10 Gyr old.<ref>{{cite journal|last1=Fridlund|first1=Malcolm|last2=Gaidos|first2=Eric|last3=Barragán|first3=Oscar|last4=Persson|first4=Carina|last5=Gandolfi|first5=Davide|last6=Cabrera|first6=Juan|last7=Hirano|first7=Teruyuki|last8=Kuzuhara|first8=Masayuki|last9=Csizmadia|first9=Sz|last10=Nowak|first10=Grzegorz|last11=Endl|first11=Michael|last12=Grziwa|first12=Sascha|last13=Korth|first13=Judith|last14=Pfaff|first14=Jeremias|last15=Bitsch|first15=Bertram|last16=Johansen|first16=Anders|last17=Mustill|first17=Alexander|last18=Davies|first18=Melvyn|last19=Deeg|first19=Hans|last20=Palle|first20=Enric|last21=Cochran|first21=William|last22=Eigmüller|first22=Philipp|last23=Erikson|first23=Anders|last24=Guenther|first24=Eike|last25=Hatzes|first25=Artie|last26=Kiilerich|first26=Amanda|last27=Kudo|first27=Tomoyuki|last28=MacQueen|first28=Philipp|last29=Narita|first29=Norio|last30=Nespral|first30=David|last31=Pätzold|first31=Martin|last32=Prieto-Arranz|first32=Jorge|last33=Rauer|first33=Heike|last34=van Eylen|first34=Vincent|title=EPIC210894022b −A short period super-Earth transiting a metal poor, evolved old star|journal=Astronomy & Astrophysics|volume=604|pages=A16|date=28 April 2017|arxiv=1704.08284|doi=10.1051/0004-6361/201730822|s2cid=39412906}}</ref> When planets form in a gaseous protoplanetary disk,<ref name=dangelo_bodenheimer_2016>{{Cite journal|last=D'Angelo|first=G.|author2= Bodenheimer, P. |title=In Situ and Ex Situ Formation Models of Kepler 11 Planets|journal=The Astrophysical Journal|year=2016|volume=828|issue=1|pages=id. 33 (32 pp.)|doi=10.3847/0004-637X/828/1/33|arxiv = 1606.08088 |bibcode = 2016ApJ...828...33D |s2cid=119203398 |doi-access=free }}</ref> they accrete [[hydrogen]]/[[helium]] envelopes.<ref name=dangelo_bodenheimer_2013>{{cite journal|last=D'Angelo|first=G.|author2= Bodenheimer, P. |title=Three-Dimensional Radiation-Hydrodynamics Calculations of the Envelopes of Young Planets Embedded in Protoplanetary Disks|journal=[[The Astrophysical Journal]]|year=2013|volume=778|issue=1|pages=77 (29 pp.)|doi=10.1088/0004-637X/778/1/77|arxiv = 1310.2211 |bibcode = 2013ApJ...778...77D |s2cid=118522228}}</ref><ref name=dangelo2014>{{cite journal|last=D'Angelo|first=G.|author2=Weidenschilling, S. J. |author3=Lissauer, J. J. |author4=Bodenheimer, P. |title=Growth of Jupiter: Enhancement of core accretion by a voluminous low-mass envelope|journal=Icarus|date=2014|volume=241|pages=298–312|arxiv=1405.7305|doi=10.1016/j.icarus.2014.06.029|bibcode=2014Icar..241..298D|s2cid=118572605}}</ref> These envelopes cool and contract over time and, depending on the mass of the planet, some or all of the hydrogen/helium is eventually lost to space.<ref name=dangelo_bodenheimer_2016 /> This means that even terrestrial planets may start off with large radii if they form early enough.<ref>{{Cite journal | doi = 10.1093/mnras/stu085 |arxiv=1401.2765 |title=Origin and loss of nebula-captured hydrogen envelopes from 'sub'- to 'super-Earths' in the habitable zone of Sun-like stars |url=https://www.researchgate.net/publication/260647400 |journal=Monthly Notices of the Royal Astronomical Society |volume=439 |issue=4 |pages=3225–3238 |year=2014 |last1=Lammer |first1=H. |last2=Stokl |first2=A. |last3=Erkaev |first3=N. V. |last4=Dorfi |first4=E. A. |last5=Odert |first5=P. |last6=Gudel |first6=M. |last7=Kulikov |first7=Y. N. |last8=Kislyakova |first8=K. G. |last9=Leitzinger |first9=M. |doi-access=free |bibcode=2014MNRAS.439.3225L
|s2cid=118620603}}</ref><ref>{{cite journal |arxiv=1001.0917 |last1=Johnson |first1=R. E. |title=Thermally-Diven Atmospheric Escape |journal=The Astrophysical Journal |volume= 716 |issue= 2 |pages=1573–1578 |year=2010 |doi=10.1088/0004-637X/716/2/1573 |bibcode=2010ApJ...716.1573J |s2cid= 36285464}}</ref><ref>{{cite journal |arxiv=1006.0021|bibcode = 2010Icar..210..539Z |doi = 10.1016/j.icarus.2010.07.013 |volume=210 |issue=2 |title=Atmospheric mass loss by stellar wind from planets around main sequence M stars |journal=Icarus |pages=539–544 |year=2010 |last1=Zendejas |first1=J. |last2=Segura |first2=A. |last3=Raga |first3=A.C. |s2cid=119243879}}</ref> An example is [[Kepler-51b]] which has only about twice the mass of Earth but is almost the size of Saturn, which is a hundred times the mass of Earth. Kepler-51b is quite young at a few hundred million years old.<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>

==Planet-hosting stars==
{{Main|Planet-hosting star}}
[[File:Morgan-Keenan spectral classification.svg|thumb|upright=1.2|alt=The Morgan-Keenan spectral classification system, showing size-and-color comparisons of M, K, G, F, A, B, and O stars|The Morgan-Keenan spectral classification]]
[[File:OGLE-2007-BLG-349.jpg|thumb|upright=1|Artist's impression of exoplanet orbiting two stars.<ref>{{cite web|title=Artist's impression of exoplanet orbiting two stars|url=http://www.spacetelescope.org/images/heic1619a/|website=www.spacetelescope.org|access-date=24 September 2016}}</ref>]]
There is at least one planet on average per star.<ref name="Nature-20120111">{{Cite journal | last1 = Cassan | first1 = A. | last2 = Kubas | first2 = D. | last3 = Beaulieu | first3 = J. -P. | last4 = Dominik | first4 = M. | last5 = Horne | first5 = K. | last6 = Greenhill | first6 = J. | last7 = Wambsganss | first7 = J. | last8 = Menzies | first8 = J. | last9 = Williams | first9 = A. | last10 = Jørgensen | doi = 10.1038/nature10684 | first10 = U. G. | last11 = Udalski | first11 = A. | last12 = Bennett | first12 = D. P. | last13 = Albrow | first13 = M. D. | last14 = Batista | first14 = V. | last15 = Brillant | first15 = S. | last16 = Caldwell | first16 = J. A. R. | last17 = Cole | first17 = A. | last18 = Coutures | first18 = C. | last19 = Cook | first19 = K. H. | last20 = Dieters | first20 = S. | last21 = Prester | first21 = D. D. | last22 = Donatowicz | first22 = J. | last23 = Fouqué | first23 = P. | last24 = Hill | first24 = K. | last25 = Kains | first25 = N. | last26 = Kane | first26 = S. | last27 = Marquette | first27 = J. -B. | last28 = Martin | first28 = R. | last29 = Pollard | first29 = K. R. | last30 = Sahu | first30 = K. C.| title = One or more bound planets per Milky Way star from microlensing observations | journal = Nature | volume = 481 | issue = 7380 | pages = 167–169 | date=11 January 2012| pmid =  22237108| bibcode=2012Natur.481..167C|arxiv = 1202.0903 | s2cid = 2614136 }}</ref> About 1 in 5 [[Solar analog|Sun-like stars]]<ref group=lower-alpha name=footnoteA /> have an "Earth-sized"<ref group=lower-alpha name=footnoteB /> planet in the [[habitable zone]].<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>

Most known exoplanets orbit stars roughly similar to the [[Sun]], i.e. [[main sequence|main-sequence stars]] of [[stellar classification|spectral categories]] F, G, or K. Lower-mass stars ([[red dwarf]]s, of [[stellar classification|spectral category]] M) are less likely to have planets massive enough to be detected by the [[radial-velocity method]].<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|author-link3=Geoffrey Marcy|author-link4=Steven S. Vogt|author-link6=Debra Fischer |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><ref name="bonfils05">{{Cite journal |doi=10.1051/0004-6361:200500193 |title=The HARPS search for southern extra-solar planets VI: A Neptune-mass planet around the nearby M dwarf Gl 581 |journal=Astronomy and Astrophysics |volume=443 |issue=3 |pages=L15–L18 |year=2005 |last1=Bonfils |first1=Xavier |last2=Forveille |first2=Thierry |last3=Delfosse |first3=Xavier |last4=Udry |first4=Stéphane |last5=Mayor |first5=Michel |last6=Perrier |first6=Christian |last7=Bouchy |first7=François |last8=Pepe |first8=Francesco |last9=Queloz |first9=Didier |last10=Bertaux |first10=Jean-Loup |bibcode=2005A&A...443L..15B |arxiv=astro-ph/0509211 |s2cid=59569803 }}</ref> Despite this, several tens of planets around red dwarfs have been discovered by the [[Kepler space telescope]], which uses the [[transit method]] to detect smaller planets.<ref>{{Cite web |date=2023-05-23 |title=Kepler / K2 - NASA Science |url=https://science.nasa.gov/mission/kepler/ |access-date=2025-08-22 |language=en-US}}</ref>

Using data from [[Kepler space telescope|Kepler]], a correlation has been found between the [[metallicity]] of a star and the probability that the star hosts a giant planet, similar to the size of [[Jupiter]]. Stars with higher metallicity are more likely to have planets, especially giant planets, than stars with lower metallicity.<ref>{{Cite journal | doi = 10.1088/0004-6256/149/1/14| title = Revealing a Universal Planet–Metallicity Correlation for Planets of Different Solar-Type Stars| journal = The Astronomical Journal| volume = 149| issue = 1| page = 14| year = 2014| last1 = Wang | first1 = J. | last2 = Fischer | first2 = D. A. | bibcode = 2015AJ....149...14W|arxiv = 1310.7830 | s2cid = 118415186}}</ref>

Some planets orbit one member of a [[binary star]] system,<ref>{{Cite web|title=Science work|url=https://www.univie.ac.at/adg/schwarz/multiple.html|access-date=2022-01-17|website=www.univie.ac.at}}</ref> and several [[circumbinary planet]]s have been discovered which orbit both members of a binary star. A few planets in [[triple star]] systems are known<ref>{{Cite web|title=STAR-DATA|url=https://www.univie.ac.at/adg/schwarz/multi.html|access-date=2022-01-17|website=www.univie.ac.at}}</ref> and one in the quadruple system [[PH1b|Kepler-64]].

== Orbital and physical parameters ==
{{Main|Exoplanet orbital and physical parameters}}

== General features ==

=== Color and brightness ===
{{See also|Sudarsky's gas giant classification}}The apparent brightness ([[apparent magnitude]]) of a planet depends on how far away the observer is, how reflective the planet is (albedo), and how much light the planet receives from its star, which depends on how far the planet is from the star and how bright the star is. So, a planet with a low albedo that is close to its star can appear brighter than a planet with a high albedo that is far from the star.<ref>[http://phl.upr.edu/library/notes/theapparentbrightnessandsizeofexoplanetsandtheirstars The Apparent Brightness and Size of Exoplanets and their Stars] {{Webarchive|url=https://web.archive.org/web/20140812200814/http://phl.upr.edu/library/notes/theapparentbrightnessandsizeofexoplanetsandtheirstars|date=12 August 2014}}, Abel Mendez, updated 30 June 2012, 12:10&nbsp;pm</ref>[[File:Color HD 189733b vs solar system.jpg|thumb|upright=1.6|alt=Color-color diagram comparing the colors of Solar System planets to exoplanet HD 189733b. HD 189733b reflects as much green as Mars and almost as much blue as Earth.|This [[color–color diagram]] compares the colors of planets in the Solar System to exoplanet [[HD 189733b]]. The exoplanet's deep blue color is produced by [[silicate]] droplets, which scatter blue light in its atmosphere.]]
In 2013, the color of an exoplanet was determined for the first time. The best-fit [[albedo]] measurements of [[HD 189733b]] suggest that it is deep dark blue.<ref>{{Cite web|last=Garner|first=Rob|date=2016-10-31|title=NASA Hubble Finds a True Blue Planet|url=http://www.nasa.gov/content/nasa-hubble-finds-a-true-blue-planet|access-date=2022-01-17|website=NASA}}</ref><ref>{{Cite journal | doi = 10.1088/2041-8205/772/2/L16|arxiv=1307.3239| title = The Deep Blue Color of HD189733b: Albedo Measurements with Hubble Space Telescope/Space Telescope Imaging Spectrograph at Visible Wavelengths| journal = The Astrophysical Journal| volume = 772| issue = 2| pages = L16| year = 2013| last1 = Evans | first1 = T. M. | last2 = Pont | first2 = F. D. R. | last3 = Sing | first3 = D. K. | last4 = Aigrain | first4 = S.|author-link4=Suzanne Aigrain | last5 = Barstow | first5 = J. K. | last6 = Désert | first6 = J. M. | last7 = Gibson | first7 = N. | last8 = Heng | first8 = K. | last9 = Knutson | first9 = H. A. | last10 = Lecavelier Des Etangs | first10 = A. |bibcode=2013ApJ...772L..16E|s2cid=38344760}}</ref> Later that same year, the colors of several other exoplanets were determined, including [[GJ 504 b]] which visually has a magenta color,<ref name="Kuzuhara2013">{{Cite journal|arxiv=1307.2886|title=Direct Imaging of a Cold Jovian Exoplanet in Orbit around the Sun-like Star GJ 504|journal=The Astrophysical Journal|volume=774|issue=11|page=11|date=2013|display-authors=etal|doi=10.1088/0004-637X/774/1/11|bibcode = 2013ApJ...774...11K |last1=Kuzuhara|first1=M.|last2=Tamura|first2=M.|last3=Kudo|first3=T.|last4=Janson|first4=M.|last5=Kandori|first5=R.|last6=Brandt|first6=T. D.|last7=Thalmann|first7=C.|last8=Spiegel|first8=D.|last9=Biller|first9=B.|last10=Carson|first10=J.|last11=Hori|first11=Y.|last12=Suzuki|first12=R. |last13=Burrows |first13=Adam |last14=Henning|first14=T.|last15=Turner|first15=E. L.|last16=McElwain|first16=M. W.|last17=Moro-Martín|first17=A.|last18=Suenaga|first18=T.|last19=Takahashi|first19=Y. H.|last20=Kwon|first20=J.|last21=Lucas|first21=P.|last22=Abe|first22=L.|last23=Brandner|first23=W.|last24=Egner|first24=S.|last25=Feldt|first25=M.|last26=Fujiwara|first26=H.|last27=Goto|first27=M.|last28=Grady|first28=C. A.|last29=Guyon|first29=O.|last30=Hashimoto|first30=J.<!-- the rest: Y. Hayano, M. Hayashi, S. S. Hayashi, K. W. Hodapp, M. Ishii, M. Iye, G. R. Knapp, T. Matsuo, S. Mayama, S. Miyama, J.-I. Morino, J. Nishikawa, T. Nishimura, T. Kotani, N. Kusakabe, T. -S. Pyo, E. Serabyn, H. Suto, M. Takami, N. Takato, H. Terada, D. Tomono, M. Watanabe, J. P. Wisniewski, T. Yamada, H. Takami, T. Usuda -->|s2cid=53343537|url=https://pure.uva.nl/ws/files/2002826/150064_Direct_Imaging_of_a_Cold_Jovian_Exoplanet.pdf}}</ref> and [[Kappa Andromedae b]], which if seen up close would appear reddish in color.<ref name="arXiv1211.3744">{{cite journal|title=Direct Imaging Discovery of a 'Super-Jupiter' Around the late B-Type Star Kappa And|date=15 November 2012|arxiv=1211.3744|author1=Carson|author2=Thalmann|author3=Janson|author4=Kozakis|author5=Bonnefoy|author6=Biller|author7=Schlieder|author8=Currie|author9=McElwain|bibcode = 2013ApJ...763L..32C |doi = 10.1088/2041-8205/763/2/L32|volume=763|issue=2|journal=The Astrophysical Journal|pages=L32|s2cid=119253577}}</ref> [[Helium planet]]s are expected to be white or grey in appearance.<ref name=SpaceDaily-2015-06-16>{{cite news |url= http://www.spacedaily.com/reports/Helium_Shrouded_Planets_May_Be_Common_in_Our_Galaxy_999.html |title= Helium-Shrouded Planets May Be Common in Our Galaxy |publisher= SpaceDaily |date= 16 June 2015 |access-date=3 August 2015}}</ref>

The darkest known planet in terms of [[geometric albedo]] is [[TrES-2b]], a [[hot Jupiter]] that reflects less than 1% of the light from its star, making it less reflective than coal or black acrylic paint. Hot Jupiters are expected to be quite dark due to sodium and potassium in their atmospheres, but it is not known why TrES-2b is so dark—it could be due to an unknown chemical compound.<ref name=darkest_news>{{cite web |url=http://www.space.com/12612-alien-planet-darkest-coal-black-kepler.html |title=Coal-Black Alien Planet Is Darkest Ever Seen |date=11 August 2011 |publisher=Space.com |access-date=12 August 2011}}</ref><ref>{{cite journal |arxiv=1108.2297 |bibcode=2011MNRAS.417L..88K |doi=10.1111/j.1745-3933.2011.01127.x |volume=417 |issue=1 |title=Detection of visible light from the darkest world |journal=Monthly Notices of the Royal Astronomical Society: Letters |pages=L88–L92 |year=2011 |last1=Kipping |first1=David M. |last2=Spiegel |first2=David S. |doi-access=free |s2cid=119287494}}</ref><ref>{{Cite journal |doi=10.1088/0004-637X/761/1/53 |arxiv=1210.4592 |title=Photometrically derived masses and radii of the planet and star in the TrES-2 system |journal=The Astrophysical Journal |volume=761 |issue=1 |page=53 |year=2012 |last1=Barclay |first1=T. |last2=Huber |first2=D. |last3=Rowe |first3=J. F. |last4=Fortney |first4=J. J. |last5=Morley |first5=C. V. |last6=Quintana |first6=E. V. |last7=Fabrycky |first7=D. C. |last8=Barentsen |first8=G. |last9=Bloemen |first9=S. |last10=Christiansen |first10=J. L. |last11=Demory |first11=B. O. |last12=Fulton |first12=B. J. |last13=Jenkins |first13=J. M. |last14=Mullally |first14=F. |last15=Ragozzine |first15=D. |last16=Seader |first16=S. E. |last17=Shporer |first17=A. |last18=Tenenbaum |first18=P. |last19=Thompson |first19=S. E. |bibcode=2012ApJ...761...53B |s2cid=18216065}}</ref>

For [[gas giant]]s, geometric albedo generally decreases with increasing metallicity or atmospheric temperature unless there are clouds to modify this effect. Increased cloud-column depth increases the albedo at optical wavelengths, but decreases it at some infrared wavelengths. Optical albedo increases with age, because older planets have higher cloud-column depths. Optical albedo decreases with increasing mass, because higher-mass giant planets have higher surface gravities, which produces lower cloud-column depths. Also, elliptical orbits can cause major fluctuations in atmospheric composition, which can have a significant effect.<ref name="wfirstreturn">{{cite arXiv|eprint=1412.6097 |last1=Burrows |first1=Adam |title=Scientific Return of Coronagraphic Exoplanet Imaging and Spectroscopy Using WFIRST |class=astro-ph.EP |year=2014 }}</ref>

There is more thermal emission than reflection at some near-infrared wavelengths for massive and/or young gas giants. So, although optical brightness is fully [[Planetary phase|phase]]-dependent, this is not always the case in the near infrared.<ref name="wfirstreturn" />

Temperatures of gas giants reduce over time and with distance from their stars. Lowering the temperature increases optical albedo even without clouds. At a sufficiently low temperature, water clouds form, which further increase optical albedo. At even lower temperatures, ammonia clouds form, resulting in the highest albedos at most optical and near-infrared wavelengths.<ref name="wfirstreturn" />

=== Magnetic field ===

In 2014, a magnetic field around [[HD 209458 b]] was inferred from the way [[hydrogen]] was evaporating from the planet. It is the first (indirect) detection of a magnetic field on an exoplanet. The magnetic field is estimated to be about one-tenth as strong as Jupiter's.<ref>{{Cite web|author1=Charles Q. Choi|date=2014-11-20|title=Unlocking the Secrets of an Alien World's Magnetic Field|url=https://www.space.com/27828-alien-planet-magnetic-field-strength.html|access-date=2022-01-17|website=Space.com|language=en}}</ref><ref>{{Cite journal|doi=10.1126/science.1257829|pmid=25414310 |title=Magnetic moment and plasma environment of HD 209458b as determined from Ly observations |journal=Science |volume=346 |issue=6212 |pages=981–984 |year=2014 |last1=Kislyakova |first1=K. G.|last2=Holmstrom |first2=M. |last3=Lammer |first3=H. |last4=Odert |first4=P. |last5=Khodachenko |first5=M. L. |bibcode=2014Sci...346..981K |arxiv = 1411.6875 |s2cid=206560188}}</ref>

The magnetic fields of exoplanets are thought to be detectable by their [[Aurora (astronomy)|auroral]] [[radio]] emissions with sensitive low-frequency radio telescopes such as [[Low-Frequency Array (LOFAR)|LOFAR]], although they have yet to be found.<ref>{{Cite journal | doi = 10.1111/j.1365-2966.2011.18528.x|arxiv=1102.2737| title = Magnetosphere-ionosphere coupling at Jupiter-like exoplanets with internal plasma sources: Implications for detectability of auroral radio emissions| journal = Monthly Notices of the Royal Astronomical Society| volume = 414| issue = 3| pages = 2125–2138| year = 2011| last1 = Nichols | first1 = J. D.|doi-access=free |bibcode=2011MNRAS.414.2125N|s2cid=56567587}}</ref><ref>{{Cite web|date=2011-04-18|title=Radio Telescopes Could Help Find Exoplanets|url=https://www.redorbit.com/news/space/2031221/radio_telescopes_could_help_find_exoplanets/|access-date=2022-01-17|website=Redorbit|language=en-US}}</ref> The radio emissions could measure the rotation rate of the interior of an exoplanet, and may yield a more accurate way to measure exoplanet rotation than by examining the motion of [[cloud]]s.<ref>{{cite web|url=http://www.ece.vt.edu/swe/lwa/memo/lwa0013.pdf|title=Radio Detection of Extrasolar Planets: Present and Future Prospects|work=NRL, NASA/GSFC, NRAO, Observatoìre de Paris|access-date=15 October 2008|archive-date=30 October 2008|archive-url=https://web.archive.org/web/20081030022342/http://www.ece.vt.edu/swe/lwa/memo/lwa0013.pdf}}</ref> However, the most sensitive radio search for [[aurora]]l emissions, thus far, from nine exoplanets with Arecibo also did not result in any discoveries.<ref>{{cite journal|last1=Route|first1=Matthew|title=ROME. IV. An Arecibo Search for Substellar Magnetospheric Radio Emissions in Purported Exoplanet-hosting Systems at 5 GHz|journal=The Astrophysical Journal|date=1 May 2024|volume=966|issue=1|page=55|doi=10.3847/1538-4357/ad30ff|arxiv=2403.02226|bibcode=2024ApJ...966...55R|doi-access=free }}</ref>

[[Earth's magnetic field]] results from its flowing liquid metallic core, but on massive super-Earths with high pressure, different compounds may form which do not match those created under terrestrial conditions. Compounds may form with greater viscosities and high melting temperatures, which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles. Forms of magnesium oxide such as {{chem2|MgSi3O12}} could be a liquid metal at the pressures and temperatures found in super-Earths and could generate a magnetic field in the mantles of super-Earths.<ref name="Kean">{{cite journal|last1=Kean|first1=Sam|title=Forbidden plants, forbidden chemistry|journal=Distillations|date=2016|volume=2|issue=2|page=5|url=https://www.sciencehistory.org/distillations/magazine/forbidden-planet-forbidden-chemistry|access-date=22 March 2018|archive-date=23 March 2018|archive-url=https://web.archive.org/web/20180323154914/https://www.sciencehistory.org/distillations/magazine/forbidden-planet-forbidden-chemistry}}</ref><ref>{{Cite web |author1=Choi |first=Charles Q. |date=2012-11-22 |title=Super-Earths Get Magnetic 'Shield' from Liquid Metal |url=https://www.space.com/18604-super-earth-planets-liquid-metal.html |access-date=2022-01-17 |website=Space.com |language=en}}</ref>

[[Hot Jupiter]]s have been observed to have a larger radius than expected. This could be caused by the interaction between the [[stellar wind]] and the planet's [[magnetosphere]] creating an electric current through the planet that heats it up ([[Joule heating]]) causing it to expand. The more magnetically active a star is, the greater the stellar wind and the larger the electric current leading to more heating and expansion of the planet. This theory matches the observation that stellar activity is correlated with inflated planetary radii.<ref>{{Cite journal | doi = 10.1088/2041-8205/765/2/L25| title = Stellar Magnetic Fields As a Heating Source for Extrasolar Giant Planets| journal = The Astrophysical Journal| volume = 765| issue = 2| pages = L25| year = 2013| last1 = Buzasi | first1 = D.|arxiv = 1302.1466 |bibcode = 2013ApJ...765L..25B | s2cid = 118978422}}</ref>

In August 2018, scientists announced the transformation of gaseous [[deuterium]] into a liquid [[metallic hydrogen]] form. This may help researchers better understand [[Gas giant|giant gas planets]], such as [[Jupiter]], [[Saturn]] and related exoplanets, since such planets are thought to contain a lot of liquid metallic hydrogen, which may be responsible for their observed powerful [[magnetic field]]s.<ref name="NYT-20180816">{{cite news |last=Chang |first=Kenneth |title=Settling Arguments About Hydrogen With 168 Giant Lasers – Scientists at Lawrence Livermore National Laboratory said they were "converging on the truth" in an experiment to understand hydrogen in its liquid metallic state. |url=https://www.nytimes.com/2018/08/16/science/metallic-hydrogen-lasers.html |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2018/08/16/science/metallic-hydrogen-lasers.html |archive-date=2022-01-01 |url-access=limited |date=16 August 2018 |work=The New York Times |access-date=18 August 2018}}{{cbignore}}</ref><ref name="SCI-20180816">{{cite journal |author=Staff |title=Under pressure, hydrogen offers a reflection of giant planet interiors – Hydrogen is the most-abundant element in the universe and the simplest, but that simplicity is deceptive |url=https://www.sciencedaily.com/releases/2018/08/180816143205.htm |date=16 August 2018 |journal=[[Science Daily]] |access-date=18 August 2018}}</ref>

Although scientists previously announced that the magnetic fields of close-in exoplanets may cause increased [[stellar flare]]s and starspots on their host stars, in 2019 this claim was demonstrated to be false in the [[HD 189733]] system. The failure to detect "star-planet interactions" in the well-studied HD 189733 system calls other related claims of the effect into question.<ref>{{cite journal|last1=Route|first1=Matthew|title=The Rise of ROME. I. A Multiwavelength Analysis of the Star-Planet Interaction in the HD 189733 System|journal=The Astrophysical Journal|date=10 February 2019|volume=872|issue=1|page=79|doi=10.3847/1538-4357/aafc25|arxiv=1901.02048|bibcode=2019ApJ...872...79R|s2cid=119350145 |doi-access=free }}</ref> A later search for radio emissions from eight exoplanets that orbit within 0.1 [[astronomical unit]]s of their host stars, conducted by the [[Arecibo Telescope|Arecibo radio telescope]] also failed to find signs of these magnetic star-planet interactions.<ref>{{cite journal|last1=Route|first1=Matthew|last2=Wolszczan|first2=Alex|title=ROME. III. The Arecibo Search for Star–Planet Interactions at 5 GHz|journal=The Astrophysical Journal|date=1 August 2023|volume=952|issue=2|page=118|doi=10.3847/1538-4357/acd9ad|arxiv=2202.08899|bibcode=2023ApJ...952..118R|doi-access=free }}</ref>

In 2019, the strength of the surface magnetic fields of 4 [[hot Jupiter]]s were estimated and ranged between 20 and 120 [[Gauss (unit)|gauss]] compared to Jupiter's surface magnetic field of 4.3 gauss.<ref>{{Cite web |author1=Rabie |first=Passant |date=2019-07-29 |title=Magnetic Fields of 'Hot Jupiter' Exoplanets Are Much Stronger Than We Thought |url=https://www.space.com/hot-jupiter-magnetic-fields-measured-for-first-time.html |access-date=2022-01-17 |website=Space.com |language=en}}</ref><ref>{{Cite journal|last1=Cauley|first1=P. Wilson|last2=Shkolnik|first2=Evgenya L.|last3=Llama|first3=Joe|last4=Lanza|first4=Antonino F.|date=Dec 2019|title=Magnetic field strengths of hot Jupiters from signals of star-planet interactions|journal=Nature Astronomy|volume=3|issue=12|pages=1128–1134|doi=10.1038/s41550-019-0840-x|arxiv=1907.09068|bibcode=2019NatAs...3.1128C|s2cid=198147426|issn=2397-3366}}</ref>

In 2023 astronomers detected what might be the first sign of magnetosphere around a [[terrestrial planet|rocky]] exoplanet in the [[YZ Ceti]] system.<ref>{{Cite web |last=O'Callaghan |first=Jonathan |date=7 August 2023 |title=Exoplanets Could Help Us Learn How Planets Make Magnetism |url=https://www.quantamagazine.org/exoplanets-could-help-us-learn-how-planets-make-magnetism-20230807/ |website=[[Quanta Magazine]] }}</ref>

=== Plate tectonics ===

In 2007, two independent teams of researchers came to opposing conclusions about the likelihood of [[plate tectonics]] on larger [[super-Earth]]s<ref>{{cite journal |doi=10.1016/j.epsl.2009.07.015 |title=Convection scaling and subduction on Earth and super-Earths |date=2009 |last1=Valencia |first1=Diana |last2=O'Connell |first2=Richard J.|journal=Earth and Planetary Science Letters |volume=286 |issue=3–4 |pages=492–502 |bibcode=2009E&PSL.286..492V}}</ref><ref>{{cite journal |doi=10.1016/j.epsl.2011.07.029|title=Plate tectonics on super-Earths: Equally or more likely than on Earth|date=2011 |last1=Van Heck |first1=H.J. |last2=Tackley |first2=P.J. |journal=Earth and Planetary Science Letters |volume=310 |issue=3–4 |pages=252–261 |bibcode=2011E&PSL.310..252V}}</ref> with one team saying that plate tectonics would be episodic or stagnant<ref>{{cite journal |doi=10.1029/2007GL030598 |title=Geological consequences of super-sized Earths|date=2007 |last1=O'Neill |first1=C. |last2=Lenardic |first2=A. |s2cid=41617531|journal=Geophysical Research Letters |volume=34|issue=19|pages=L19204 |bibcode=2007GeoRL..3419204O|doi-access=free }}</ref> and the other team saying that plate tectonics is very likely on super-Earths even if the planet is dry.<ref>{{Cite journal |first1=Diana |last1=Valencia |first2=Richard J.|last2=O'Connell |first3=Dimitar D |last3=Sasselov |date=November 2007 |title=Inevitability of Plate Tectonics on Super-Earths|journal=Astrophysical Journal Letters |volume=670 |issue=1 |pages=L45–L48 |doi=10.1086/524012 |arxiv=0710.0699|bibcode=2007ApJ...670L..45V|s2cid=9432267}}</ref>

If super-Earths have more than 80 times as much water as Earth, then they become [[ocean planet]]s with all land completely submerged. However, if there is less water than this limit, then the deep water cycle would move enough water between the oceans and mantle to allow continents to exist.<ref>{{Cite web|title=Super Earths Likely To Have Both Oceans and Continents – Astrobiology|url=http://astrobiology.com/2014/01/super-earths-likely-to-have-both-oceans-and-continents.html|access-date=2022-01-17|website=astrobiology.com|date=7 January 2014 }}</ref><ref>{{Cite journal |doi=10.1088/0004-637X/781/1/27 |title=Water Cycling Between Ocean and Mantle: Super-Earths Need Not Be Waterworlds |journal=The Astrophysical Journal |volume = 781| issue=1 |page = 27 |year=2014 |last1=Cowan |first1=N. B. |last2=Abbot |first2=D. S. |bibcode=2014ApJ...781...27C |arxiv=1401.0720 |s2cid=56272100}}</ref>

=== Volcanism ===
Large surface temperature variations on [[55 Cancri e]] have been attributed to possible volcanic activity releasing large clouds of dust which blanket the planet and block thermal emissions.<ref>{{cite magazine |author=Lemonick |first=Michael D. |date=6 May 2015 |title=Astronomers May Have Found Volcanoes 40 Light-Years From Earth |url=http://news.nationalgeographic.com/2015/05/150506-volcano-planet-space-cancri-astronomy/ |archive-url=https://web.archive.org/web/20150509051828/http://news.nationalgeographic.com/2015/05/150506-volcano-planet-space-cancri-astronomy/ |archive-date=9 May 2015 |access-date=8 November 2015 |magazine=National Geographic}}</ref><ref>{{cite journal |arxiv=1505.00269 |bibcode=2016MNRAS.455.2018D |doi=10.1093/mnras/stv2239 |volume=455 |issue=2 |title=Variability in the super-Earth 55 Cnc e |journal=Monthly Notices of the Royal Astronomical Society |pages=2018–2027 |year=2015 |last1=Demory |first1=Brice-Olivier |last2=Gillon |first2=Michael |last3=Madhusudhan |first3=Nikku |last4=Queloz |first4=Didier |doi-access=free |s2cid=53662519}}</ref>

Tidal heating caused by [[gravity|gravitational]] tug of neighboring planets might lead to the emergence of volcanic activities on a terrestrial exoplanet.<ref>{{cite 
web |url= https://www.sci.news/astronomy/volcano-covered-exo-earth-lp-791-18d-11930.html |title=Volcano-Covered Terrestrial Exoplanet Found Orbiting Nearby Star |website=Sci.News}}</ref><ref name="Bello-Arufe2025">{{cite journal |last1=Bello-Arufe |first1=Aaron |last2=Damiano |first2=Mario |display-authors=etal |date=January 2025 |title=Evidence for a volcanic atmosphere on the sub-Earth L98-59b |journal=[[The Astrophysical Journal Letters]] |volume= 980|issue= 2|pages= L26|doi= 10.3847/2041-8213/adaf22|doi-access=free |arxiv=2501.18680 |bibcode=2025ApJ...980L..26B }}</ref>

=== Rings ===
In 2007, the star [[V1400 Centauri]] was occulted by an object (either a planet or brown dwarf) surrounded by an extensive disc of debris. The object, designated [[J1407b]], was long believed to host a vast planetary [[ring system]] much larger than [[Saturn's rings]].<ref>{{Cite web|title=Scientists Discover a Saturn-like Ring System Eclipsing a Sun-like Star|url=https://www.spacedaily.com/reports/Scientists_Discover_a_Saturn_like_Ring_System_Eclipsing_a_Sun_like_Star_999.html|access-date=2022-01-17|website=www.spacedaily.com}}</ref><ref>{{Cite journal | doi = 10.1088/0004-6256/143/3/72| title = Planetary Construction Zones in Occultation: Discovery of an Extrasolar Ring System Transiting a Young Sun-Like Star and Future Prospects for Detecting Eclipses by Circumsecondary and Circumplanetary Disks| journal = The Astronomical Journal| volume = 143| issue = 3| page = 72| year = 2012| last1 = Mamajek | first1 = E. E. | last2 = Quillen | first2 = A. C. | last3 = Pecaut | first3 = M. J. | last4 = Moolekamp | first4 = F. | last5 = Scott | first5 = E. L. | last6 = Kenworthy | first6 = M. A. | last7 = Cameron | first7 = A. C. | last8 = Parley | first8 = N. R. | bibcode=2012AJ....143...72M|arxiv = 1108.4070 | s2cid = 55818711}}</ref> Follow-up observations found the supposed ring system could instead be a circumplanetary disk.<ref>{{cite journal |last1=Rieder |first1=Steven |last2=Kenworthy |first2=Matthew A. |date=November 2016 |title=Constraints on the size and dynamics of the J1407b ring system |journal=Astronomy & Astrophysics |volume=596 |page=5 |arxiv=1609.08485 |bibcode=2016A&A...596A...9R |doi=10.1051/0004-6361/201629567 |s2cid=118413749 |id=A9 |doi-access=free}}</ref><ref>{{cite journal |last1=Kenworthy |first1=M. A. |last2=Klaasen |first2=P. D. |last3=Min |first3=M. |last4=van der Marel |first4=N.|author4-link=Nienke van der Marel |last5=Bohn |first5=A. J. |last6=Kama |first6=M. |display-authors=etal |date=January 2020 |title=ALMA and NACO observations towards the young exoring transit system J1407 (V1400 Cen) |journal=Astronomy & Astrophysics |volume=633 |page=6 |arxiv=1912.03314 |bibcode=2020A&A...633A.115K |doi=10.1051/0004-6361/201936141 |id=A115 |doi-access=free}}</ref>

There is strong evidence of a ring system around [[HIP 41378 f]], given the planet's measured radius is too large for its mass, the radius measurement might have been affected by a ring system around the planet.<ref name="Saillenfest2023">{{Cite journal |last1=Saillenfest |first1=M. |last2=Sulis |first2=S. |last3=Charpentier |first3=P. |last4=Santerne |first4=A. |year=2023 |title=Oblique rings from migrating exomoons: A possible origin for long-period exoplanets with enlarged radii |journal=Astronomy and Astrophysics |volume=675 |doi=10.1051/0004-6361/20234674 |doi-broken-date=12 July 2025}}</ref><ref>{{cite journal |last1=Alam |first1=Munazza K. |last2=Kirk |first2=James |last3=Dressing |first3=Courtney D. |last4=López-Morales |first4=Mercedes |last5=Ohno |first5=Kazumasa |last6=Gao |first6=Peter |last7=Akinsanmi |first7=Babatunde |last8=Santerne |first8=Alexandre |last9=Grouffal |first9=Salomé |last10=Adibekyan |first10=Vardan |last11=Barros |first11=Susana C. C. |last12=Buchhave |first12=Lars A. |last13=Crossfield |first13=Ian J. M. |last14=Dai |first14=Fei |last15=Deleuil |first15=Magali |year=2022 |title=The First Near-infrared Transmission Spectrum of HIP 41378 f, A Low-mass Temperate Jovian World in a Multiplanet System |journal=The Astrophysical Journal Letters |volume=927 |issue=1 |pages=L5 |arxiv=2201.02686 |bibcode=2022ApJ...927L...5A |doi=10.3847/2041-8213/ac559d |s2cid=245837282 |doi-access=free |last16=Giacalone |first16=Steven |last17=Lillo-Box |first17=Jorge |last18=Marley |first18=Mark |last19=Mayo |first19=Andrew W. |last20=Mortier |first20=Annelies |last21=Santos |first21=Nuno C. |last22=Sousa |first22=Sérgio G. |last23=Turtelboom |first23=Emma V. |last24=Wheatley |first24=Peter J. |last25=Vanderburg |first25=Andrew M.}}</ref>

The rings of the Solar System's gas giants are aligned with their planet's equator. However, for exoplanets that orbit close to their star, tidal forces from the star would lead to the outermost rings of a planet being aligned with the planet's orbital plane around the star. A planet's innermost rings would still be aligned with the planet's equator so that if the planet has a [[axial tilt|tilted rotational axis]], then the different alignments between the inner and outer rings would create a warped ring system.<ref>{{cite journal |arxiv=1104.3863|bibcode = 2011ApJ...734..117S |doi = 10.1088/0004-637X/734/2/117 | volume=734 |issue = 2 |title=Warm Saturns: On the Nature of Rings around Extrasolar Planets That Reside inside the Ice Line |journal=The Astrophysical Journal |page=117|year = 2011 |last1 = Schlichting |first1 = Hilke E. |last2 = Chang |first2=Philip |s2cid=42698264}}</ref>

=== Moons ===
{{Main|Exomoon}}There is evidence that moons around other planets, commonly referred to exomoons, may exist. None has been confirmed so far.{{cn|date=July 2025}}

In 2012 a candidate exomoon was detected around WASP-12b via periodic light variations in the planet's [[light curve]].<ref>[http://www.ria.ru/science/20120206/558647431.html Российские астрономы впервые открыли луну возле экзопланеты] (in Russian) - "Studying of a curve of change of shine of WASP-12b has brought to the Russian astronomers unusual result: regular splashes were found out.<...> Though stains on a star surface also can cause similar changes of shine, observable splashes are very similar on duration, a profile and amplitude that testifies for benefit of exomoon existence."</ref> Subsequent observations found this object might actually be a [[Trojan_(celestial_body)|trojan planet]].<ref>{{citation |last1=Kislyakova |first1=K. G. |title=On the ultraviolet anomalies of the WASP-12 and HD 189733 systems: Trojan satellites as a plasma source |journal=Monthly Notices of the Royal Astronomical Society |volume=461 |issue=1 |pages=988–999 |year=2016 |arxiv=1605.02507 |bibcode=2016MNRAS.461..988K |doi=10.1093/mnras/stw1110 |s2cid=119205132 |last2=Pilat-Lohinger |first2=E. |last3=Funk |first3=B. |last4=Lammer |first4=H. |last5=Fossati |first5=L. |last6=Eggl |first6=S. |last7=Schwarz |first7=R. |last8=Boudjada |first8=M. Y. |last9=Erkaev |first9=N. V. |doi-access=free}}</ref>

In December 2013, a candidate exomoon was detected in the microlensing event [[MOA-2011-BLG-262L|MOA-2011-BLG-262]], it was believed to be either a {{Earth mass|0.5|sym=y|link=y}} exomoon around a Jupiter-sized free-floating planet or a Neptune-mass planet around a red dwarf,<ref>{{Cite journal | doi = 10.1088/0004-637X/785/2/155|arxiv=1312.3951| title = MOA-2011-BLG-262Lb: A sub-Earth-mass moon orbiting a gas giant or a high-velocity planetary system in the galactic bulge| journal = The Astrophysical Journal| volume = 785| issue = 2| page = 155| year = 2014| last1 = Bennett | first1 = D. P.| last2 = Batista | first2 = V.| last3 = Bond | first3 = I. A.| last4 = Bennett | first4 = C. S.| last5 = Suzuki | first5 = D.| last6 = Beaulieu | first6 = J. -P. | last7 = Udalski | first7 = A.| last8 = Donatowicz | first8 = J.| last9 = Bozza | first9 = V.| last10 = Abe | first10 = F.| last11 = Botzler | first11 = C. S.| last12 = Freeman | first12 = M.| last13 = Fukunaga | first13 = D.| last14 = Fukui | first14 = A.| last15 = Itow | first15 = Y.| last16 = Koshimoto | first16 = N.| last17 = Ling | first17 = C. H.| last18 = Masuda | first18 = K.| last19 = Matsubara | first19 = Y.| last20 = Muraki | first20 = Y.| last21 = Namba | first21 = S.| last22 = Ohnishi | first22 = K.| last23 = Rattenbury | first23 = N. J.| last24 = Saito | first24 = T. | last25 = Sullivan | first25 = D. J.| last26 = Sumi | first26 = T.| last27 = Sweatman | first27 = W. L.| last28 = Tristram | first28 = P. J.| last29 = Tsurumi | first29 = N.| last30 = Wada | first30 = K.| display-authors = etal|bibcode=2014ApJ...785..155B|s2cid=118327512}}</ref> but follow-up observations confirmed the latter scenario.<ref name="Terry2024">{{Cite journal |arxiv=2410.09147 |first1=Sean K. |last1=Terry |first2=Jean-Philippe |last2=Beaulieu |title=A Candidate High-velocity Exoplanet System in the Galactic Bulge |date=2025 |last3=Bennett |first3=David P. |last4=Bhattacharya |first4=Aparna |last5=Hulberg |first5=Jon |last6=Huston |first6=Macy J. |last7=Koshimoto |first7=Naoki |last8=Blackman |first8=Joshua W. |last9=Bond |first9=Ian A.|journal=The Astronomical Journal |volume=169 |issue=3 |page=131 |doi=10.3847/1538-3881/ad9b0f |doi-access=free |bibcode=2025AJ....169..131T }}</ref>

On 3 October 2018, evidence suggesting a large exomoon orbiting [[Kepler-1625b]] was reported,<ref>{{Cite journal|last1=Teachey|first1=Alex|last2=Kipping|first2=David M.|date=1 October 2018|title=Evidence for a large exomoon orbiting Kepler-1625b|journal=Science Advances|language=en|volume=4|issue=10|article-number=eaav1784|doi=10.1126/sciadv.aav1784|pmid=30306135|pmc=6170104|issn=2375-2548|bibcode=2018SciA....4.1784T|arxiv=1810.02362}}</ref> and in 2021 evidence of an exomoon around Kepler-1708b was also reported.<ref>{{Cite news |title=Scientists think they've found a big, weird moon in a far-off star system |url=https://www.npr.org/2022/01/13/1072570125/kepler-large-exomoon |access-date=2022-03-28 |work=NPR.org |language=en}}</ref> Their existence, however, remain doubtful,<ref>{{cite journal |last1=Heller |first1=René |last2=Hippke |first2=Michael |date=2024 |title=Large exomoons unlikely around Kepler-1625 b and Kepler-1708 B |url=https://ui.adsabs.harvard.edu/abs/2024NatAs...8..193H/abstract |journal=Nature Astronomy |volume=8 |issue=2 |page=193 |arxiv=2312.03786 |bibcode=2024NatAs...8..193H |doi=10.1038/s41550-023-02148-w}}</ref> but follow-up observations may confirm these exomoons.<ref>{{Cite arXiv |eprint=2401.10333 |first1=David |last1=Kipping |first2=Alex |last2=Teachey |title=A Reply to: Large Exomoons unlikely around Kepler-1625 b and Kepler-1708 b |date=2024-01-18 |last3=Yahalomi |first3=Daniel A. |last4=Cassese |first4=Ben |last5=Quarles |first5=Billy |last6=Bryson |first6=Steve |last7=Hansen |first7=Brad |last8=Szulágyi |first8=Judit |last9=Burke |first9=Chri|class=astro-ph.EP }}</ref>

The detection of [[sodium]] in hot Jupiters such as [[WASP-76b]], [[HD 189733 b]] or [[WASP-49b]] is likely due to a [[Io (moon)|Io]]-like exomoon around these planets.<ref>{{Cite journal |last1=Oza |first1=Apurva V. |last2=Johnson |first2=Robert E. |last3=Lellouch |first3=Emmanuel |last4=Schmidt |first4=Carl |last5=Schneider |first5=Nick |last6=Huang |first6=Chenliang |last7=Gamborino |first7=Diana |last8=Gebek |first8=Andrea |last9=Wyttenbach |first9=Aurelien |last10=Demory |first10=Brice-Olivier |last11=Mordasini |first11=Christoph |last12=Saxena |first12=Prabal |last13=Dubois |first13=David |last14=Moullet |first14=Arielle |last15=Thomas |first15=Nicolas |date=2019-08-28 |title=Sodium and Potassium Signatures of Volcanic Satellites Orbiting Close-in Gas Giant Exoplanets |journal=[[The Astrophysical Journal]] |volume=885 |issue=2 |page=168 |arxiv=1908.10732 |bibcode=2019ApJ...885..168O |doi=10.3847/1538-4357/ab40cc |s2cid=201651224 |doi-access=free}}</ref><ref>{{cite journal |last1=Gebek |first1=Andrea |last2=Oza |first2=Apurva |date=29 July 2020 |title=Alkaline exospheres of exoplanet systems: evaporative transmission spectra |url=https://academic.oup.com/mnras/article-abstract/497/4/5271/5877918?redirectedFrom=fulltext |journal=Monthly Notices of the Royal Astronomical Society |volume=497 |issue=4 |pages=5271–5291 |arxiv=2005.02536 |bibcode=2020MNRAS.497.5271G |doi=10.1093/mnras/staa2193 |s2cid=218516741 |access-date=8 December 2020 |doi-access=free}}</ref>

=== Atmospheres ===
{{main|Exoplanet atmosphere}}
[[File:Cloudy versus clear atmospheres on two exoplanets.jpg|thumb|upright=1.6|Clear versus cloudy atmospheres on two exoplanets.<ref>{{cite web|title=Cloudy versus clear atmospheres on two exoplanets|url=https://www.spacetelescope.org/images/opo1722a/|website=www.spacetelescope.org|access-date=6 June 2017}}</ref>]]

Atmospheres have been detected around several exoplanets. The first to be observed was [[HD 209458 b]] in 2001.<ref>{{cite journal|last=Charbonneau|first=David|display-authors=etal|year=2002|title=Detection of an Extrasolar Planet Atmosphere|journal=The Astrophysical Journal|volume=568|issue=1|pages=377–384|arxiv=astro-ph/0111544|bibcode=2002ApJ...568..377C|doi=10.1086/338770|s2cid=14487268}}</ref>

[[File:PIA18410-TitanSunsetStudies-CassiniSpacecraft-20140527.jpg|thumb|upright=1.4|alt=Artist's concept of the ''Cassini'' spacecraft in front of a sunset on Saturn's moon Titan|Sunset studies on [[Titan (moon)|Titan]] by [[Cassini (spacecraft)|''Cassini'']] help understand exoplanet [[atmosphere]]s (artist's concept).]]
As of February 2014, more than fifty [[Transit method|transiting]] and five [[Direct imaging|directly imaged]] exoplanet atmospheres have been observed,<ref>{{cite book |last1=Madhusudhan|first1=Nikku|page=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 [[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>

In May 2017, glints of light from [[Earth]], seen as twinkling from an orbiting satellite a million miles away, were found to be [[Reflection (physics)|reflected light]] from [[ice crystals]] in the [[Atmosphere of Earth|atmosphere]].<ref name="NYT-20170519">{{cite news |last=St. Fleur |first=Nicholas |title=Spotting Mysterious Twinkles on Earth From a Million Miles Away |url=https://www.nytimes.com/2017/05/19/science/dscovr-satellite-ice-glints-earth-atmosphere.html |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2017/05/19/science/dscovr-satellite-ice-glints-earth-atmosphere.html |archive-date=2022-01-01 |url-access=limited |date=19 May 2017 |work=The New York Times |access-date=20 May 2017}}{{cbignore}}</ref><ref name="GRL-201760515">{{cite journal |last1=Marshak |first1=Alexander |last2=Várnai |first2=Tamás |last3=Kostinski |first3=Alexander |title=Terrestrial glint seen from deep space: oriented ice crystals detected from the Lagrangian point|date=15 May 2017 |journal=[[Geophysical Research Letters]] |doi=10.1002/2017GL073248 |volume=44 |issue=10 |pages=5197–5202|bibcode = 2017GeoRL..44.5197M |s2cid=109930589 |url=https://zenodo.org/record/1229066|hdl=11603/13118 |hdl-access=free }}</ref> The technology used to determine this may be useful in studying the atmospheres of distant worlds, including those of exoplanets.{{cn|date=July 2025}}

==== Comet-like tails ====
[[Kepler-1520b]] is a small rocky planet, very close to its star, that is evaporating and leaving a trailing tail of cloud and dust like a [[comet]].<ref>{{Cite web|last=University|first=Leiden|title=Evaporating exoplanet stirs up dust|url=https://phys.org/news/2012-08-evaporating-exoplanet.html|access-date=2022-01-17|website=phys.org|language=en}}</ref> The dust could be ash erupting from volcanos and escaping due to the small planet's low surface-gravity, or it could be from metals that are vaporized by the high temperatures of being so close to the star with the metal vapor then condensing into dust.<ref>{{Cite web|date=2012-05-18|title=New-found exoplanet is evaporating away|url=https://tgdaily.com/science/space/63469-new-found-exoplanet-is-evaporating-away/|access-date=2022-01-17|website=TGDaily|language=en-US}}</ref>

In June 2015, scientists reported that the atmosphere of [[GJ 436 b]] was evaporating, resulting in a giant cloud around the planet and, due to radiation from the host star, a long trailing tail {{convert|9|e6mi|e6km|order=flip|abbr=unit}} long.<ref name="NYT-20150625">{{cite news |last=Bhanoo |first=Sindya N. |title=A Planet with a Tail Nine Million Miles Long |url=https://www.nytimes.com/interactive/projects/cp/summer-of-science-2015/latest/exoplanet-tail |date=25 June 2015 |work=[[The New York Times]] |access-date=25 June 2015}}</ref>

=== Insolation pattern ===
[[Tidally locked]] planets in a 1:1 [[spin-orbit resonance]] would have their star always shining directly overhead on one spot, which would be hot with the opposite hemisphere receiving no light and being freezing cold. Such a planet could resemble an eyeball, with the hotspot being the pupil.<ref>{{Cite web|last=Raymond|first=Sean|date=2015-02-20|title=Forget "Earth-Like"—We'll First Find Aliens on Eyeball Planets|url=http://nautil.us/blog/forget-earth_likewell-first-find-aliens-on-eyeball-planets|access-date=2022-01-17|website=Nautilus|archive-date=23 June 2017|archive-url=https://web.archive.org/web/20170623082602/http://nautil.us/blog/forget-earth_likewell-first-find-aliens-on-eyeball-planets}}</ref> Planets with an [[Orbital eccentricity|eccentric orbit]] could be locked in other resonances. 3:2 and 5:2 resonances would result in a double-eyeball pattern with hotspots in both eastern and western hemispheres.<ref>{{cite journal |doi=10.1016/j.icarus.2014.12.017|bibcode = 2015Icar..250..395D | volume=250 | title=Insolation patterns on eccentric exoplanets |journal=Icarus |pages=395–399|year = 2015 |last1 = Dobrovolskis |first1 = Anthony R.}}</ref> Planets with both an eccentric orbit and a [[axial tilt|tilted axis of rotation]] would have more complicated insolation patterns.<ref>{{cite journal |doi=10.1016/j.icarus.2013.06.026|bibcode = 2013Icar..226..760D | volume=226 |issue = 1 | title=Insolation on exoplanets with eccentricity and obliquity|journal=Icarus |pages=760–776|year = 2013 |last1 = Dobrovolskis |first1 = Anthony R.}}</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:WASP-33b.jpg|thumb|upright=1.6|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>]]
Measuring the intensity of the light it receives from its parent star can estimate the temperature of an exoplanet. For example, the planet [[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 [[albedo]], and because factors such as the [[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 = 183–186 | year = 2007 | last1 = Knutson | first1 = H. A.  | last2 = Charbonneau | first2 = D.  | last3 = Allen | first3 = L. E. |author3-link=Lori Allen (astronomer) | 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 ==
{{see also|Astrobiology|Circumstellar habitable zone|Planetary habitability}}
As more planets are discovered, the field of [[exoplanetology]] continues to grow into a deeper study of extrasolar worlds, and will ultimately tackle the prospect of [[Astrobiology|life on planets]] beyond the [[Solar System]].<ref name="Ollivier2014">{{cite journal |date=2014|last1= Ollivier |first1=Marc |last2=Maurel |first2=Marie-Christine |title= Planetary Environments and Origins of Life |journal= Bio Web of Conferences |doi=10.1051/bioconf/20140200001|volume=2 |page=00001|doi-access=free }}</ref> At cosmic distances, [[life]] can only be detected if it is developed at a planetary scale and strongly modified the planetary environment, in such a way that the modifications cannot be explained by classical physico-chemical processes (out of equilibrium processes).<ref name="Ollivier2014"/> For example, molecular oxygen ({{O2}}) in the [[atmosphere of Earth]] is a result of [[photosynthesis]] by living plants and many kinds of microorganisms, so it can be used as an [[Biomarker|indication of life]] on exoplanets, although small amounts of oxygen could also be produced by non-biological means.<ref name='NAOJ2014'>{{cite news |url=http://astrobiology.com/2015/09/oxygen-is-not-definitive-evidence-of-life-on-extrasolar-planets.html |title=Oxygen Is Not Definitive Evidence of Life on Extrasolar Planets |work=NAOJ|publisher=Astrobiology Web |date=10 September 2015 |access-date=11 September 2015}}</ref> Furthermore, a potentially habitable planet must orbit a stable [[star]] at a distance within which [[planetary-mass object]]s with sufficient [[atmospheric pressure]] can support [[liquid water]] at their surfaces.<ref name=kopparapu-2013>{{cite journal  |title=A revised estimate of the occurrence rate of terrestrial planets in the habitable zones around kepler m-dwarfs |author=Kopparapu, Ravi Kumar |journal=The Astrophysical Journal Letters |date=2013 |volume=767 |issue=1 |doi=10.1088/2041-8205/767/1/L8 |arxiv=1303.2649 |pages=L8|bibcode = 2013ApJ...767L...8K |s2cid=119103101}}</ref><ref name="SCI-20130503">{{cite journal |last1=Cruz |first1=Maria |last2=Coontz |first2=Robert |title=Exoplanets – Introduction to Special Issue |journal=[[Science (journal)|Science]] |volume=340 |page=565|doi=10.1126/science.340.6132.565 |pmid=23641107 |issue=6132 |date=2013 |doi-access=free}}</ref>

=== Habitable zone ===
{{main|Habitable zone}}
[[File:Diagram of habitable zone rocky exoplanets, from NASA Exoplanet Archive and Gaia DR3 data.png|alt=A diagram depicting habitable zone boundaries across star type. The y-axis is stellar temperature, with the Sun (5772 Kelvin) at the top. The x-axis is the percentage of starlight that reaches the planet, ranging from 25% of Earth's starlight to 150% of Earth's starlight on the inner edge of the habitable zone. The image plots 42 exoplanets, most of which orbit red dwarfs. The coldest planets around red dwarfs are depicted as icy "eyeball" planets due to tidal locking, while most of the other planets around red dwarfs are purple, due to speculations about purple photosynthesizing creatures. Habitable one planets around yellow stars are depicted as green or blue. Earth is plotted near the top, with only Kepler-452 b close to its position.|thumb|A diagram depicting [[habitable zone]] boundaries across [[Stellar classification|star type]] with September 2024 data. [[Earth]] is plotted alongside 42 potentially rocky exoplanets within the 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 a 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: [[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>{{Cite web|last=Choi |first=Charles Q. |date=1 September 2011 |website=Astrobiology Magazine |title=Alien Life More Likely on 'Dune' Planets|url=http://www.astrobio.net/exclusive/4188/alien-life-more-likely-on-%25E2%2580%2598dune%25E2%2580%2599-planets|archive-url=https://web.archive.org/web/20131202223111/http://www.astrobio.net/exclusive/4188/alien-life-more-likely-on-%25E2%2580%2598dune%25E2%2580%2599-planets |archive-date=2 December 2013 }}</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 = 577–581| 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 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 and axial tilt|rotation rate]] is one of the major factors determining the [[#Atmospheric circulation|circulation of the atmosphere]] and hence the pattern of clouds: slowly rotating planets create thick clouds that [[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 [[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 = 607–610| 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 }}</ref>

[[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>

Planets in the habitable zones of stars with [[Metallicity|low metallicity]] are more habitable for complex life on land than high metallicity stars because the stellar spectrum of high metallicity stars is less likely to cause the formation of ozone thus enabling more ultraviolet rays to reach the planet's surface.<ref name="SA-20230419">{{cite news |last=Starr |first=Michelle |title=Scientists Think They've Narrowed Down The Star Systems Most Likely to Host Life |url=https://www.sciencealert.com/scientists-think-theyve-narrowed-down-the-star-systems-most-likely-to-host-life |date=19 April 2023 |work=[[ScienceAlert]] |access-date=19 April 2023 }}</ref><ref name="NC-20230418">{{cite journal |author=Shapiro, Anna V. |display-authors=et al. |title=Metal-rich stars are less suitable for the evolution of life on their planets |date=18 April 2023 |journal=[[Nature Communications]] |volume=14 |issue=1893 |article-number=1893 |doi=10.1038/s41467-023-37195-4 |pmid=37072387 |pmc=10113254 |bibcode=2023NatCo..14.1893S }}</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>{{Cite news|date=2014-01-07|title=Further away planets 'can support life' say researchers|language=en-GB|work=BBC News|url=https://www.bbc.com/news/uk-scotland-north-east-orkney-shetland-25639306|access-date=2023-02-12}}</ref> even [[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 [[universe]] the temperature of the [[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 [[Habitability of natural satellites|habitable moons]].<ref>{{Cite web|first=Andy |last=Ridgway |date=29 July 2015 |title=Home, sweet exomoon: The new frontier in the search for ET|url=https://www.newscientist.com/article/mg22730320-300-home-sweet-exomoon-the-new-frontier-in-the-search-for-et/|access-date=2023-02-12|website=New Scientist|language=en-US}}</ref>

=== Ice ages and snowball states ===
{{See also|Ice age|Snowball Earth}}
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 [[Solution weathering|carbon dioxide weathering]], resulting in a build-up of carbon dioxide in the atmosphere from volcanic emissions. This creates a [[greenhouse effect]] which thaws the planet again. Planets with a large [[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>{{Cite web|last=Kelley |first=Peter |date=15 April 2014 |title=Astronomers: 'Tilt-a-worlds' could harbor life|url=https://www.washington.edu/news/2014/04/15/astronomers-tilt-a-worlds-could-harbor-life/|access-date=2023-02-12|website=UW News|language=en}}</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>{{Cite web|last=Kelley |first=Peter |date=18 July 2013 |title=A warmer planetary haven around cool stars, as ice warms rather than cools|url=https://www.washington.edu/news/2013/07/18/a-warmer-planetary-haven-around-cool-stars-as-ice-warms-rather-than-cools/|access-date=2023-02-12|website=UW News|language=en}}</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. |author-link2=Cecilia Bitz| 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 [[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 [[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 [[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. | doi-access = free| bibcode = 2008MNRAS.391..237J|arxiv = 0808.2770 | s2cid = 19930771}}</ref>

=== Potentially habitable planets ===
{{See also|List of potentially habitable exoplanets|List of nearest terrestrial exoplanet candidates}}

A review in 2015 identified exoplanets [[Kepler-62f]], [[Kepler-186f]] and [[Kepler-442b]] as the best candidates for being potentially habitable.<ref name="centauridreams">{{cite web |author=Gilster |first1=Paul |last2=LePage |first2=Andrew |date=2015-01-30 |title=A Review of the Best Habitable Planet Candidates |url=http://www.centauri-dreams.org/?p=32470 |access-date=2015-07-24 |publisher=Centauri Dreams, Tau Zero Foundation}}</ref> These are at a distance of 1000, 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 [[red dwarf]] star.<ref>{{cite web|url=https://www.nasa.gov/news-release/nasas-kepler-telescope-discovers-first-earth-size-planet-in-habitable-zone/|title=NASA's Kepler Telescope Discovers First Earth-Size Planet in 'Habitable Zone'|publisher=[[NASA]]|date=17 April 2014}}</ref>
When looking at the nearest terrestrial exoplanet candidates, [[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 |author=Bignami |first=Giovanni F. |url=https://books.google.com/books?id=crvpCQAAQBAJ&pg=PA110 |title=The Mystery of the Seven Spheres: How Homo sapiens will Conquer Space |publisher=Springer |year=2015 |isbn=978-3-319-17004-6 |page=110}}</ref>

{{#chart:Exoplanets_by_radius.chart|data=Exoplanets_by_radius.tab}}

==== Earth-size planets ====
{{See also|Earth analog}}
* In November 2013, it was estimated that 22±8% of Sun-like<ref group="lower-alpha" name="footnoteA"/> stars in the Milky Way galaxy may have an Earth-sized<ref group="lower-alpha" name="footnoteB"/> planet in the habitable<ref group=lower-alpha name=footnoteC/> zone.<ref name="ucb1in5" /><ref name="earthsunhzprev" /> Assuming 200 billion stars in the Milky Way,<ref group="lower-alpha" name="footnoteD"/> that would be 11 billion potentially habitable Earths, rising to 40 billion if [[red dwarf]]s are included.<ref name="LATimes-20131104"/>
* [[Kepler-186f]], a 1.2-Earth-radius planet in the habitable zone of a [[red dwarf]], was reported in April 2014.
* Proxima Centauri b, a planet in the habitable zone of [[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]] 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 [[Kepler-452b]] is the first discovered near-Earth-size planet in the [[Circumstellar habitable zone|"habitable zone"]] around a [[G star|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>
{| class="wikitable" style="margin:0.5em auto; width:450px; align=right"
! Notable Exoplanets – [[Kepler (spacecraft)|Kepler Space Telescope]]
|-
| style="font-size:88%" | [[File:PIA19827-Kepler-SmallPlanets-HabitableZone-20150723.jpg|600px]]
{{center|Comparison of small planets found by ''[[Kepler (spacecraft)|Kepler]]'' in the [[habitable zone]] of their host stars.}}
|}

== Planetary system ==
{{Main|Planetary system}}

Exoplanets are often members of planetary systems of multiple planets around a star. The planets interact with each other gravitationally and sometimes form resonant systems where the orbital periods of the planets are in integer ratios. The [[Kepler-223]] system contains four planets in an 8:6:4:3 [[orbital resonance]].<ref>{{cite web|last=Emspak|first=Jesse|title=Kepler Finds Bizarre Systems|url=http://www.ibtimes.com/articles/117984/20110302/kepler-finds-strange-worlds-fastest-planet.htm|work=International Business Times|date=March 2, 2011|publisher=International Business Times Inc.|access-date=March 2, 2011}}</ref>

Some [[hot Jupiter]]s orbit their stars in the [[Retrograde and prograde motion#Exoplanets|opposite direction]] to their stars' rotation.<ref name="2010question">{{cite web|url=http://www.astro.gla.ac.uk/nam2010/pr10.php|title=NAM2010 at the University of Glasgow|access-date=2010-04-15|archive-date=2011-07-16|archive-url=https://web.archive.org/web/20110716051715/http://www.astro.gla.ac.uk/nam2010/pr10.php}}</ref> One proposed explanation is that hot Jupiters tend to form in dense clusters, where [[Perturbation (astronomy)|perturbations]] are more common and [[gravitational capture]] of planets by neighboring stars is possible.<ref>{{cite web |author=Sutter |first=Paul M. |date=December 9, 2022 |title=Trading spaces: How swapping stars create hot Jupiters |url=https://phys.org/news/2022-12-spaces-swapping-stars-hot-jupiters.html |agency=Universe Today}}</ref>

== Search projects ==
* [[Extremely Large Telescope#Instrumentation|ANDES]] – The ArmazoNes High Dispersion Echelle Spectrograph, a planet finding and planet characterisation spectrograph, is expected to be fitted onto [[Extremely Large Telescope|ESO's ELT 39.3m telescope]]. ANDES was formally known as HIRES, which itself was created after a merger of the consortia behind the earlier CODEX (optical high-resolution) and SIMPLE (near-infrared high-resolution) spectrograph concepts.{{cn|date=July 2025}}
* [[CoRoT]] – Space telescope that found the first transiting rocky planet.<ref>{{Cite journal |last1=Moutou |first1=Claire |last2=Deleuil |first2=Magali |last3=Guillot |first3=Tristan |last4=Baglin |first4=Annie |last5=Bordé |first5=Pascal |last6=Bouchy |first6=Francois |last7=Cabrera |first7=Juan |last8=Csizmadia |first8=Szilárd |last9=Deeg |first9=Hans J. |date=2013-11-01 |title=CoRoT: Harvest of the exoplanet program |url=https://www.sciencedirect.com/science/article/pii/S0019103513001383 |journal=Icarus |volume=226 |issue=2 |pages=1625–1634 |doi=10.1016/j.icarus.2013.03.022 |issn=0019-1035|arxiv=1306.0578 |bibcode=2013Icar..226.1625M }}</ref>
* [[ESPRESSO]] – A rocky planet-finding, and stable spectroscopic observing, spectrograph mounted on ESO's 4 × 8.2 m [[Very Large Telescope|VLT]] telescope, sited on the levelled summit of [[Cerro Paranal]] in the [[Atacama Desert]] of northern Chile.
* [[High Accuracy Radial Velocity Planet Searcher|HARPS]] – High-precision [[Echelle grating|echelle]] planet-finding [[spectrograph]] installed on the [[ESO 3.6 m Telescope|ESO's 3.6m telescope]] at [[La Silla Observatory]] in [[Chile]].{{cn|date=July 2025}}
* [[Kepler space telescope|Kepler]] – Mission to look for large numbers of exoplanets using the transit method.{{cn|date=July 2025}}
* [[Transiting Exoplanet Survey Satellite|TESS]] – Mission to search for new exoplanets, active from 2018 to 2020 and rotating to observe stars from all over the sky. {{As of|2025|03|22}}, TESS had identified 7,525 candidate exoplanets, of which 618 had been confirmed.<ref>{{cite web |title=Transiting Exoplanets Survey Satellite (TESS) |url=https://exoplanetarchive.ipac.caltech.edu/index.html|work=Exoplanet Exploration: Planets Beyond our Solar System |publisher=CalTech}} {{PD-notice}}</ref>

== See also ==
{{Portal|Biology|Astronomy|Stars|Spaceflight|Outer space|Solar System|Science}}
* [[Detecting Earth from distant star-based systems]]
* [[Extrasolar Planets Encyclopedia]]
* [[Extrasolar planets in fiction]]
* [[Habitable zone for complex life]]
* [[Lists of exoplanets]]
* [[NASA Exoplanet Archive]]
* [[Planetary capture]]
{{Clear}}

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

== References ==
{{Reflist}}

== Further reading ==
* {{Cite book |last=Boss |first=Alan |year=2009 |title=The Crowded Universe: The Search for Living Planets |publisher=Basic Books |bibcode=2009cusl.book.....B|ref=none}} {{ISBN|978-0-465-00936-7}} (Hardback); {{ISBN|978-0-465-02039-3}} (Paperback).
* {{Cite book |last=Dorminey |first=Bruce |year=2001 |title=Distant Wanderers |title-link=Distant Wanderers (book) |publisher=Springer-Verlag |ref=none}} {{ISBN|978-0-387-95074-7}} (Hardback); {{ISBN|978-1-4419-2872-6}} (Paperback).
* {{Cite book |last=Jayawardhana |first=Ray |year=2011 |title=Strange New Worlds: The Search for Alien Planets and Life Beyond Our Solar System |location=Princeton, NJ |publisher=Princeton University Press |isbn=978-0-691-14254-8 |ref=none}}  (Hardcover.)
* {{cite book |last1=van Dishoeck |first1=Ewine F. |page=835 |last2= Bergin |first2=Edwin A. |last3= Lis |first3=Dariusz C. |last4= Lunine |first4=Jonathan I. |year=2014 |chapter=Water: From Clouds to Planets |title=Protostars and Planets VI |arxiv=1401.8103 |bibcode=2014prpl.conf..835V |doi=10.2458/azu_uapress_9780816531240-ch036 |isbn=978-0-8165-3124-0 |s2cid=55875067|ref=none}}
* {{Cite book |last1=Villard |first1=Ray |last2=Cook |first2=Lynette R. |year=2005 |title=Infinite Worlds: An Illustrated Voyage to Planets Beyond Our Sun |publisher=University of California Press |title-link=Infinite Worlds: An Illustrated Voyage to Planets Beyond Our Sun|ref=none}} {{ISBN|978-0-520-23710-0}}
* {{Cite book |last=Yaqoob |first=Tahir |year=2011 |title=Exoplanets and Alien Solar Systems |publisher=New Earth Labs (Education and Outreach) |isbn=978-0-9741689-2-0|ref=none}} Paperback.

== External links ==
{{Commons category|Exoplanets}}
{{Wikiversity|Observational astronomy/Extrasolar planet}}
{{Scholia|topic}}
* [https://exoplanet.eu/home/ The Extrasolar Planets Encyclopaedia] ([[Paris Observatory]])
* [http://exoplanetarchive.ipac.caltech.edu/ NASA Exoplanet Archive]

{{Exoplanet}}
{{Exoplanet search projects}}
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