Editing Exoplanet (section)

{{Short description|Planet outside 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 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]] |accessdate=2 April 2022}}</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 [[Extraterrestrial life|potential for life]].<ref name="SA-20230123">{{cite news |last=O'Callaghan |first=Jonthan |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]] |accessdate=23 January 2023 }}</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> In several cases, [[List of multiplanetary systems|multiple planets]] have been observed around a 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">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]] (also known as PSR B1257+12 A or PSR B1257+12 b), 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.

Almost all planets detected so far are within the Milky Way. However, there is evidence that [[extragalactic planet]]s, exoplanets located in other galaxies, may exist.<ref name="NG-20180205">{{cite web |last=Zachos |first=Elaine |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|url-status=dead|archive-date=28 April 2021|date=5 February 2018 |work=[[National Geographic Society]] |access-date=5 February 2018 }}</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 [[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>

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>

[[Rogue planets]] are those that are not in [[planetary system]]s. 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|url-status=dead|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>