Editing Planet (section)

== Exoplanets ==
{{Main|Exoplanet}}
[[File:Exoplanet detections per year.png|alt=Exoplanet detections per year |thumb|upright=1.2|Exoplanet detections per year as of August 2023 (by [[NASA Exoplanet Archive]])<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=24 June 2022 |archive-date=30 April 2012 |archive-url=https://web.archive.org/web/20120430080407/https://exoplanetarchive.ipac.caltech.edu/exoplanetplots/ |url-status=live }}</ref>]]
An [[exoplanet]] is a planet outside the Solar System. {{Extrasolar planet counts|full}} Known exoplanets range in size from gas giants [[WASP-17b|about twice as large as Jupiter]] down to [[Kepler-37b|just over the size of the Moon]]. Analysis of [[gravitational microlensing]] data suggests a minimum average of 1.6 bound planets for every star in the [[Milky Way]].<ref name="nature.com">{{cite journal |display-authors=4 |last1=Cassan |first1=Arnaud | first2=D. | last2=Kubas | first3=J.-P. | last3=Beaulieu | first4=M. | last4=Dominik | first5=K. | last5=Horne | first6=J. | last6=Greenhill | first7=J. | last7=Wambsganss | first8=J. | last8=Menzies | first9=A. | last9=Williams | first10=U. G. | last10=Jørgensen | first11=A. | last11=Udalski | first12=D. P. | last12=Bennett | first13=M. D. | last13=Albrow | first14=V. | last14=Batista | first15=S. | last15=Brillant | first16=J. A. R. | last16=Caldwell | first17= A. | last17=Cole | first18=Ch. | last18=Coutures | first19=K. H. | last19=Cook | first20=S. | last20=Dieters | first21=D. | last21=Dominis Prester | first22=J. | last22=Donatowicz | first23=P. | last23=Fouqué | first24=K. | last24=Hill | first25=N. | last25=Kains |title=One or more bound planets per Milky Way star from microlensing observations |journal=Nature |date=12 January 2012 |volume=481 |pages=167–169 |doi=10.1038/nature10684 |issue=7380 |bibcode=2012Natur.481..167C |pmid=22237108 |arxiv=1202.0903|s2cid=2614136 }}</ref>

In early 1992, radio astronomers [[Aleksander Wolszczan]] and [[Dale Frail]] announced the discovery of two planets orbiting the [[pulsar]] [[PSR 1257+12]].<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. Researchers suspect they formed from a disk remnant left over from the [[supernova]] that produced the pulsar.<ref>{{Cite journal |title=Planets Around the Pulsar PSR B1257+12 |url=https://adsabs.harvard.edu/full/2008ASPC..398....3W |access-date=13 May 2022 |journal=Extreme Solar Systems |bibcode=2008ASPC..398....3W |last1=Wolszczan |first1=Alex |year=2008 |volume=398 |pages=3+ |archive-date=13 May 2022 |archive-url=https://web.archive.org/web/20220513200823/https://adsabs.harvard.edu/full/2008ASPC..398....3W |url-status=live }}</ref>

The first confirmed discovery of an exoplanet orbiting an ordinary [[main sequence|main-sequence]] star occurred on 6 October 1995, when [[Michel Mayor]] and [[Didier Queloz]] of the [[University of Geneva]] announced the detection of [[51 Pegasi b]], an exoplanet around [[51 Pegasi]].<ref name=whatworlds>{{cite web |url=http://www.cbc.ca/news/technology/what-worlds-are-out-there-a-field-guide-to-exoplanets-1.3734359 |title=What worlds are out there? |work=[[Canadian Broadcasting Corporation]] |language=en |date=25 August 2016 |access-date=5 June 2017 |archive-date=25 August 2016 |archive-url=https://web.archive.org/web/20160825161047/http://www.cbc.ca/news/technology/what-worlds-are-out-there-a-field-guide-to-exoplanets-1.3734359 |url-status=live }}</ref> From then until the [[Kepler space telescope]] mission, most of the known exoplanets were gas giants comparable in mass to Jupiter or larger as they were more easily detected. The [[List of exoplanets discovered using the Kepler space telescope|catalog of Kepler candidate planets]] consists mostly of planets the size of Neptune and smaller, down to smaller than Mercury.<ref>{{Cite web |last=Chen |first=Rick |date=23 October 2018 |title=Top Science Results from the Kepler Mission |url=http://www.nasa.gov/kepler/topscience |access-date=11 July 2022 |website=NASA |quote=The most common size of planet Kepler found doesn't exist in our solar system—a world between the size of Earth and Neptune—and we have much to learn about these planets. |archive-date=11 July 2022 |archive-url=https://web.archive.org/web/20220711224819/https://www.nasa.gov/kepler/topscience/ |url-status=dead }}</ref><ref name="Barclay-2013">{{Cite journal |last1=Barclay |first1=Thomas |last2=Rowe |first2=Jason F. |last3=Lissauer |first3=Jack J. |last4=Huber |first4=Daniel |last5=Fressin |first5=François |last6=Howell |first6=Steve B. |last7=Bryson |first7=Stephen T. |last8=Chaplin |first8=William J. |last9=Désert |first9=Jean-Michel |last10=Lopez |first10=Eric D. |last11=Marcy |first11=Geoffrey W. |display-authors=4 |date=28 February 2013 |title=A sub-Mercury-sized exoplanet |url=http://www.nature.com/articles/nature11914 |journal=Nature |language=en |volume=494 |issue=7438 |pages=452–454 |doi=10.1038/nature11914 |pmid=23426260 |arxiv=1305.5587 |bibcode=2013Natur.494..452B |s2cid=205232792 |issn=0028-0836 |access-date=11 July 2022 |archive-date=19 October 2022 |archive-url=https://web.archive.org/web/20221019020640/https://www.nature.com/articles/nature11914 |url-status=live }}</ref>

In 2011, the [[Kepler space telescope]] team reported the discovery of the first Earth-sized exoplanets orbiting a [[Solar analog|Sun-like star]], [[Kepler-20e]] and [[Kepler-20f]].<ref name="NASA-20111220">{{cite web|last=Johnson|first=Michele|title=NASA Discovers First Earth-size Planets Beyond Our Solar System|url=http://www.nasa.gov/mission_pages/kepler/news/kepler-20-system.html|publisher=[[NASA]]|date=20 December 2011|access-date=20 December 2011|archive-date=16 May 2020|archive-url=https://web.archive.org/web/20200516230313/https://www.nasa.gov/mission_pages/kepler/news/kepler-20-system.html|url-status=dead}}</ref><ref name="Nature-20111220">{{cite journal |last=Hand |first=Eric |title=Kepler discovers first Earth-sized exoplanets |doi=10.1038/nature.2011.9688 |date=20 December 2011 |journal=[[Nature (journal)|Nature]]|s2cid=122575277 }}</ref><ref name="NYT-20111220">{{cite news |last=Overbye |first=Dennis |title=Two Earth-Size Planets Are Discovered |url=https://www.nytimes.com/2011/12/21/science/space/nasas-kepler-spacecraft-discovers-2-earth-size-planets.html |date=20 December 2011 |newspaper=The New York Times |access-date=21 December 2011 |archive-date=20 December 2011 |archive-url=https://web.archive.org/web/20111220234625/http://www.nytimes.com/2011/12/21/science/space/nasas-kepler-spacecraft-discovers-2-earth-size-planets.html |url-status=live }}</ref> Since that time, more than 100 planets have been identified that are approximately the [[Earth radius|same size as Earth]], 20 of which orbit in the [[circumstellar habitable zone|habitable zone]] of their star—the range of orbits where a terrestrial planet could sustain liquid water on its surface, given enough atmospheric pressure.<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 |page=L8|bibcode = 2013ApJ...767L...8K|s2cid=119103101 }}</ref><ref>{{cite web|first=Traci|last=Watson|url=https://www.usatoday.com/story/news/2016/05/10/kepler-finds-new-planets/84187098/|title=NASA discovery doubles the number of known planets|date=10 May 2016|work=[[USA Today]]|access-date=10 May 2016|archive-date=10 May 2016|archive-url=https://web.archive.org/web/20160510184735/http://www.usatoday.com/story/news/2016/05/10/kepler-finds-new-planets/84187098/|url-status=live}}</ref><ref>{{Cite web |url=http://phl.upr.edu/projects/habitable-exoplanets-catalog |title=The Habitable Exoplanets Catalog |website=Planetary Habitability Laboratory |publisher=University of Puerto Rico at Arecibo |access-date=12 July 2022 |archive-date=20 October 2011 |archive-url=https://web.archive.org/web/20111020180543/http://phl.upr.edu/projects/habitable-exoplanets-catalog |url-status=live }}</ref> One in five Sun-like stars is thought to have an Earth-sized planet in its habitable zone, which suggests that the nearest would be expected to be within 12&nbsp;[[light-year]]s distance from Earth.{{efn|name=1in5earthsized|Here, "Earth-sized" means 1–2 Earth radii, and "habitable zone" means the region with 0.25 to 4 times Earth's stellar flux (corresponding to 0.5–2 AU for the Sun). Data for [[G-type star]]s like the Sun is not available. This statistic is an extrapolation from data on [[K-type star]]s.<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|access-date=7 November 2013|archive-url=https://web.archive.org/web/20141107081158/http://newscenter.berkeley.edu/2013/11/04/astronomers-answer-key-question-how-common-are-habitable-planets/|archive-date=7 November 2014}}</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>}} The frequency of occurrence of such terrestrial planets is one of the variables in the [[Drake equation]], which estimates the number of [[Extraterrestrial life|intelligent, communicating civilizations]] that exist in the Milky Way.<ref>{{cite news | last=Drake |first=Frank |title=The Drake Equation Revisited |publisher=Astrobiology Magazine |date=29 September 2003 |url=http://www.astrobio.net/index.php?option=com_retrospection&task=detail&id=610 |archive-url=https://web.archive.org/web/20110628180502/http://www.astrobio.net/index.php?option=com_retrospection&task=detail&id=610 |archive-date=28 June 2011 |access-date=23 August 2008 |url-status=usurped}}</ref>

There are types of planets that do not exist in the Solar System: [[super-Earth]]s and [[mini-Neptune]]s, which have masses between that of Earth and Neptune. Objects less than about twice the mass of Earth are expected to be rocky like Earth; beyond that, they become a mixture of volatiles and gas like Neptune.<ref name="ChenKipping">{{cite journal |last1=Chen |first1=Jingjing |last2=Kipping |first2=David |date=2016 |title=Probabilistic Forecasting of the Masses and Radii of Other Worlds |journal=The Astrophysical Journal |volume=834 |issue=1 |page=17 |arxiv=1603.08614 |doi=10.3847/1538-4357/834/1/17 |s2cid=119114880 |doi-access=free |bibcode=2017ApJ...834...17C }}</ref> The planet [[Gliese 581c]], with a mass 5.5–10.4 times the mass of Earth,<ref>{{cite journal |doi=10.1051/0004-6361/200912172 |bibcode=2009A&A...507..487M |journal=[[Astronomy and Astrophysics]] |volume=507 |issue=1 |year=2009 |pages=487–494 |last1=Mayor |first1=Michel |last2=Bonfils |first2=Xavier |last3=Forveille |first3=Thierry |last4=Delfosse |first4=Xavier |last5=Udry |first5=Stéphane |last6=Bertaux |first6=Jean-Loup |last7=Beust |first7=Hervé |last8=Bouchy |first8=François |last9=Lovis |first9=Christophe |last10=Pepe |first10=Francesco |last11=Perrier |first11=Christian |last12=Queloz |first12=Didier |last13=Santos |first13=Nuno C. |title=The HARPS search for southern extra-solar planets, XVIII. An Earth-mass planet in the GJ 581 planetary system |arxiv=0906.2780 |display-authors=3 |url=http://obswww.unige.ch/~udry/Gl581_preprint.pdf |s2cid=2983930 |archive-url=https://web.archive.org/web/20090521052641/http://obswww.unige.ch/~udry/Gl581_preprint.pdf |archive-date=21 May 2009 }}</ref> attracted attention upon its discovery for potentially being in the habitable zone,<ref name="BBC1">{{cite news |url=http://news.bbc.co.uk/1/hi/sci/tech/6589157.stm |title=New 'super-Earth' found in space |access-date=25 April 2007 |date=25 April 2007 |work=BBC News |archive-date=10 November 2012 |archive-url=https://web.archive.org/web/20121110172300/http://news.bbc.co.uk/2/hi/science/nature/6589157.stm |url-status=live }}</ref> though later studies concluded that it is actually too close to its star to be habitable.<ref name="blo07">{{cite journal|bibcode=2007A&A...476.1365V|author=von Bloh|display-authors=etal|date=2007|title=The Habitability of Super-Earths in Gliese 581 |journal=[[Astronomy and Astrophysics]]|volume=476|issue=3|pages=1365–1371|doi=10.1051/0004-6361:20077939 |arxiv = 0705.3758 |s2cid=14475537}}</ref> Planets more massive than Jupiter are also known, extending seamlessly into the realm of brown dwarfs.<ref name=Hatzes/>

Exoplanets have been found that are much closer to their parent star than any planet in the Solar System is to the Sun. Mercury, the closest planet to the Sun at 0.4&nbsp;[[astronomical unit|AU]], takes 88&nbsp;days for an orbit, but [[ultra-short period planet]]s can orbit in less than a day. The [[Kepler-11]] system has five of its planets in shorter orbits than Mercury's, all of them much more massive than Mercury. There are [[hot Jupiter]]s, such as 51 Pegasi b,<ref name=whatworlds/> that orbit very close to their star and may evaporate to become [[chthonian planet]]s, which are the leftover cores. There are also exoplanets that are much farther from their star. Neptune is 30&nbsp;AU from the Sun and takes 165&nbsp;years to orbit, but there are exoplanets that are thousands of AU from their star and take more than a million&nbsp;years to orbit (e.g. [[COCONUTS-2b]]).<ref>{{Cite journal |last1=Zhang |first1=Zhoujian |last2=Liu |first2=Michael C. |last3=Claytor |first3=Zachary R. |last4=Best |first4=William M. J. |last5=Dupuy |first5=Trent J. |last6=Siverd |first6=Robert J. | display-authors=4 |date=1 August 2021 |title=The Second Discovery from the COCONUTS Program: A Cold Wide-orbit Exoplanet around a Young Field M Dwarf at 10.9 pc |journal=The Astrophysical Journal Letters |volume=916 |issue=2 |page=L11 |doi=10.3847/2041-8213/ac1123 |arxiv=2107.02805 |bibcode=2021ApJ...916L..11Z |hdl=20.500.11820/4f26e8e5-5d42-4259-bc20-fcb093d664b6 |s2cid=236464073 |issn=2041-8205 |doi-access=free }}</ref>

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