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-#REDIRECT [[Ares]]
+[[File:Planet_collage_to_scale.jpg|thumb|upright=1.5|The eight planets of the [[Solar System]] with size to scale (up to down, left to right): [[Saturn]], [[Jupiter]], [[Uranus]], [[Neptune]] (outer planets), [[Earth]], [[Venus]], [[Mars]], and [[Mercury (planet)|Mercury]] (inner planets)]]
+A '''planet''' is a large, [[Hydrostatic equilibrium|rounded]] [[Astronomical object|astronomical body]] that is generally required to be in [[orbit]] around a [[star]], [[stellar remnant]], or [[brown dwarf]], and is not one itself.<ref>{{Cite journal |last=Lecavelier des Etangs |first=A. |last2=Lissauer |first2=Jack J. |date=2022-06-01 |title=The IAU working definition of an exoplanet |url=https://www.sciencedirect.com/science/article/pii/S138764732200001X |journal=New Astronomy Reviews |volume=94 |pages=101641 |doi=10.1016/j.newar.2022.101641 |issn=1387-6473}}</ref> The [[Solar System]] has eight planets by the most restrictive definition of the term: the [[terrestrial planet]]s [[Mercury (planet)|Mercury]], [[Venus]], [[Earth]], and [[Mars]], and the [[giant planet]]s [[Jupiter]], [[Saturn]], [[Uranus]], and [[Neptune]]. The best available theory of planet formation is the [[nebular hypothesis]], which posits that an [[interstellar cloud]] collapses out of a [[nebula]] to create a young [[protostar]] orbited by a [[protoplanetary disk]]. Planets grow in this disk by the gradual accumulation of material driven by [[gravity]], a process called [[accretion (astrophysics)|accretion]].
+The word ''planet'' comes from the Greek {{lang|grc|[[wikt:πλανήτης#Ancient Greek|πλανήται]]}} ({{Transliteration|grc|planḗtai}}) {{gloss|wanderers}}. In [[Classical antiquity|antiquity]], this word referred to the [[Sun]], [[Moon]], and five points of light visible to the naked eye that moved across the background of the stars—namely, Mercury, Venus, Mars, Jupiter, and Saturn. Planets have historically had religious associations: [[History of astronomy#Early History|multiple cultures]] identified celestial bodies with gods, and these connections with mythology and [[folklore]] persist in the schemes for naming newly discovered Solar System bodies. Earth itself was recognized as a planet when [[heliocentrism]] supplanted [[Geocentric model|geocentrism]] during the 16th and 17th centuries.
+With the development of the [[telescope]], the meaning of ''planet'' broadened to include objects only visible with assistance: the [[natural satellite|moons]] of the planets beyond Earth; the [[ice giant]]s Uranus and Neptune; [[Ceres (dwarf planet)|Ceres]] and other bodies later recognized to be part of the [[asteroid belt]]; and [[Pluto (dwarf planet)|Pluto]], later found to be the largest member of the collection of icy bodies known as the [[Kuiper belt]]. The discovery of other large objects in the Kuiper belt, particularly [[Eris (dwarf planet)|Eris]], spurred debate about how exactly to define a planet. In 2006, the [[International Astronomical Union]] (IAU) adopted a definition of a planet in the Solar System, placing the four terrestrial planets and the four giant planets in the planet category; Ceres, Pluto, and Eris are in the category of [[dwarf planet]].<ref name="IAU">{{cite web |title=IAU 2006 General Assembly: Result of the IAU Resolution votes |url=http://www.iau.org/news/pressreleases/detail/iau0603/ |publisher=International Astronomical Union |date=2006 |access-date=30 December 2009 |archive-date=29 April 2014 |archive-url=https://web.archive.org/web/20140429212224/http://iau.org/news/pressreleases/detail/iau0603/ |url-status=live }}</ref><ref name="WSGESP">{{cite web|date=2001 |title=Working Group on Extrasolar Planets (WGESP) of the International Astronomical Union |work=IAU |url=http://www.dtm.ciw.edu/boss/definition.html |access-date=23 August 2008 |archive-url=https://web.archive.org/web/20060916161707/http://www.dtm.ciw.edu/boss/definition.html |archive-date=16 September 2006 }}</ref><ref name=planetarysociety>{{cite web |first=Emily |last=Lakdawalla |author-link=Emily Lakdawalla |url=https://www.planetary.org/worlds/what-is-a-planet |title=What Is A Planet? |website=The Planetary Society |date=21 April 2020 |access-date=3 April 2022 |archive-date=22 January 2022 |archive-url=https://web.archive.org/web/20220122142140/https://www.planetary.org/worlds/what-is-a-planet |url-status=live }}</ref> Many [[Planetary science|planetary scientists]] have nonetheless continued to apply the term ''planet'' more broadly, including dwarf planets as well as rounded satellites like the Moon.<ref>{{Cite web|url=https://www.sciencenews.org/article/pluto-planet-vote-status-definition-demotion|title=The definition of planet is still a sore point – especially among Pluto fans|date=24 August 2021|first=Lisa|last=Grossman|website=[[Science News]]|access-date=10 July 2022|archive-date=10 July 2022|archive-url=https://web.archive.org/web/20220710033107/https://www.sciencenews.org/article/pluto-planet-vote-status-definition-demotion|url-status=live}}</ref>
+Further advances in astronomy led to the discovery of over 5,900 planets outside the Solar System, termed [[exoplanet]]s. These often show unusual features that the Solar System planets do not show, such as [[hot Jupiter]]s—giant planets that orbit close to their parent stars, like [[51 Pegasi b]]—and extremely [[Orbital eccentricity|eccentric orbits]], such as [[HD 20782 b]]. The discovery of brown dwarfs and planets larger than Jupiter also spurred debate on the definition, regarding where exactly to draw the line between a planet and a star. Multiple exoplanets have been found to orbit in the [[circumstellar habitable zone|habitable zones]] of their stars (where liquid water can potentially exist on a [[planetary surface]]), but Earth remains the only planet known to support [[life]].
+== Formation ==
+{{Main|Nebular hypothesis}}
+{{multiple image
+| direction         = horizontal
+| header            = Artists' impressions
+| image1            = The_Mysterious_Case_of_the_Disappearing_Dust.jpg
+| caption1          = A protoplanetary disk
+| image2            = PIA18469-AsteroidCollision-NearStarNGC2547-ID8-2013.jpg
+| caption2          = Protoplanets colliding during planet formation
+| total_width       = 400
+}}
+It is not known with certainty how planets are formed. The prevailing theory is that they coalesce during the collapse of a [[nebula]] into a thin disk of gas and dust. A [[protostar]] forms at the core, surrounded by a rotating [[protoplanetary disk]]. Through [[Accretion (astrophysics)|accretion]] (a process of sticky collision) dust particles in the disk steadily accumulate [[mass]] to form ever-larger bodies. Local concentrations of mass known as [[planetesimal]]s form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become increasingly dense until they collapse inward under gravity to form [[protoplanet]]s.<ref>{{cite journal | first=G. W. |last=Wetherill |title=Formation of the Terrestrial Planets |journal=Annual Review of Astronomy and Astrophysics |date=1980 |volume=18 | issue=1 |pages=77–113 |bibcode=1980ARA&A..18...77W |doi=10.1146/annurev.aa.18.090180.000453 |issn=0066-4146}}</ref> After a planet reaches a mass somewhat larger than Mars's mass, it begins to accumulate an extended [[atmosphere]],<ref name=dangelo_bodenheimer_2013>{{cite journal|last1=D'Angelo|first1=G.|last2=Bodenheimer|first2=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> greatly increasing the capture rate of the planetesimals by means of [[Drag (physics)|atmospheric drag]].<ref>{{cite journal | last1=Inaba | first1=S. | last2=Ikoma | first2=M. |title=Enhanced Collisional Growth of a Protoplanet that has an Atmosphere |journal=Astronomy and Astrophysics |date=2003 |volume=410 | issue=2 |pages=711–723 |bibcode=2003A&A...410..711I |doi = 10.1051/0004-6361:20031248|doi-access=free }}</ref><ref name=dangelo2014>{{cite journal|last1=D'Angelo|first1=G.|last2=Weidenschilling | first2=S. J. |last3=Lissauer | first3=J. J. |last4=Bodenheimer | first4=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> Depending on the accretion history of solids and gas, a [[giant planet]], an [[ice giant]], or a [[terrestrial planet]] may result.<ref name=lhdb2009>{{cite journal|last1=Lissauer|first1=J. J.|last2=Hubickyj | first2=O. |last3=D'Angelo | first3=G. |last4=Bodenheimer | first4=P. |title=Models of Jupiter's growth incorporating thermal and hydrodynamic constraints| journal=Icarus|year=2009|volume=199|issue=2| pages=338–350|arxiv=0810.5186|doi=10.1016/j.icarus.2008.10.004|bibcode=2009Icar..199..338L |s2cid=18964068}}</ref><ref name=ddl2011>{{cite book|last1=D'Angelo|first1=G.|last2=Durisen|first2=R. H.|last3=Lissauer|first3=J. J.|chapter=Giant Planet Formation|bibcode=2010exop.book..319D|title=Exoplanets|publisher=University of Arizona Press, Tucson, AZ|editor-first=S.|editor-last=Seager|pages=319–346|date=2011|chapter-url=http://www.uapress.arizona.edu/Books/bid2263.htm|arxiv=1006.5486|access-date=1 May 2016|archive-date=30 June 2015|archive-url=https://web.archive.org/web/20150630164645/http://www.uapress.arizona.edu/Books/bid2263.htm|url-status=live}}</ref><ref name=chambes2011>{{cite book|last=Chambers|first=J.|chapter=Terrestrial Planet Formation|bibcode=2010exop.book..297C|title=Exoplanets|publisher=University of Arizona Press|location=Tucson, AZ|editor-first=S.|editor-last=Seager|pages=297–317|date=2011|chapter-url=http://www.uapress.arizona.edu/Books/bid2263.htm|access-date=1 May 2016|archive-date=30 June 2015|archive-url=https://web.archive.org/web/20150630164645/http://www.uapress.arizona.edu/Books/bid2263.htm|url-status=live}}</ref> It is thought that the [[regular satellite]]s of Jupiter, Saturn, and Uranus formed in a similar way;<ref name="arxiv0812">{{cite book |author1=Canup, Robin M. |author1-link=Robin Canup |author2=Ward, William R. |title=Origin of Europa and the Galilean Satellites |publisher=[[University of Arizona Press]] |date=2008 |arxiv=0812.4995|bibcode = 2009euro.book...59C |page=59|isbn=978-0-8165-2844-8}}</ref><ref name=dangelo_podolak_2015>{{cite journal|last1=D'Angelo|first1=G.| last2=Podolak | first2=M.|title=Capture and Evolution of Planetesimals in Circumjovian Disks|journal=The Astrophysical Journal|date=2015|volume=806|issue=1|pages=29pp|doi=10.1088/0004-637X/806/2/203|arxiv = 1504.04364 |bibcode = 2015ApJ...806..203D |s2cid=119216797}}</ref> however, [[Triton (moon)|Triton]] was likely [[gravitational capture|captured]] by Neptune,<ref name="Agnor06">{{Cite journal| doi = 10.1038/nature04792| url = http://extranet.on.br/rodney/curso2010/aula9/tritoncapt_hamilton.pdf| title = Neptune's capture of its moon Triton in a binary–planet gravitational encounter| journal = Nature| volume = 441| issue = 7090| pages = 192–4| year = 2006| last1 = Agnor| first1 = C. B.| last2 = Hamilton| first2 = D. P.| pmid = 16688170| bibcode = 2006Natur.441..192A| s2cid = 4420518| access-date = 1 May 2022| archive-date = 14 October 2016| archive-url = https://web.archive.org/web/20161014134243/http://extranet.on.br/rodney/curso2010/aula9/tritoncapt_hamilton.pdf}}</ref> and Earth's Moon<ref name="taylor1998">{{cite web |url=http://www.psrd.hawaii.edu/Dec98/OriginEarthMoon.html |title=Origin of the Earth and Moon |last=Taylor |first=G. Jeffrey |date=31 December 1998 |work=Planetary Science Research Discoveries |publisher=Hawai'i Institute of Geophysics and Planetology |access-date=7 April 2010 |url-status=live |archive-url=https://web.archive.org/web/20100610011142/http://www.psrd.hawaii.edu/Dec98/OriginEarthMoon.html |archive-date=10 June 2010}}</ref> and Pluto's Charon might have formed in collisions.<ref name="Stern_2015">
+{{cite journal |title=The Pluto system: Initial results from its exploration by New Horizons |first1=S.A. |last1=Stern |first2=F. |last2=Bagenal |first3=K. |last3=Ennico |first4=G.R. |last4=Gladstone |first5=W.M. |last5=Grundy |first6=W.B. |last6=McKinnon |first7=J.M. |last7=Moore |first8=C.B. |last8=Olkin |first9=J.R. |last9=Spencer |display-authors=4 |journal=Science |date=16 October 2015 |pmid=26472913 |page=aad1815 |volume=350 |issue=6258 |doi=10.1126/science.aad1815 |arxiv=1510.07704 |bibcode=2015Sci...350.1815S |s2cid=1220226 }}</ref>
+When the protostar has grown such that it ignites to form a star, the surviving disk is removed from the inside outward by [[photoevaporation]], the [[solar wind]], [[Poynting–Robertson effect|Poynting–Robertson drag]] and other effects.<ref>{{cite thesis | last = Dutkevitch |first = Diane |date =1995 |url =http://www.astro.umass.edu/theses/dianne/thesis.html |archive-url =https://web.archive.org/web/20071125124958/http://www.astro.umass.edu/theses/dianne/thesis.html |archive-date=25 November 2007 |title =The Evolution of Dust in the Terrestrial Planet Region of Circumstellar Disks Around Young Stars |type=PhD thesis|publisher=University of Massachusetts Amherst |access-date = 23 August 2008 |bibcode=1995PhDT..........D}}</ref><ref>{{cite journal | last1=Matsuyama | first1=I. | last2=Johnstone | first2=D. | last3=Murray | first3=N. |title=Halting Planet Migration by Photoevaporation from the Central Source |journal=The Astrophysical Journal |date = 2005 |volume=585 |issue=2 |pages=L143–L146 |bibcode=2003ApJ...585L.143M |doi = 10.1086/374406|arxiv = astro-ph/0302042 | s2cid=16301955 }}</ref> Thereafter there still may be many protoplanets orbiting the star or each other, but over time many will collide, either to form a larger, combined protoplanet or release material for other protoplanets to absorb.<ref>{{cite journal | last1=Kenyon |first1=Scott J. | last2=Bromley | first2=Benjamin C. |journal=Astronomical Journal |volume=131 | issue=3 |pages=1837–1850 | date=2006 |doi=10.1086/499807 |title= Terrestrial Planet Formation. I. The Transition from Oligarchic Growth to Chaotic Growth | bibcode=2006AJ....131.1837K|arxiv = astro-ph/0503568 |s2cid=15261426 }}</ref> Those objects that have become massive enough will capture most matter in their orbital neighbourhoods to become planets. Protoplanets that have avoided collisions may become [[natural satellite]]s of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets or [[small Solar System body|small bodies]].<ref>{{Cite journal |last1=Martin |first1=R. G. |last2=Livio |first2=M. |date=1 January 2013 |title=On the formation and evolution of asteroid belts and their potential significance for life |journal=Monthly Notices of the Royal Astronomical Society: Letters |language=en |volume=428 |issue=1 |pages=L11–L15 |doi=10.1093/mnrasl/sls003 |issn=1745-3925|doi-access=free |arxiv=1211.0023 }}</ref><ref>{{Cite journal |last=Peale |first=S. J. |date=September 1999 |title=Origin and Evolution of the Natural Satellites |url=https://www.annualreviews.org/doi/10.1146/annurev.astro.37.1.533 |journal=Annual Review of Astronomy and Astrophysics |language=en |volume=37 |issue=1 |pages=533–602 |doi=10.1146/annurev.astro.37.1.533 |bibcode=1999ARA&A..37..533P |issn=0066-4146 |access-date=13 May 2022 |archive-date=13 May 2022 |archive-url=https://web.archive.org/web/20220513181131/https://www.annualreviews.org/doi/10.1146/annurev.astro.37.1.533 }}</ref>
+{{Multiple image
+| direction         = horizontal
+| align             = right
+| width             = 200
+| image1            = 15-044a-SuperNovaRemnant-PlanetFormation-SOFIA-20150319.jpg
+| image2            = 15-044b-SuperNovaRemnant-PlanetFormation-SOFIA-20150319.jpg
+| footer_align      = center
+| footer            = [[Supernova remnant]] ejecta producing planet-forming material
+}}
+The energetic impacts of the smaller planetesimals (as well as [[radioactive decay]]) will heat up the growing planet, causing it to at least partially melt. The interior of the planet begins to differentiate by density, with higher density materials sinking toward the [[planetary core|core]].<ref>{{cite journal | journal=Icarus |date=1987 |volume=69 | issue=2 |pages=239–248 |last1=Ida |first1=Shigeru | last2=Nakagawa | first2=Yoshitsugu | last3=Nakazawa | first3=Kiyoshi |title= The Earth's core formation due to the Rayleigh-Taylor instability |doi=10.1016/0019-1035(87)90103-5 |bibcode=1987Icar...69..239I}}</ref> Smaller terrestrial planets lose most of their atmospheres because of this accretion, but the lost gases can be replaced by outgassing from the [[mantle (geology)|mantle]] and from the subsequent impact of [[comet]]s<ref>{{cite journal | last=Kasting |first=James F. |title=Earth's early atmosphere |journal=Science |date=1993 |volume=259 |bibcode=1993Sci...259..920K |doi=10.1126/science.11536547 |pmid=11536547 |issue=5097 | pages=920–926|s2cid=21134564 }}</ref> (smaller planets will lose any atmosphere they gain through various [[Atmospheric escape|escape mechanisms]]<ref>{{Cite web |last=Chuang |first=F. |date=6 June 2012 |title=FAQ – Atmosphere |url=https://www.psi.edu/epo/faq/atmosphere.html |access-date=13 May 2022 |website=Planetary Science Institute |language=en |archive-date=23 March 2022 |archive-url=https://web.archive.org/web/20220323224518/https://www.psi.edu/epo/faq/atmosphere.html |url-status=live }}</ref>).
+With the discovery and observation of [[planetary system]]s around stars other than the Sun, it is becoming possible to elaborate, revise or even replace this account. The level of [[metallicity]]—an astronomical term describing the abundance of [[chemical element]]s with an [[atomic number]] greater than 2 ([[helium]])—appears to determine the likelihood that a star will have planets.<ref>{{cite journal |bibcode=2005ApJ...622.1102F |doi=10.1086/428383 |title=The Planet-Metallicity Correlation |journal=The Astrophysical Journal |volume=622 |issue=2 |page=1102 |year=2005 |last1=Fischer |first1=Debra A. |last2=Valenti |first2=Jeff|doi-access=free }}</ref><ref>{{Cite journal |arxiv=1310.7830 |last1=Wang |first1=Ji |title=Revealing a Universal Planet-Metallicity Correlation for Planets of Different Sizes Around Solar-Type Stars |journal=The Astronomical Journal |volume=149 |issue=1 |page=14 |last2=Fischer |first2=Debra A. |year=2013 |doi=10.1088/0004-6256/149/1/14 |bibcode=2015AJ....149...14W|s2cid=118415186 }}</ref> Hence, a metal-rich [[population I star]] is more likely to have a substantial planetary system than a metal-poor, [[population II star]].<ref>{{Cite book |last1=Harrison |first1=Edward Robert |url=https://books.google.com/books?id=kNxeHD2cbLYC |title=Cosmology: The Science of the Universe |date=2000 |publisher=Cambridge University Press |isbn=978-0-521-66148-5 |page=114 |language=en |access-date=13 May 2022 |archive-date=14 December 2023 |archive-url=https://web.archive.org/web/20231214142630/https://books.google.com/books?id=kNxeHD2cbLYC |url-status=live }}</ref>
+== Planets in the Solar System ==
+{{Main|Solar System}}
+According to the [[IAU definition of planet|IAU definition]], there are eight planets in the Solar System, which are (in increasing distance from the Sun):<ref name="IAU"/> Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Jupiter is the largest, at 318 [[Earth mass]]es, whereas Mercury is the smallest, at 0.055 Earth masses.<ref name="JPL-physical-parameters">{{cite web |title=Planetary Physical Parameters |url=https://ssd.jpl.nasa.gov/planets/phys_par.html |url-status=live |archive-url=https://web.archive.org/web/20221004115344/https://ssd.jpl.nasa.gov/planets/phys_par.html |archive-date=4 October 2022 |access-date=11 July 2022 |website=Solar System Dynamics |publisher=Jet Propulsion Laboratory}}</ref>
+The planets of the Solar System can be divided into categories based on their composition. [[Terrestrial planet|Terrestrials]] are similar to Earth, with bodies largely composed of [[Rock (geology)|rock]] and metal: Mercury, Venus, Earth, and Mars. Earth is the largest terrestrial planet.<ref name=Lewis59>{{cite book |first=John S. |last=Lewis |date=2004 |title=Physics and Chemistry of the Solar System |page=59 |edition=2nd |publisher=Academic Press |isbn=978-0-12-446744-6}}</ref> [[Giant planet]]s are significantly more massive than the terrestrials: Jupiter, Saturn, Uranus, and Neptune.<ref name=Lewis59/> They differ from the terrestrial planets in composition. The [[gas giant]]s, Jupiter and Saturn, are primarily composed of [[hydrogen]] and helium and are the most massive planets in the Solar System. Saturn is one third as massive as Jupiter, at 95 Earth masses.<ref name=M19>{{cite web |last1=Marley |first1=Mark |title=Not a Heart of Ice |date=2 April 2019 |url=https://www.planetary.org/articles/not-a-heart-of-ice |website=planetary.org |publisher=The Planetary Society |access-date=5 May 2022 |language=en |archive-date=12 August 2019 |archive-url=https://web.archive.org/web/20190812095448/http://www.planetary.org/blogs/guest-blogs/2019/not-a-heart-of-ice.html |url-status=live }}</ref> The [[ice giant]]s, Uranus and Neptune, are primarily composed of low-boiling-point materials such as water, [[methane]], and [[ammonia]], with thick atmospheres of hydrogen and helium. They have a significantly lower mass than the gas giants (only 14 and 17 Earth masses).<ref name=M19/>
+[[File:Solar System true color (captions).jpg|center|thumb|600x600px|The Sun's, planets', dwarf planets' and moons' size to scale, labelled. Distance of objects is not to scale. The asteroid belt lies between the orbits of Mars and Jupiter, the Kuiper belt lies beyond Neptune's orbit.]]
+Dwarf planets are gravitationally rounded, but have not cleared their orbits of other [[Small Solar System body|bodies]]. In increasing order of average distance from the Sun, the ones generally agreed among astronomers are {{dp|Ceres}}, {{dp|Orcus}}, {{dp|Pluto}}, {{dp|Haumea}}, {{dp|Quaoar}}, {{dp|Makemake}}, {{dp|Gonggong}}, {{dp|Eris}}, and {{dp|Sedna}}.<ref name=Grundy2019/><ref name=JWST/> Ceres is the largest object in the [[asteroid belt]], located between the orbits of Mars and Jupiter. The other eight all orbit beyond Neptune. Orcus, Pluto, Haumea, Quaoar, and Makemake orbit in the [[Kuiper belt]], which is a second belt of small Solar System bodies beyond the orbit of Neptune. Gonggong and Eris orbit in the [[scattered disc]], which is somewhat further out and, unlike the Kuiper belt, is unstable towards interactions with Neptune. Sedna is the largest known [[detached object]], a population that never comes close enough to the Sun to interact with any of the classical planets; the origins of their orbits are still being debated. All nine are similar to terrestrial planets in having a solid surface, but they are made of ice and rock rather than rock and metal. Moreover, all of them are smaller than Mercury, with Pluto being the largest known dwarf planet and Eris being the most massive.<ref name="Brown Schaller 2007">{{cite journal| doi = 10.1126/science.1139415| last1 = Brown| first1 = Michael E.| author-link = Michael E. Brown| last2 = Schaller| first2 = Emily L.| s2cid = 21468196| date = 15 June 2007| title = The Mass of Dwarf Planet Eris| journal = Science| volume = 316| issue = 5831| page = 1585| pmid = 17569855| bibcode = 2007Sci...316.1585B| url = http://hubblesite.org/pubinfo/pdf/2007/24/pdf.pdf| access-date = 27 September 2015| archive-url = https://web.archive.org/web/20160304053122/http://hubblesite.org/pubinfo/pdf/2007/24/pdf.pdf| archive-date = 4 March 2016}}</ref><ref>{{cite web |url=https://www.nasa.gov/feature/how-big-is-pluto-new-horizons-settles-decades-long-debate |title=How Big Is Pluto? New Horizons Settles Decades-Long Debate |website=NASA |date=7 August 2017 |access-date=5 May 2022 |archive-date=9 November 2019 |archive-url=https://web.archive.org/web/20191109182908/https://www.nasa.gov/feature/how-big-is-pluto-new-horizons-settles-decades-long-debate/ |url-status=dead }}</ref>
+There are at least nineteen [[planetary-mass moon]]s or satellite planets—moons large enough to take on ellipsoidal shapes:<ref name=planetarysociety/>
+* One satellite of Earth: the [[Moon]]
+* Four [[Moons of Jupiter|satellites of Jupiter]]: [[Io (moon)|Io]], [[Europa (moon)|Europa]], [[Ganymede (moon)|Ganymede]], and [[Callisto (moon)|Callisto]]
+* Seven [[Moons of Saturn|satellites of Saturn]]: [[Mimas (moon)|Mimas]], [[Enceladus]], [[Tethys (moon)|Tethys]], [[Dione (moon)|Dione]], [[Rhea (moon)|Rhea]], [[Titan (moon)|Titan]], and [[Iapetus (moon)|Iapetus]]
+* Five [[Moons of Uranus|satellites of Uranus]]: [[Miranda (moon)|Miranda]], [[Ariel (moon)|Ariel]], [[Umbriel]], [[Titania (moon)|Titania]], and [[Oberon (moon)|Oberon]]
+* One [[Moons of Neptune|satellite of Neptune]]: [[Triton (moon)|Triton]]
+* One [[Moons of Pluto|satellite of Pluto]]: [[Charon (moon)|Charon]]
+The Moon, Io, and Europa have compositions similar to the terrestrial planets; the others are made of ice and rock like the dwarf planets, with [[Tethys (moon)|Tethys]] being made of almost pure ice. Europa is often considered an icy planet, though, because its surface ice layer makes it difficult to study its interior.<ref name=planetarysociety/><ref>{{cite book |first=John S. |last=Lewis |date=2004 |title=Physics and Chemistry of the Solar System |page=425 |edition=2nd |publisher=Academic Press |isbn=978-0-12-446744-6}}</ref> Ganymede and Titan are larger than Mercury by radius, and Callisto almost equals it, but all three are much less massive. Mimas is the smallest object generally agreed to be a [[Geophysical definition of planet|geophysical planet]], at about six&nbsp;millionths of Earth's mass, though there are many larger bodies that may not be geophysical planets (e.g. {{dp|Salacia}}).<ref name=Grundy2019/>
+==Reference==
+{{reflist}}