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[[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 name="exodef"/> 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 | |||
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| 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 | |||
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}} | |||
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 | |||
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| 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 | |||
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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> | |||