Editing Planet (section)

=== Defining the term ''planet'' ===
{{Further|Definition of planet}}
{{Anchor|Definition and similar concepts}}A growing number of astronomers argued for Pluto to be declassified as a planet, because many similar objects approaching its size had been found in the same region of the Solar System (the [[Kuiper belt]]) during the 1990s and early 2000s. Pluto was found to be just one "small" body in a population of thousands.<ref name="plutoplanet">{{cite journal |last1=Basri |first1=Gibor |last2=Brown |first2=Michael E. |date=2006 |title=Planetesimals to Brown Dwarfs: What is a Planet? |url=http://www.gps.caltech.edu/~mbrown/papers/ps/basribrown.pdf |url-status=live |journal=Annual Review of Earth and Planetary Sciences |volume=34 |pages=193–216 |arxiv=astro-ph/0608417 |bibcode=2006AREPS..34..193B |doi=10.1146/annurev.earth.34.031405.125058 |s2cid=119338327 |archive-url=https://web.archive.org/web/20080704213644/http://www.gps.caltech.edu/~mbrown/papers/ps/basribrown.pdf |archive-date=4 July 2008 |access-date=4 August 2008}}</ref> They often referred to the demotion of the asteroids as a precedent, although that had been done based on their geophysical differences from planets rather than their being in a belt.<ref name="metzger22" /> Some of the larger [[trans-Neptunian object]]s, such as [[50000 Quaoar|Quaoar]], [[90377 Sedna|Sedna]], [[Eris (dwarf planet)|Eris]], and [[Haumea]],<ref name="planeta">{{cite web|title=Estados Unidos "conquista" Haumea|work=[[ABC.es|ABC]]|date=20 September 2008|url=http://www.abc.es/20080920/nacional-sociedad/estados-unidos-conquista-haumea-20080920.html|access-date=18 September 2008|language=es|archive-date=6 October 2017|archive-url=https://web.archive.org/web/20171006013056/http://www.abc.es/20080920/nacional-sociedad/estados-unidos-conquista-haumea-20080920.html|url-status=live}}</ref> were heralded in the popular press as the [[tenth planet]].

The announcement of Eris in 2005, an object 27% more massive than Pluto, created the impetus for an official definition of a planet,<ref name="plutoplanet" /> as considering Pluto a planet would logically have demanded that Eris be considered a planet as well. Since different procedures were in place for naming planets versus non-planets, this created an urgent situation because under the rules Eris could not be named without defining what a planet was.<ref name="metzger22" /> At the time, it was also thought that the size required for a trans-Neptunian object to become round was about the same as that required for the moons of the giant planets (about 400&nbsp;km diameter), a figure that would have suggested about 200 round objects in the Kuiper belt and thousands more beyond.<ref name="Brown">{{cite web |author-link=Michael E. Brown |title=The Dwarf Planets |first=Michael E. |last=Brown |publisher=California Institute of Technology, Department of Geological Sciences |url=http://web.gps.caltech.edu/~mbrown/dwarfplanets/ |access-date=26 January 2008 |archive-date=19 July 2011 |archive-url=https://web.archive.org/web/20110719164447/http://web.gps.caltech.edu/~mbrown/dwarfplanets/ |url-status=live }}</ref><ref name="BrownList">{{cite web |last1=Brown |first1=Mike |author1-link=Michael E. Brown |title=How Many Dwarf Planets Are There in the Outer Solar System? |url=http://web.gps.caltech.edu/~mbrown/dps.html |publisher=California Institute of Technology |access-date=11 August 2022 |archive-url=https://web.archive.org/web/20220719141419/http://web.gps.caltech.edu/~mbrown/dps.html |archive-date=19 July 2022 |date=23 February 2021}}</ref> Many astronomers argued that the public would not accept a definition creating a large number of planets.<ref name="metzger22" />

{{Quote box
| title = The [[International Astronomical Union]]'s<br />definition of a planet in the [[Solar System]]
| quote = {{ordered list|
Object is in [[orbit]] around the Sun|
Object has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a [[hydrostatic equilibrium]] (nearly round) shape|
Object has [[cleared the neighbourhood]] around its orbit}}

Source: {{cite news|url=http://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf|title=IAU 2006 General Assembly: Resolutions 5 and 6|date=24 August 2006|publisher=IAU|access-date=23 June 2009}}
| width = 375px
}}To acknowledge the problem, the [[International Astronomical Union]] (IAU) set about creating the [[IAU definition of planet|definition of planet]] and produced one in August 2006. Under this definition, the Solar System is considered to have eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune). Bodies that fulfill the first two conditions but not the third are classified as [[dwarf planet]]s, provided they are not [[natural satellite]]s of other planets. Originally an IAU committee had proposed a definition that would have included a larger number of planets as it did not include (c) as a criterion.<ref>{{cite news |last=Rincon |first=Paul |date=16 August 2006 |title=Planets plan boosts tally 12 |url=http://news.bbc.co.uk/1/hi/sci/tech/4795755.stm |url-status=live |archive-url=https://web.archive.org/web/20070302051348/http://news.bbc.co.uk/1/hi/sci/tech/4795755.stm |archive-date=2 March 2007 |access-date=23 August 2008 |department=[[BBC News]] |publisher=[[British Broadcasting Corporation]]}}</ref> After much discussion, it was decided via a vote that those bodies should instead be classified as dwarf planets.<ref name="Pluto loses status as a planet-2006" /><ref>{{cite journal |last=Green |first=D. W. E. |date=13 September 2006 |title=(134340) Pluto, (136199) Eris, and (136199) Eris I (Dysnomia) |url=http://www.cbat.eps.harvard.edu/iauc/08700/08747.html |journal=IAU Circular |publisher=Central Bureau for Astronomical Telegrams, International Astronomical Union |issue=8747 |page=1 |bibcode=2006IAUC.8747....1G |id=Circular No. 8747 |archive-url=https://web.archive.org/web/20080624225029/http://www.cfa.harvard.edu/iau/special/08747.pdf |archive-date=24 June 2008 |access-date=5 July 2011}}</ref>

==== Criticisms and alternatives to IAU definition ====
{{See also|List of gravitationally rounded objects of the Solar System}}
[[File:25 solar system objects smaller than Earth.jpg|thumb|upright=1.6|The planetary-mass moons to scale, compared with Mercury, Venus, Earth, Mars, and Pluto. Sub-planetary [[Proteus (moon)|Proteus]] and [[Nereid (moon)|Nereid]] (about the same size as Mimas) have been included for comparison. Unimaged [[Dysnomia (moon)|Dysnomia]] (intermediate in size between Tethys and Enceladus) is not shown; it is in any case probably not a solid body.<ref name="Brown2023">{{cite journal |last1=Brown |first1=Michael E. |last2=Butler |first2=Bryan |date=October 2023 |title=Masses and densities of dwarf planet satellites measured with ALMA |journal=The Planetary Science Journal |volume=4 |issue=10 |pages=6 |arxiv=2307.04848 |bibcode=2023PSJ.....4..193B |doi=10.3847/PSJ/ace52a |s2cid= |id=193 |doi-access=free}}</ref>]]

The IAU definition has not been universally used or accepted. In [[planetary geology]], celestial objects are [[geophysical definition of planet|defined as planets by geophysical characteristics]]. A celestial body may acquire a dynamic (planetary) geology at approximately the mass required for its mantle to become plastic under its own weight. This leads to a state of [[hydrostatic equilibrium]] where the body acquires a stable, round shape, which is adopted as the hallmark of planethood by geophysical definitions. For example:<ref name="Stern_Levison_2002">{{citation
 | last1=Stern | first1=S. Alan
 | last2=Levison | first2=Harold F.
 | editor1-first=H. | editor1-last=Rickman
 | title=Regarding the criteria for planethood and proposed planetary classification schemes
 | journal=Highlights of Astronomy | volume=12
 | pages=205–213 | date=2002
 | publisher=Astronomical Society of the Pacific
 | location=San Francisco
 | bibcode=2002HiA....12..205S
 | isbn=978-1-58381-086-6
 | doi=10.1017/S1539299600013289
 | doi-access=free
 }} See p. 208.</ref>

{{blockquote|a substellar-mass body that has never undergone nuclear fusion and has enough gravitation to be round due to hydrostatic equilibrium, regardless of its orbital parameters.<ref name=Astronomy>{{cite web
 |url          = http://www.astronomy.com/magazine/2018/05/an-organically-grown-planet-definition
 |title        = An organically grown planet definition — Should we really define a word by voting?
 |last1        = Runyon
 |first1       = Kirby D.
 |last2        = Stern
 |first2       = S. Alan
 |date         = 17 May 2018
 |website      = [[Astronomy (magazine)|Astronomy]]
 |access-date  = 12 October 2019
 |archive-date = 10 October 2019
 |archive-url  = https://web.archive.org/web/20191010153028/http://astronomy.com/magazine/2018/05/an-organically-grown-planet-definition
 |url-status   = live
}}</ref>}}

In the Solar System, this mass is generally less than the mass required for a body to clear its orbit; thus, some objects that are considered "planets" under geophysical definitions are not considered as such under the IAU definition, such as Ceres and Pluto.<ref name=planetarysociety/> (In practice, the requirement for hydrostatic equilibrium is universally relaxed to a requirement for rounding and compaction under self-gravity; Mercury is not actually in hydrostatic equilibrium,<ref name="Mercury">Sean Solomon, Larry Nittler & Brian Anderson, eds. (2018) ''Mercury: The View after MESSENGER''. Cambridge Planetary Science series no. 21, Cambridge University Press, pp. 72–73.</ref> but is universally included as a planet regardless.)<ref>{{Cite tweet |user=plutokiller |last=Brown |first=Mike |number=1624127764969459713 |title=The real answer here is to not get too hung up on definitions, which I admit is hard when the IAU tries to make them sound official and clear, but, really, we all understand the intent of the hydrostatic equilibrium point, and the intent is clearly to include Merucry & the moon}}</ref> Proponents of such definitions often argue that location should not matter and that planethood should be defined by the intrinsic properties of an object.<ref name=planetarysociety/> Dwarf planets had been proposed as a category of small planet (as opposed to [[Minor planet|planetoid]]s as sub-planetary objects) and planetary geologists continue to treat them as planets despite the IAU definition.<ref name=Grundy2019>{{cite journal
 |first1=W.M. |last1=Grundy
 |first2=K.S. |last2=Noll
 |first3=M.W. |last3=Buie
 |first4=S.D. |last4=Benecchi
 |first5=D. |last5=Ragozzine
 |first6=H.G. |last6=Roe
 | display-authors=4
 |title=The Mutual Orbit, Mass, and Density of Transneptunian Binary Gǃkúnǁʼhòmdímà ({{mp|(229762) 2007 UK|126}})
 |url=http://www2.lowell.edu/~grundy/abstracts/2019.G-G.html
 |journal=Icarus
 |doi=10.1016/j.icarus.2018.12.037
 |date=December 2018
 |volume=334
 |page=30
 |bibcode=2019Icar..334...30G
 |s2cid=126574999
 |archive-url=https://web.archive.org/web/20190407045339/http://www2.lowell.edu/~grundy/abstracts/preprints/2019.G-G.pdf
 |archive-date=7 April 2019}}</ref>

The number of dwarf planets even among known objects is not certain. In 2019, Grundy et al. argued based on the low densities of some mid-sized trans-Neptunian objects that the limiting size required for a trans-Neptunian object to reach equilibrium was in fact much larger than it is for the icy moons of the giant planets, being about 900–1000&nbsp;km diameter.<ref name=Grundy2019/> There is general consensus on Ceres in the asteroid belt<ref>{{Cite journal |last1=Raymond |first1=C. A. |last2=Ermakov |first2=A. I. |last3=Castillo-Rogez |first3=J. C. |last4=Marchi |first4=S. |last5=Johnson |first5=B. C. |last6=Hesse |first6=M. A. |last7=Scully |first7=J. E. C. |last8=Buczkowski |first8=D. L. |last9=Sizemore |first9=H. G. |last10=Schenk |first10=P. M. |last11=Nathues |first11=A. |display-authors=4 |date=August 2020 |title=Impact-driven mobilization of deep crustal brines on dwarf planet Ceres |url=https://www.nature.com/articles/s41550-020-1168-2 |journal=Nature Astronomy |language=en |volume=4 |issue=8 |pages=741–747 |bibcode=2020NatAs...4..741R |doi=10.1038/s41550-020-1168-2 |s2cid=211137608 |issn=2397-3366 |access-date=27 June 2022 |archive-date=21 June 2022 |archive-url=https://web.archive.org/web/20220621225448/https://www.nature.com/articles/s41550-020-1168-2 |url-status=live }}</ref> and on the eight trans-Neptunians that probably cross this threshold—{{dp|Orcus}}, {{dp|Pluto}}, {{dp|Haumea}}, {{dp|Quaoar}}, {{dp|Makemake}}, {{dp|Gonggong}}, {{dp|Eris}}, and {{dp|Sedna}}.<ref>{{Cite journal |last1=Barr |first1=Amy C. |last2=Schwamb |first2=Megan E. |date=1 August 2016 |title=Interpreting the densities of the Kuiper belt's dwarf planets |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=460 |issue=2 |pages=1542–1548 |doi=10.1093/mnras/stw1052 |issn=0035-8711|doi-access=free |arxiv=1603.06224 }}</ref><ref name=JWST>{{cite journal|last1=Emery|first1=J. P. |first2=I. |last2=Wong |first3=R. |last3=Brunetto |first4=J. C. |last4=Cook |first5=N. |last5=Pinilla-Alonso |first6=J. A. |last6=Stansberry |first7=B. J. |last7=Holler |first8=W. M. |last8=Grundy |first9=S. |last9=Protopapa |first10=A. C. |last10=Souza-Feliciano |first11=E. |last11=Fernández-Valenzuela |first12=J. I. |last12=Lunine |first13=D. C. |last13=Hines |author-link=|date=2024|title=A Tale of 3 Dwarf Planets: Ices and Organics on Sedna, Gonggong, and Quaoar from JWST Spectroscopy|journal=Icarus |volume=414 |doi=10.1016/j.icarus.2024.116017 |arxiv=2309.15230|bibcode=2024Icar..41416017E }}</ref>

Planetary geologists may include the nineteen known [[planetary-mass moon]]s as "satellite planets", including Earth's Moon and Pluto's [[Charon (moon)|Charon]], like the early modern astronomers.<ref name=planetarysociety/><ref name="News.discovery.com">{{cite web |url=http://news.discovery.com/space/should-large-moons-be-called-satellite-planets.html |title=Should Large Moons Be Called 'Satellite Planets'?|last1=Villard|first1=Ray|website=Discovery News|publisher=[[Discovery, Inc.]] |date=14 May 2010 |access-date=4 November 2011 |archive-date=5 May 2012 |archive-url=https://web.archive.org/web/20120505221146/http://news.discovery.com/space/should-large-moons-be-called-satellite-planets.html }}</ref> Some go even further and include as planets relatively large, geologically evolved bodies that are nonetheless not very round today, such as Pallas and Vesta;<ref name="planetarysociety" /> rounded bodies that were completely disrupted by impacts and re-accreted like Hygiea;<ref>{{Cite web|url=https://www.space.com/asteroid-hygiea-may-be-smallest-dwarf-planet.html|title=Asteroid Hygiea May be the Smallest Dwarf Planet in the Solar System|website=[[Space.com]]|publisher=[[Purch Group]]|last1=Urrutia|first1=Doris Elin|date=28 October 2019|access-date=28 August 2022|archive-date=5 November 2019|archive-url=https://web.archive.org/web/20191105002904/https://www.space.com/asteroid-hygiea-may-be-smallest-dwarf-planet.html|url-status=live}}</ref><ref>{{Cite news|url=https://www.sciencenews.org/article/hygiea-may-be-solar-system-smallest-dwarf-planet|title=The solar system may have a new smallest dwarf planet: Hygiea|website=[[Science News]]|publisher=[[Society for Science]]|date=28 October 2019|access-date=28 August 2022|archive-date=31 August 2022|archive-url=https://web.archive.org/web/20220831083918/https://www.sciencenews.org/article/hygiea-may-be-solar-system-smallest-dwarf-planet|url-status=live}}</ref><ref name=Yang2020>{{citation|arxiv=2007.08059|title=Binary asteroid (31) Euphrosyne: Ice-rich and nearly spherical|year=2020|doi=10.1051/0004-6361/202038372|last1=Yang|first1=B.|last2=Hanuš|first2=J.|last3=Carry|first3=B.|last4=Vernazza|first4=P.|last5=Brož|first5=M.|last6=Vachier|first6=F.|last7=Rambaux|first7=N.|last8=Marsset|first8=M.|last9=Chrenko|first9=O.|last10=Ševeček|first10=P.|last11=Viikinkoski|first11=M.|last12=Jehin|first12=E.|last13=Ferrais|first13=M.|last14=Podlewska-Gaca|first14=E.|last15=Drouard|first15=A.|last16=Marchis|first16=F.|last17=Birlan|first17=M.|last18=Benkhaldoun|first18=Z.|last19=Berthier|first19=J.|last20=Bartczak|first20=P.|last21=Dumas|first21=C.|last22=Dudziński|first22=G.|last23=Ďurech|first23=J.|last24=Castillo-Rogez|first24=J.|last25=Cipriani|first25=F.|last26=Colas|first26=F.|last27=Fetick|first27=R.|last28=Fusco|first28=T.|last29=Grice|first29=J.|last30=Jorda|first30=L.|journal=Astronomy & Astrophysics|volume=641|page=A80|bibcode=2020A&A...641A..80Y|s2cid=220546126|display-authors=29}}</ref> or even everything at least the diameter of Saturn's moon [[Mimas (moon)|Mimas]], the smallest planetary-mass moon. (This may even include objects that are not round but happen to be larger than Mimas, like Neptune's moon [[Proteus (moon)|Proteus]].)<ref name=planetarysociety/>

Astronomer [[Jean-Luc Margot]] proposed a mathematical criterion that determines whether an object can clear its orbit during the lifetime of its host star, based on the mass of the planet, its semimajor axis, and the mass of its host star.<ref>{{cite news |last=Netburn |first=Deborah |date=13 November 2015 |title=Why we need a new definition of the word 'planet' |url=http://www.latimes.com/science/sciencenow/la-sci-sn-new-planet-definition-margot-20151113-htmlstory.html |url-status=live |archive-url=https://web.archive.org/web/20210603084521/https://www.latimes.com/science/sciencenow/la-sci-sn-new-planet-definition-margot-20151113-htmlstory.html |archive-date=3 June 2021 |access-date=24 July 2016 |newspaper=[[Los Angeles Times]]}}</ref> The formula produces a value called {{mvar|π}} that is greater than 1 for planets.{{efn|name=not-confuse-π|Margot's parameter<ref name=Margot/> is not to be confused with the [[Pi|famous mathematical constant]] {{nowrap|{{mvar|π}}≈3.14159265 ... .}} }} The eight known planets and all known exoplanets have {{mvar|π}} values above 100, while Ceres, Pluto, and Eris have {{mvar|π}} values of 0.1, or less. Objects with {{mvar|π}} values of 1 or more are expected to be approximately spherical, so that objects that fulfill the orbital-zone clearance requirement around Sun-like stars will also fulfill the roundness requirement<ref name="Margot">
{{cite journal |last=Margot |first=Jean-Luc |author-link=Jean-Luc Margot |year=2015 |title=A quantitative criterion for defining planets |journal=[[The Astronomical Journal]] |volume=150 |issue=6 |page=185 |arxiv=1507.06300 |bibcode=2015AJ....150..185M |doi=10.1088/0004-6256/150/6/185 |s2cid=51684830}}
</ref> – though this may not be the case around very low-mass stars.<ref name="Margot 2024"/> In 2024, Margot and collaborators proposed a revised version of the criterion with a uniform clearing timescale of 10&nbsp;billion years (the approximate main-sequence lifetime of the Sun) or 13.8&nbsp;billion years (the [[age of the universe]]) to accommodate planets orbiting brown dwarfs.<ref name="Margot 2024">{{cite journal |last1=Margot |first1=Jean-Luc |last2=Gladman |first2=Brett |last3=Yang |first3=Tony |title=Quantitative Criteria for Defining Planets |journal=The Planetary Science Journal |date=1 July 2024 |volume=5 |issue=7 |pages=159 |doi=10.3847/PSJ/ad55f3|doi-access=free |arxiv=2407.07590 |bibcode=2024PSJ.....5..159M }}</ref>