Editing Planet (section)

==== Mass ====
{{Main|Planetary mass}}
A planet's defining physical characteristic is that it is massive enough for the force of its own gravity to dominate over the [[electromagnetic force]]s binding its physical structure, leading to a state of [[hydrostatic equilibrium]]. This effectively means that all planets are spherical or spheroidal. Up to a certain mass, an object can be irregular in shape, but beyond that point, which varies depending on the chemical makeup of the object, gravity begins to pull an object towards its own centre of mass until the object collapses into a sphere.<ref>{{cite web |title=The Dwarf Planets |url=http://www.gps.caltech.edu/~mbrown/dwarfplanets/ |author-link=Michael E. Brown |last=Brown |first=Michael E. |work=California Institute of Technology |date=2006 |access-date=1 February 2008 |archive-date=16 January 2011 |archive-url=https://web.archive.org/web/20110116181239/http://www.gps.caltech.edu/~mbrown/dwarfplanets/ |url-status=live }}</ref>

Mass is the prime attribute by which planets are distinguished from stars. No objects between the masses of the Sun and Jupiter exist in the Solar System, but there are exoplanets of this size. The lower [[stellar mass]] limit is estimated to be around 75 to 80 times that of Jupiter ({{Jupiter mass|link=yes}}). Some authors advocate that this be used as the upper limit for planethood, on the grounds that the internal physics of objects does not change between approximately one Saturn mass (beginning of significant self-compression) and the onset of hydrogen burning and becoming a [[red dwarf]] star.<ref name=ChenKipping/> Beyond roughly 13 {{Jupiter mass}} (at least for objects with solar-type [[isotopic abundance]]), an object achieves conditions suitable for [[nuclear fusion]] of [[deuterium]]: this has sometimes been advocated as a boundary,<ref name=exoworkdef/> even though deuterium burning does not last very long and most brown dwarfs have long since finished burning their deuterium.<ref name=Hatzes/> This is not universally agreed upon: the [[Extrasolar Planets Encyclopaedia|exoplanets Encyclopaedia]] includes objects up to 60 {{Jupiter mass}},<ref name=corot/> and the [[Exoplanet Data Explorer]] up to 24 {{Jupiter mass}}.<ref name=eod/>

The smallest known exoplanet with an accurately known mass is [[PSR B1257+12A]], one of the first exoplanets discovered, which was found in 1992 in orbit around a [[pulsar]]. Its mass is roughly half that of the planet Mercury.<ref name="konacki2003">{{cite journal | author=Konacki, M. | author2=Wolszczan, A. | title=Masses and Orbital Inclinations of Planets in the PSR B1257+12 System | journal=The Astrophysical Journal | volume=591 | issue=2 | pages=L147–L150 | date=2003 | doi=10.1086/377093 | bibcode=2003ApJ...591L.147K|arxiv = astro-ph/0305536 | s2cid=18649212 }}</ref> Even smaller is [[WD 1145+017 b]], orbiting a white dwarf; its mass is roughly that of the dwarf planet Haumea, and it is typically termed a minor planet.<ref>{{cite book |last=Veras |first=Dimitri |chapter=Planetary Systems Around White Dwarfs |date=2021 |url=https://oxfordre.com/planetaryscience/view/10.1093/acrefore/9780190647926.001.0001/acrefore-9780190647926-e-238 |title=Oxford Research Encyclopedia of Planetary Science |publisher=Oxford University Press |language=en |arxiv=2106.06550 |doi=10.1093/acrefore/9780190647926.013.238 |isbn=978-0-19-064792-6 |access-date=12 July 2022 |archive-date=6 June 2022 |archive-url=https://web.archive.org/web/20220606003104/https://oxfordre.com/planetaryscience/view/10.1093/acrefore/9780190647926.001.0001/acrefore-9780190647926-e-238 |url-status=live }}</ref> The smallest known planet orbiting a main-sequence star other than the Sun is [[Kepler-37b]], with a mass (and radius) that is probably slightly higher than that of the Moon.<ref name="Barclay-2013" /> The smallest object in the Solar System generally agreed to be a geophysical planet is Saturn's moon Mimas, with a radius about 3.1% of Earth's and a mass about 0.00063% of Earth's.<ref name="Jacobson2022">{{cite journal |last1=Jacobson |first1=Robert. A. |title=The Orbits of the Main Saturnian Satellites, the Saturnian System Gravity Field, and the Orientation of Saturn's Pole* |journal=The Astronomical Journal |date=1 November 2022 |volume=164 |issue=5 |page=199 |doi=10.3847/1538-3881/ac90c9|bibcode=2022AJ....164..199J |s2cid=252992162 |doi-access=free }}</ref> Saturn's smaller moon [[Phoebe (moon)|Phoebe]], currently an irregular body of 1.7% Earth's radius<ref name=Thomas2010>{{cite journal| doi = 10.1016/j.icarus.2010.01.025| last1 = Thomas| first1 = P. C.| date = July 2010| title = Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission| journal = Icarus| volume = 208| issue = 1| pages = 395–401| url = http://www.ciclops.org/media/sp/2011/6794_16344_0.pdf| bibcode = 2010Icar..208..395T| access-date = 7 May 2023| archive-date = 23 December 2018| archive-url = https://web.archive.org/web/20181223003125/http://www.ciclops.org/media/sp/2011/6794_16344_0.pdf| url-status = dead}}</ref> and 0.00014% Earth's mass,<ref name="Jacobson2022"/> is thought to have attained hydrostatic equilibrium and differentiation early in its history before being battered out of shape by impacts.<ref name=planetlike>{{cite web |date=26 April 2012 |author=Jia-Rui C. Cook and Dwayne Brown |title=Cassini Finds Saturn Moon Has Planet-Like Qualities |url=http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20120426/ |publisher=JPL/NASA |archive-url=https://web.archive.org/web/20120427192715/http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20120426/ | archive-date=27 April 2012 |url-status=dead}}</ref> Some asteroids may be fragments of [[protoplanet]]s that began to accrete and differentiate, but suffered catastrophic collisions, leaving only a metallic or rocky core today,<ref name=Gaffey1984>{{cite journal
  |last=Gaffey |first=Michael
  |title=Rotational spectral variations of asteroid (8) Flora: Implications for the nature of the S-type asteroids and for the parent bodies of the ordinary chondrites
  |journal=Icarus
  |volume=60
  |issue=1 |pages=83–114 |date=1984
  |doi=10.1016/0019-1035(84)90140-4
  |bibcode=1984Icar...60...83G}}</ref><ref name=Hardersen-Gaffey-Abell-2005>
{{cite journal
 |first1=Paul S.     |last1=Hardersen
 |first2=Michael J.  |last2=Gaffey
 |first3=Paul A.     |last3=Abell
 |name-list-style=amp
 |year=2005
 |title=Near-IR spectral evidence for the presence of iron-poor orthopyroxenes on the surfaces of six M-type asteroid
 |journal=Icarus
 |volume=175 |issue=1 |page=141
 |bibcode=2005Icar..175..141H  |doi=10.1016/j.icarus.2004.10.017
}}
</ref><ref name=Asphaug-Reufer-2014>
{{cite journal
 |first1=E. |last1=Asphaug
 |first2=A. |last2=Reufer
 |year=2014
 |title=Mercury and other iron-rich planetary bodies as relics of inefficient accretion
 |journal=Nature Geoscience
 |volume=7  |issue=8  |pages=564–568
 |bibcode=2014NatGe...7..564A   |doi=10.1038/NGEO2189
}}</ref> or a reaccumulation of the resulting debris.<ref name=Yang2020/>