View source for Astronomy:Planetary-mass moon

{{short description|Planetary-mass bodies that are also natural satellites}}
{{Solar System sidebar}}
[[File:Large Moons (4089199369).jpg|thumb|300px|Planetary-mass moons larger than Pluto, the largest Solar dwarf planet.]]

A '''planetary-mass moon''' is a [[Astronomy:Planetary-mass object|planetary-mass object]] that is also a [[Astronomy:Natural satellite|natural satellite]]. They are large and ellipsoidal (sometimes spherical) in shape. Moons may be in hydrostatic equilibrium due to tidal or radiogenic heating, in some cases forming a subsurface ocean. Two moons in the Solar System are larger than the planet [[Astronomy:Mercury (planet)|Mercury]] (though less massive): [[Astronomy:Ganymede (moon)|Ganymede]] and [[Astronomy:Titan (moon)|Titan]], and seven are larger and more massive than the dwarf planets {{dp|Pluto}} and {{dp|Eris}}. 

The concept of ''satellite planets'' – the idea that planetary-mass objects, including moons, are [[Astronomy:Planet|planet]]s – is used by some planetary scientists, such as [[Biography:Alan Stern|Alan Stern]], who are more concerned with whether a celestial body has [[Earth:Planetary geology|planetary geology]] (that is, whether it is a planetary body) than its solar or non-solar orbit (planetary dynamics).<ref name="stern">{{cite web |url=http://news.discovery.com/space/should-large-moons-be-called-satellite-planets.html#post-a-comment |archive-url=https://web.archive.org/web/20141025001402/http://news.discovery.com/space/should-large-moons-be-called-satellite-planets.htm |title=Should Large Moons Be Called 'Satellite Planets'? |publisher=News.discovery.com |date=2010-05-14 |archive-date=2014-10-25}}</ref> This conceptualization of planets as three classes of objects (classical planets, [[Astronomy:Dwarf planet|dwarf planet]]s and satellite planets) has not been accepted by the [[Organization:International Astronomical Union|International Astronomical Union]] (the IAU).

==Early history==
The distinction between a satellite and a classical planet was not recognized until after the heliocentric model of the Solar System was established. When in 1610 [[Biography:Galileo Galilei|Galileo]] discovered the first satellites of another planet (the four [[Astronomy:Galilean moons|Galilean moons]] of Jupiter), he referred to them as "four planets flying around the star of Jupiter at unequal intervals and periods with wonderful swiftness."<ref>{{cite book|title=Siderius Nuncius|author=Galileo Galilei|publisher=University of Chicago Press|year=1989|page=26|others=Albert van Helden}}</ref> Similarly, [[Biography:Christiaan Huygens|Christiaan Huygens]], upon discovering Saturn's largest moon [[Astronomy:Titan (moon)|Titan]] in 1655, employed the terms "planeta" (planet), "Stella" (star), "luna" (moon), and the more modern "satellite" (attendant) to describe it.<ref>{{cite book|title=Systema Saturnium: Sive de Causis Miradorum Saturni Phaenomenon, et comite ejus Planeta Novo|author= Christiani Hugenii (Christiaan Huygens)|publisher=Adriani Vlacq|year= 1659|pages= 1–50}}</ref> Giovanni Cassini, in announcing his discovery of Saturn's moons Iapetus and [[Astronomy:Rhea (moon)|Rhea]] in 1671 and 1672, described them as ''Nouvelles Planetes autour de Saturne'' ("New planets around Saturn").<ref>{{cite book|title=Decouverte de deux Nouvelles Planetes autour de Saturne|author=Giovanni Cassini|publisher=Sabastien Mabre-Craniusy|year= 1673| pages=6–14}}</ref> However, when the ''Journal de Scavans'' reported Cassini's discovery of two new Saturnian moons (Tethys and Dione) in 1686, it referred to them strictly as "satellites", though sometimes to Saturn as the "primary planet".<ref name="Cassini1686–1692">{{cite journal| doi = 10.1098/rstl.1686.0013| last = Cassini | first = G. D.|  year = 1686–1692| title = An Extract of the Journal Des Scavans. On April 22 st. N. 1686. Giving an Account of Two New Satellites of Saturn, Discovered Lately by Mr. Cassini at the Royal Observatory in Paris| journal = Philosophical Transactions of the Royal Society of London| volume = 16| issue = 179–191| pages = 79–85| jstor = 101844| bibcode = 1686RSPT...16...79C | doi-access = free}}<!-- This journal became the Philosophical Transactions of the Royal Society of London in 1775. Are there any earlier publications? Two years seems a long time... --></ref> When William Herschel announced his discovery of two objects in orbit around Uranus ([[Astronomy:Titania (moon)|Titania]] and [[Astronomy:Oberon (moon)|Oberon]]) in 1787, he referred to them as "satellites" and "secondary planets".<ref>{{cite book|title=An Account of the Discovery of Two Satellites Around the Georgian Planet. Read at the Royal Society|author=William Herschel|publisher=J. Nichols|year=1787|pages=1–4}}</ref> All subsequent reports of natural satellite discoveries used the term "satellite" exclusively,<ref>See primary citations in Timeline of discovery of Solar System planets and their moons</ref> though the 1868 book ''Smith's Illustrated Astronomy'' referred to satellites as "secondary planets".<ref name="smith">{{cite book
 |first = Asa | last = Smith
 |year=1868
 |title=Smith's Illustrated Astronomy
 |publisher=Nichols & Hall
 |url=https://archive.org/details/smithsillustrat00smitgoog |page = [https://archive.org/details/smithsillustrat00smitgoog/page/n26 23] |quote = secondary planet Herschel. }}</ref>

==Modern concept==
{{image frame
|content={{Graph:Chart
   | width=200
   | height=200
   | type=rect
   | x= Ganymede, Titan, Callisto, Io, Moon, Europa, Triton
   | y1= 148.2,134.5,107.6,89.32,73.46,48.00,21.39
   | showValues=format:.1f,offset:1
   | xAxisAngle = 45
}}
|width=250
|caption=Comparative masses of the seven largest moons. Values are ×10<sup>21</sup>&nbsp;kg. The moons smaller than Triton would be barely visible at this scale.
|border=no
}}
{{image frame
|content={{Graph:Chart
   | width=300
   | height=300
   | type=rect
   | x= Triton, Titania, Oberon,  Rhea, Iapetus, Charon, Umbriel, Ariel, Dione, Tethys, Dysnomia, Enceladus, Miranda,  Mimas
   | y1= 21.39,    3.40,   3.08, 2.307,   1.806,  1.586,    1.28,  1.25, 1.095,  0.617,      0.4,     0.108,   0.066, 0.0375
   | showValues=format:.2f,offset:1
   | xAxisAngle = 45
}}
|width=340
|caption=The masses of the mid-sized moons, compared to Triton. Values are ×10<sup>21</sup>&nbsp;kg. Dysnomia is given a value at the center of the known range (0.3–0.5). Unmeasured Vanth and Ilmarë are excluded. Enceladus, Miranda, and Mimas are nearly invisible at this scale.
|border=no
}}
In the modern era, Alan Stern considers satellite planets to be one of three categories of planets, along with [[Astronomy:Dwarf planet|dwarf planet]]s and classical planets.<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'? |publisher=News.discovery.com |date=May 14, 2010 |access-date=November 4, 2011 |archive-url=https://web.archive.org/web/20110720105357/http://news.discovery.com/space/should-large-moons-be-called-satellite-planets.html |archive-date= July 20, 2011}}</ref> The term ''planemo'' ("planetary-mass object") covers all three populations.<ref>{{cite journal |last1=Basri |first1=Gibor |last2=Brown |first2=Michael E. |date=2006 |title=Planetesimals to Brown Dwarfs: What is a Planet? |url=http://siba.unipv.it/fisica/articoli/A/Annual%20Review%20Earth%20Planetary%20Sciences_vol.34_2006_pp.193-216.pdf |journal=Annual Review of Earth and Planetary Sciences |volume=34 |pages=193–216 |doi=10.1146/annurev.earth.34.031405.125058 |arxiv=astro-ph/0608417 |bibcode=2006AREPS..34..193B |s2cid=119338327 |archive-url=https://web.archive.org/web/20130731133553/http://siba.unipv.it/fisica/articoli/A/Annual%20Review%20Earth%20Planetary%20Sciences_vol.34_2006_pp.193-216.pdf |archive-date=July 31, 2013 }}</ref>
Stern's and the IAU's definition of 'planet' depends on [[Astronomy:Hydrostatic equilibrium|hydrostatic equilibrium]] – on the body's mass being sufficient to render it plastic, so that it relaxes into an ellipsoid under its own gravity. The IAU definition specifies that the mass is great enough to overcome 'rigid-body forces', and it does not address objects that may be in hydrostatic equilibrium due to a subsurface ocean or (in the case of Io) due to magma caused by tidal heating. Many of the larger icy moons could have subsurface oceans.<ref name="Hussmann Sohl et al. 2006"/> 

The seven largest moons are more massive than the dwarf planet [[Astronomy:Pluto|Pluto]], which is known to be in hydrostatic equilibrium. (They are also known to be more massive than {{dp|Eris}}, a dwarf planet even more massive than Pluto.) These seven are Earth's [[Astronomy:Moon|Moon]], the four Galilean moons of Jupiter ([[Astronomy:Io (moon)|Io]], [[Astronomy:Europa (moon)|Europa]], [[Astronomy:Ganymede (moon)|Ganymede]] and [[Astronomy:Callisto (moon)|Callisto]]), and the largest moons of Saturn ([[Astronomy:Titan (moon)|Titan]]) and of Neptune ([[Astronomy:Triton (moon)|Triton]]). Ganymede and Titan are additionally larger than the planet Mercury, and Callisto is almost as large. All of these moons are ellipsoidal. That said, the two moons larger than Mercury have less than half its mass, and it is mass, along with composition and internal temperature, that determine whether a body is plastic enough to be in hydrostatic equilibrium. Io, Europa, Ganymede, Titan, and Triton are generally believed to be in hydrostatic equilibrium, but Earth's Moon is known not to be in hydrostatic equilibrium, and the situation for Callisto is unclear.

Another dozen moons are ellipsoidal as well, indicating that they achieved equilibrium at some point in their histories. However, it has been shown that some of these moons are no longer in equilibrium, due to them becoming increasingly rigid as they cooled over time.

Neptune's second-largest moon [[Astronomy:Proteus (moon)|Proteus]] (Neptune VIII) has occasionally been included by authors discussing or advocating geophysical conceptions of the 'planet'.<ref name=planetarysociety>Emily Lakdawalla et al., [https://www.planetary.org/worlds/what-is-a-planet What Is A Planet?] {{Webarchive|url=https://web.archive.org/web/20220122142140/https://www.planetary.org/worlds/what-is-a-planet |date=2022-01-22 }} The Planetary Society, 21 April 2020</ref><ref>{{cite web|last=Williams |first=Matt |url=https://phys.org/news/2017-02-geophysical-planet-definition.html |title=A geophysical planet definition |publisher=Phys.org |date= |access-date=2022-05-25}}</ref> It is larger than Mimas but is quite far from being round.

==Current equilibrium moons==
Determining whether a moon is currently in hydrostatic equilibrium requires close observation, and is easier to disprove than to prove. 

Earth's entirely rocky moon solidified out of equilibrium billions of years ago,<ref name="Nimmo2017"/> but most of the other six moons larger than Pluto, four of which are predominantly icy, are assumed to still be in equilibrium. (Ice has less tensile strength than rock, and is deformed at lower pressures and temperatures than rock.) The evidence is perhaps strongest for [[Astronomy:Ganymede (moon)|Ganymede]], which has a magnetic field that indicates the fluid movement of electrically conducting material in its interior, though whether that fluid is a metallic core or a subsurface ocean is unknown.<ref>Planetary Science Decadal Survey Community White Paper, [https://www.lpi.usra.edu/decadal/opag/GanymedeScience.pdf Ganymede science questions and future exploration] {{Webarchive|url=https://web.archive.org/web/20220121050907/https://www.lpi.usra.edu/decadal/opag/GanymedeScience.pdf |date=2022-01-21 }}</ref>
One of the mid-sized moons of Saturn ([[Astronomy:Rhea (moon)|Rhea]]) may also be in equilibrium,<ref name=Thomas2010>P.C. Thomas (2010) [http://www.ciclops.org/media/sp/2011/6794_16344_0.pdf 'Sizes, shapes, and derived properties of the Saturnian satellites after the Cassini nominal mission'] {{Webarchive|url=https://web.archive.org/web/20181223003125/http://www.ciclops.org/media/sp/2011/6794_16344_0.pdf |date=2018-12-23 }}, ''Icarus'' 208: 395–401</ref><ref name="Hussmann Sohl et al. 2006"/> 
as may a couple of the moons of Uranus ([[Astronomy:Titania (moon)|Titania]] and [[Astronomy:Oberon (moon)|Oberon]]).<ref name="Hussmann Sohl et al. 2006">
{{cite journal| doi = 10.1016/j.icarus.2006.06.005| last1 = Hussmann| first1 = Hauke| last2 = Sohl| first2 = Frank| last3 = Spohn| first3 = Tilman| date = November 2006| title = Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-Neptunian objects| journal = Icarus| volume = 185| issue = 1| pages = 258–273| url = https://www.researchgate.net/publication/225019299| bibcode = 2006Icar..185..258H| ref = {{sfnRef|Hussmann Sohl et al.|2006}}}}</ref>
However, the other ellipsoidal moons of Saturn (Mimas, [[Astronomy:Enceladus|Enceladus]], Tethys, Dione and Iapetus) are no longer in equilibrium.<ref name=Thomas2010/> In addition to not being in equilibrium, Mimas and Tethys have very low densities and it has been suggested that they may have non-negligible internal porosity,<ref>{{cite journal |last1=Leliwa-Kopystyński |first1=J. |last2=Kossacki |first2=K. J. |date=2000 |title=Evolution of porosity in small icy bodies |url= |journal=Planetary and Space Science |volume=48 |issue=7–8 |pages=727–745 |doi=10.1016/S0032-0633(00)00038-6 |access-date=}}</ref><ref>{{cite web |url=https://meetingorganizer.copernicus.org/EPSC2022/EPSC2022-732.html |title=Red Streaks on Tethys: Evidence for Recent Activity |last1=Schenk |first1=Paul |last2=Buratti |first2=Bonnie |last3=Clark |first3=Roger |last4=Byrne |first4=Paul |last5=McKinnon |first5=William |last6=Matsuyama |first6=Isamu |last7=Nimmo |first7=Francis |last8=Scipioni |first8=Francesca |date=2022 |website=copernicus.org |publisher=
Europlanet Science Congress 2022 |access-date=20 November 2022 |quote=}}</ref> in which case they would not be satellite planets. The situation for Uranus's three smaller ellipsoidal moons ([[Astronomy:Umbriel (moon)|Umbriel]], [[Astronomy:Ariel (moon)|Ariel]] and [[Astronomy:Miranda (moon)|Miranda]]) is unclear, as is that of Pluto's moon [[Astronomy:Charon (moon)|Charon]].<ref name="Nimmo2017">
{{cite journal |last1=Nimmo |first1=Francis |display-authors=0 |title=Mean radius and shape of Pluto and Charon from New Horizons images|journal=Icarus |date=2017 |volume=287  |pages=12–29 |doi=10.1016/j.icarus.2016.06.027|bibcode=2017Icar..287...12N |arxiv=1603.00821|s2cid=44935431 }}</ref>

The TNO moons Eris I [[Astronomy:Dysnomia (moon)|Dysnomia]], Orcus I [[Astronomy:Vanth (moon)|Vanth]], and possibly [[Astronomy:174567 Varda|Varda]] I Ilmarë are at least the size of Mimas, the smallest ellipsoidal moon of Saturn. However, trans-Neptunian objects appear to become solid bodies at a larger size (around 900–1000&nbsp;km diameter) than the moons of Saturn and Uranus (around 400&nbsp;km diameter). Both Dysnomia and Vanth are dark bodies smaller than 900–1000&nbsp;km, and Dysnomia is known to be low-density, suggesting that it cannot be solid. Consequently, these bodies have been excluded.<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
 |title=The Mutual Orbit, Mass, and Density of Transneptunian Binary Gǃkúnǁʼhòmdímà ({{mp|(229762) 2007 UK|126}})
 |url=http://www2.lowell.edu/users/grundy/abstracts/2019.G-G.html
 |journal=Icarus
 |doi=10.1016/j.icarus.2018.12.037
 |year= 2019
 |volume=334
 |pages=30–38
 |s2cid=126574999
 |archive-url=https://web.archive.org/web/20190407045339/http://www2.lowell.edu/~grundy/abstracts/preprints/2019.G-G.pdf
 |url-status=live
 |archive-date=7 April 2019}}</ref>

===List===
:[[Image:Yes check.svg|15px|Yes]] – believed to be in equilibrium
:[[Image:X mark.svg|15px|No]] – confirmed not to be in equilibrium
:[[Image:Commons-emblem-query.svg|15px|Maybe]] – uncertain evidence

{| class="wikitable"
! colspan="3"| Satellites of planets
|-
| style="background-color: #cc9;"   | [[Astronomy:Moon|Satellite]] of [[Astronomy:Earth|Earth]]
| style="background-color: #ffe8e8;"| [[Astronomy:Moons of Jupiter|Satellites]] of [[Astronomy:Jupiter|Jupiter]]
| style="background-color: #cfc;"   | [[Astronomy:Moons of Uranus|Satellites]] of [[Astronomy:Uranus|Uranus]]
|-
| style="background-color: #ffc;"   | [[Astronomy:Moons of Saturn|Satellites]] of [[Astronomy:Saturn|Saturn]]
| style="background-color: #def;"   | [[Astronomy:Moons of Neptune|Satellites]] of [[Astronomy:Neptune|Neptune]]
|
|-
! colspan="3"| Satellites of generally agreed dwarf planets
|-
| style="background-color: #f5deb3;"| [[Astronomy:Moons of Pluto|Satellites]] of [[Astronomy:Pluto|Pluto]]
|
|
|}

{| class="wikitable sortable"
|+List of ellipsoidal moons<ref>Most figures are from the NASA/JPL list of [https://ssd.jpl.nasa.gov/?sat_phys_par Planetary Satellite Physical Parameters] {{Webarchive|url=https://web.archive.org/web/20190104060318/https://ssd.jpl.nasa.gov/?sat_phys_par |date=2019-01-04 }}<!--accessed 2019 Sept 15-->, apart from the masses of the Uranian moons, which are from Jacobson (2014).<!--,[https://iopscience.iop.org/article/10.1088/0004-6256/148/5/76/pdf] and the Ilmare data.[https://arxiv.org/abs/1505.00510]--></ref>
! colspan=2 | Moon
! rowspan=2 class="unsortable"| Image
! colspan=2 | Radius
! colspan=2 | Mass
! data-sort-type="number" | Density
! data-sort-type="number" | Surface gravity
! rowspan=2 data-sort-type="number" | Year of<br />discovery
! rowspan=2 |  Hydrostatic<br />equilibrium?
|-
! Name
! {{small|Designation}}
! data-sort-type="number" | ([[Kilometre|km]])
! data-sort-type="number" | (R<sub>☾</sub>)
! data-sort-type="number" | (10<sup>21</sup>&nbsp;kg)
! data-sort-type="number" | (M<sub>☾</sub>)
! data-sort-type="number" | (g/cm<sup>3</sup>)
! data-sort-type="number" | ([[Physics:Standard gravity|''g'']])
|-  style="background:#ffe8e8; text-align:center;"
|[[Astronomy:Ganymede (moon)|Ganymede]] || {{small|Jupiter III}}
| style="background:black;"| [[File:Ganymede g1 true-edit1.jpg|50px|center]]
|{{val|2634.1|0.3}}
|156.4%
|{{val|148.2}}
|201.8%
|{{val|1.942|0.005}}
|0.146
| 1610
| [[Image:Yes check.svg|15px|Yes]]

|-  style="background:#ffc; text-align:center;"
|[[Astronomy:Titan (moon)|Titan]] || {{small|Saturn VI}}
| style="background:black;"| [[File:Titan in true color.jpg|50px|center]]
| {{val|2574.7|0.1}}
|148.2%
|{{val|134.5}}
|183.2%
|{{val|1.882|0.001}}
|0.138
| 1655
| [[Image:Commons-emblem-query.svg|15px|Maybe]]<ref>{{cite journal |last1=Durante |first1=Daniele |last2=Hemingway |first2=D. J. |first3=P. |last3=Racioppa |first4=L. |last4=Iess |first5=D. J. |last5=Stevenson |date=2019 |title=Titan's gravity field and interior structure after Cassini |url=http://douglashemingway.com/publications/Durante2019.pdf |journal=Icarus |volume=326 |pages=123–132 |doi=10.1016/j.icarus.2019.03.003 |hdl=11573/1281269 |s2cid=127984873 |access-date=3 April 2022}}</ref>

|-  style="background:#ffe8e8; text-align:center;"
|[[Astronomy:Callisto (moon)|Callisto]] || {{small|Jupiter IV}}
| style="background:black;"| [[File:Callisto.jpg|50px|center]]
|{{val|2410.3|1.5}}
|138.8%
|{{val|107.6}}
|146.6%
|{{val|1.834|0.003}}
|0.126
| 1610
| [[Image:Commons-emblem-query.svg|15px|Maybe]]<ref name="Castillo-Rogez2011">{{cite journal |last1=Castillo-Rogez |first1=J. C.|display-authors=0 |title=How differentiated is Callisto |journal=42nd Lunar and Planetary Science Conference |date=2011 |page=2580 |url=https://www.lpi.usra.edu/meetings/lpsc2011/pdf/2580.pdf |access-date=2 January 2020}}</ref>

|-  style="background:#ffe8e8; text-align:center;"
|[[Astronomy:Io (moon)|Io]] || {{small|Jupiter I}}
| style="background:black;"| [[File:Io highest resolution true color.jpg|50px|center]]
|{{val|1821.6|.5}}
|104.9%
|{{val|89.3}}
|121.7%
|{{val|3.528|0.006}}
|0.183
| 1610
| [[Image:Yes check.svg|15px|Yes]]

|-  style="background:#cc9; text-align:center;"
|[[Astronomy:Moon|Moon]] (Luna) || {{small|Earth I}}
| style="background:black;"| [[File:FullMoon2010.jpg|50px|center]]
|{{val|1737.05}}
| 100%
|{{val|73.4}}
| 100%
|{{val|3.344|0.005}}
|0.165
| Prehistoric
| [[Image:X mark.svg|15px|No]]<ref>{{cite journal |last1=Garrick-Bethell |first1=I. |last2=Wisdom |first2=J |last3=Zuber |first3=MT |title=Evidence for a Past High-Eccentricity Lunar Orbit |journal=Science |date=4 August 2006 |volume=313 |issue=5787 |pages=652–655 |doi=10.1126/science.1128237 |pmid=16888135 |bibcode=2006Sci...313..652G |s2cid=317360 }}</ref>

|-  style="background:#ffe8e8; text-align:center;"
|[[Astronomy:Europa (moon)|Europa]] || {{small|Jupiter II}}
| style="background:black;"| [[File:Europa-moon.jpg|50px|center]]
|{{val|1560.8|.5}}
|89.9%
|{{val|48.0}}
|65.4%
|{{val|3.013|0.005}}
|0.134
| 1610
| [[Image:Yes check.svg|15px|Yes]]

|-  style="background:#def; text-align:center;"
|[[Astronomy:Triton (moon)|Triton]] || {{small|Neptune I}}
| style="background:black;"| [[File:Triton moon mosaic Voyager 2 (large).jpg|50px|center]]
|{{val|1353.4|0.9}}
|79.9%
|{{val|21.4}}
|29.1%
|{{val|2.059|.005}}
|0.080
| 1846
| [[Image:Yes check.svg|15px|Yes]]

|-  style="background:#cfc; text-align:center;"
|[[Astronomy:Titania (moon)|Titania]] || {{small|Uranus III}}
| style="background:black;"| [[File:Titania (moon) color cropped.jpg|50px|center]]
|{{val|788.9|1.8}}
|45.4%
|{{val|3.40|.06}}
|4.6% 
|{{val|1.66|0.04}}
|0.040
| 1787
|[[Image:Commons-emblem-query.svg|15px|Maybe]]<ref name="Hussmann Sohl et al. 2006"/>

|-  style="background:#ffc; text-align:center;"
|[[Astronomy:Rhea (moon)|Rhea]] || {{small|Saturn V}}
| style="background:black;"| [[File:PIA07763 Rhea full globe5.jpg|50px|center]]
|{{val|764.3|1.0}}
|44.0%
|{{val|2.31}}
|3.1%
|{{val|1.233|0.005}}
|0.027
| 1672
|[[Image:Commons-emblem-query.svg|15px|Maybe]]<ref name=Thomas2010/>

|-  style="background:#cfc; text-align:center;"
|[[Astronomy:Oberon (moon)|Oberon]] || {{small|Uranus IV}}
| style="background:black;"| [[File:Voyager 2 picture of Oberon.jpg|50px|center]]
|{{val|761.4|2.6}}
|43.8%
|{{val|3.08|.09}}
|4.2%
|{{val|1.56|0.06}}
|0.036
| 1787
|[[Image:Commons-emblem-query.svg|15px|Maybe]]<ref name="Hussmann Sohl et al. 2006"/>

|-  style="background:#ffc; text-align:center;"
|Iapetus || {{small|Saturn VIII}}
| style="background:black;"| [[File:Iapetus as seen by the Cassini probe - 20071008.jpg|50px|center]]
|{{val|735.6|1.5}}
|42.3%
|{{val|1.81}}
|2.5%
|{{val|1.083|0.007}}
|0.022
| 1671
|[[Image:X mark.svg|15px|No]]<ref name=Thomas2010/>

|-  style="background:#f5deb3; text-align:center;"
|[[Astronomy:Charon (moon)|Charon]] || {{small|Pluto I}}
| style="background:black;"| [[File:Charon in True Color - High-Res.jpg|50px|center]]
|{{val|603.6|1.4}}
|34.7%
|{{val|1.53}}
|2.1%
|{{val|1.664|0.012}}
|0.029
| 1978
|[[Image:Commons-emblem-query.svg|15px|Maybe]]<ref name="Nimmo2017"/>

|-  style="background:#cfc; text-align:center;"
|[[Astronomy:Umbriel (moon)|Umbriel]] || {{small|Uranus II}}
| style="background:black;"| [[File:PIA00040 Umbrielx2.47.jpg|50px|center]]
|{{val|584.7|2.8}}
|33.7%
|{{val|1.28|0.03}}
|1.7%
|{{val|1.46|0.09}}
|0.023
| 1851
|

|-  style="background:#cfc; text-align:center;"
|[[Astronomy:Ariel (moon)|Ariel]] || {{small|Uranus I}}
| style="background:black;"| [[File:Ariel (moon).jpg|50px|center]]
|{{val|578.9|0.6}}
|33.3%
|{{val|1.25|0.02}}
|1.7%
|{{val|1.59|0.09}}
|0.028
| 1851
|

|-  style="background:#ffc; text-align:center;"
|Dione || {{small|Saturn IV}}
| style="background:black;"| [[File:Dione in natural light.jpg|50px|center]]
|{{val|561.4|0.4}}
|32.3%
|{{val|1.10}}
|1.5%
|{{val|1.476|0.004}}
|0.024
| 1684
|[[Image:X mark.svg|15px|No]]<ref name=Thomas2010/>
|-  style="background:#ffc; text-align:center;"
|Tethys || {{small|Saturn III}}
| style="background:black;"| [[File:PIA18317-SaturnMoon-Tethys-Cassini-20150411.jpg|50px|center]]
|{{val|533.0|0.7}}
|30.7%
|{{val|0.617}}
|0.84%
|{{val|0.973|0.004}}

|0.015
| 1684
|[[Image:X mark.svg|15px|No]]<ref name=Thomas2010/>


|-  style="background:#ffc; text-align:center;"
|[[Astronomy:Enceladus|Enceladus]] || {{small|Saturn II}}
|bgcolor=black| [[File:PIA17202-SaturnMoon-Enceladus-ApproachingFlyby-20151028.jpg|50px|center]]
| {{val|252.1|0.2}}
| 14.5%
| {{val|0.108}}
| 0.15%
| {{val|1.608|0.003}}
|0.011
| 1789
|[[Image:X mark.svg|15px|No]]<ref name=Thomas2010/>

|-  style="background:#cfc; text-align:center;"
|[[Astronomy:Miranda (moon)|Miranda]] || {{small|Uranus V}}
|bgcolor=black| [[File:Miranda - January 24 1986 (30906319004).jpg|50px|center]]
| {{val|235.8|0.7}}
| 13.6%
| {{val|0.064|0.003}}
| 0.09%
| {{val|1.21|0.11}}
|0.008
| 1948
|

|-  style="background:#ffc; text-align:center;"
|Mimas || {{small|Saturn I}}
|bgcolor=black| [[File:Mimas Cassini.jpg|50px|center]]
| {{val|198.2|0.4}}
| 11.4%
| {{val|0.038}}
| 0.05%
| {{val|1.150|0.004}}
|0.006
| 1789
|[[Image:X mark.svg|15px|No]]<ref name=Thomas2010/>
|}

(Saturn VII is [[Astronomy:Hyperion (moon)|Hyperion]], which is not gravitationally rounded; it is smaller than Mimas.)

===Atmospheres===

Titan has a denser atmosphere (1.4 bar) than Earth; it is the only known moon with a significant atmosphere. Triton (14 μbar), Io (1.9 {{Not a typo|nbar}}), and Callisto (26 {{Not a typo|pbar}}) have very thin atmospheres, but still enough to have collisions between atmospheric molecules. Other planetary-mass moons only have [[Earth:Exosphere|exosphere]]s at most.<ref name=atmosphere>[https://www.planetary.org/articles/04081101-a-moon-with-atmosphere A Moon with Atmosphere] {{Webarchive|url=https://web.archive.org/web/20220208140112/https://www.planetary.org/articles/04081101-a-moon-with-atmosphere |date=2022-02-08 }}, Emily Lakdwalla, ''The Planetary Society'' (8 April 2015)</ref> Exospheres have been detected around Earth's Moon, Europa, Ganymede,<ref name=atmosphere/> Enceladus,<ref name="Dougherty">{{cite journal|doi=10.1126/science.1120985|title=Identification of a Dynamic Atmosphere at Enceladus with the Cassini Magnetometer|date=2006|journal=Science|volume=311|pages=1406–9|pmid=16527966|issue=5766|bibcode=2006Sci...311.1406D|last1=Dougherty|first1=M. K.|last2=Khurana|first2=K. K.|s2cid=42050327|display-authors=0}}</ref> Dione,<ref name="Ghosh">{{cite news | first = Pallab | last = Ghosh | title = Oxygen envelops Saturn's icy moon | date = 2 March 2012 | url = https://www.bbc.co.uk/news/science-environment-17225127 | work = BBC News | access-date = 2012-03-02}}</ref> and Rhea.<ref name="Teolis2010">
{{Cite journal | last1 = Teolis | first1 = B. D. | last2 = Jones | first2 = G. H. | last3 = Miles | first3 = P. F. | last4 = Tokar | first4 = R. L. | last5 = Magee | first5 = B. A. | last6 = Waite | first6 = J. H. | last7 = Roussos | first7 = E. | last8 = Young | first8 = D. T. | last9 = Crary | first9 = F. J. | last10 = Coates | first10 = A. J. | last11 = Johnson | first11 = R. E. | last12 = Tseng | first12 = W. - L. | last13 = Baragiola | first13 = R. A. | title = Cassini Finds an Oxygen-Carbon Dioxide Atmosphere at Saturn's Icy Moon Rhea | journal = Science| volume = 330 | issue = 6012| pages = 1813–1815| year = 2010 | pmid = 21109635 | doi = 10.1126/science.1198366|bibcode = 2010Sci...330.1813T | s2cid = 206530211 | doi-access = free }}
</ref> An exosphere around Titania is a possibility, though it has not been confirmed.<ref name="Widemann Sicardy et al. 2009">
{{cite journal| doi = 10.1016/j.icarus.2008.09.011| last1 = Widemann | first1 = T.| last2 = Sicardy | first2 = B.| last3 = Dusser | first3 = R.| last4 = Martinez | first4 = C.| last5 = Beisker | first5 = W.| last6 = Bredner | first6 = E.| last7 = Dunham | first7 = D.| last8 = Maley | first8 = P.| last9 = Lellouch | first9 = E.| date = February 2009| last10 = Arlot | first10 = J. -E.| last11 = Berthier | first11 = J.| last12 = Colas | first12 = F.| last13 = Hubbard | first13 = W. B.| last14 = Hill | first14 = R.| last15 = Lecacheux | first15 = J.| last16 = Lecampion | first16 = J. -F.| last17 = Pau | first17 = S.| last18 = Rapaport | first18 = M.| last19 = Roques | first19 = F.| last20 = Thuillot | first20 = W.| last21 = Hills | first21 = C. R.| last22 = Elliott | first22 = A. J.| last23 = Miles | first23 = R.| last24 = Platt | first24 = T.| last25 = Cremaschini | first25 = C.| last26 = Dubreuil | first26 = P.| last27 = Cavadore | first27 = C.| last28 = Demeautis | first28 = C.| last29 = Henriquet | first29 = P.| last30 = Labrevoir | first30 = O.| title = Titania's radius and an upper limit on its atmosphere from the September 8, 2001 stellar occultation| journal = Icarus| volume = 199| issue = 2| pages = 458–476| url = http://www.lesia.obspm.fr/perso/thomas-widemann/eprint/Widemann_etal2009.pdf| bibcode = 2009Icar..199..458W| display-authors = 29 }}
</ref>

==See also==
*List of gravitationally rounded objects of the Solar System
*[[Astronomy:List of Solar System objects by size|List of Solar System objects by size]]

==References==
{{reflist}}

==Further reading==
*[https://arxiv.org/abs/2110.15285 Moons Are Planets: Scientific Usefulness Versus Cultural Teleology in the Taxonomy of Planetary Science], Philip T. Metzger, William M. Grundy, Mark Sykes, S. Alan Stern, James F. Bell III, Charlene E. Detelich, Kirby D. Runyon, Michael Summers, 22 Oct 2021

{{Solar System moons (compact)}}
[[Category:Planetary science]]
[[Category:Planets]]
[[Category:Moons]]
[[Category:Lists of moons]]

{{Sourceattribution|Planetary-mass moon}}