Planetary-mass moon

Short description: Planetary-mass bodies that are also natural satellites

Planetary-mass moons larger than Pluto, the largest Solar dwarf planet.

A planetary-mass moon is a planetary-mass object that is a natural satellite of another non-stellar celestial object. Because of their mass, these moons are large and ellipsoidal (sometimes spherical) in shape due to hydrostatic equilibrium caused by internal partial melting and differentiation and/or from tidal or radiogenic heating, in some cases forming a subsurface ocean.

Planetary-mass moons are sometimes called satellite planets by some planetary scientists such as Alan Stern, who are more concerned with whether a celestial body has planetary geology (that is, whether it is a planetary body) than its solar or non-solar orbit (planetary dynamics).[1] Thus they consider planetary-mass moons to be a subset of the planets. This conceptualization of planets as three classes of objects (classical planets, dwarf planets and satellite planets) has not been accepted by the International Astronomical Union (the IAU).

Two moons in the Solar System, Ganymede and Titan, are larger than the terrestrial planet Mercury, and a third, Callisto, is just slightly smaller than it, although all three are less massive than Mercury. Additionally, seven moons — Ganymede, Titan, Callisto, Io, Luna (Earth's Moon), Europa, and Triton — are larger and more massive than the largest and most massive dwarf planets Pluto and Eris.

## Contents

* 1 Early history
* 2 Modern concept
* 3 Possible equilibrium moons
* 3.1 List
* 3.2 Atmospheres
* 4 See also
* 5 References
* 6 Further reading

## 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 Galileo discovered the first satellites of another planet (the four 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."[2] Similarly, Christiaan Huygens, upon discovering Saturn's largest moon Titan in 1655, employed the terms "planeta" (planet), "Stella" (star), "luna" (moon), and the more modern "satellite" (attendant) to describe it.[3] Giovanni Cassini, in announcing his discovery of Saturn's moons Iapetus and Rhea in 1671 and 1672, described them as Nouvelles Planetes autour de Saturne ("New planets around Saturn").[4] 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".[5] When William Herschel announced his discovery of two objects in orbit around Uranus (Titania and Oberon) in 1787, he referred to them as "satellites" and "secondary planets".[6] All subsequent discovery reports of natural satellite used the term "satellite" exclusively,[7] though the 1868 book Smith's Illustrated Astronomy referred to satellites as "secondary planets".[8]

## Modern concept

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Comparative masses of the seven largest moons.

Ganymede (148.2%)

Titan (134.5%)

Callisto (107.6%)

Io (89.32%)

Moon (73.46%)

Europa (48.00%)

Triton (21.39%)

[other] (17%)

The masses of the mid-sized moons, compared to Triton.

Alan Stern considers satellite planets to be one of three categories of planets, along with dwarf planets and classical planets.[9] The term planemo ("planetary-mass object") covers all three populations.[10] Stern's and the IAU's definition of 'planet' depends on 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.[11]

The seven largest moons are more massive than the dwarf planet Pluto, which is known to be in hydrostatic equilibrium (they are also known to be more massive than Eris, a dwarf planet even more massive than Pluto). These seven are Earth's Moon, the four Galilean moons of Jupiter (Io, Europa, Ganymede and Callisto), and the largest moons of Saturn (Titan) and of Neptune (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 Proteus, (Neptune VIII) has occasionally been included by authors discussing or advocating geophysical conceptions of the 'planet'.[12][13] It is larger than Mimas but is quite far from being round.

## Possible 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,[14] 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 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.[15] One of the mid-sized moons of Saturn (Rhea) may also be in equilibrium,[16][11] as may a couple of the moons of Uranus (Titania and Oberon).[11] However, the other ellipsoidal moons of Saturn (Mimas, Enceladus, Tethys, Dione and Iapetus) are no longer in equilibrium.[16] 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,[17][18] in which case they would not be satellite planets. The situation for Uranus's three smaller ellipsoidal moons (Umbriel, Ariel and Miranda) is unclear, as is that of Pluto's moon Charon.[14]

The TNO moons Eris I Dysnomia, Orcus I Vanth, and possibly 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 km diameter) than the moons of Saturn and Uranus (around 400 km diameter). Both Dysnomia and Vanth are dark bodies smaller than 900–1000 km, and Dysnomia is known to be low-density, suggesting that it cannot be solid. Consequently, these bodies have been excluded.[19]

### List

– believed to be in equilibrium
– confirmed not to be in equilibrium
– uncertain evidence
Satellites of planets

Satellites of Earth | Satellites of Jupiter | Satellites of Uranus
Satellites of Saturn | Satellites of Neptune |
Satellites of generally agreed dwarf planets
Satellites of Pluto |  |
List of ellipsoidal moons[20] Moon  | Image  | Radius  | Mass  | Density  | Surface gravity  | Year of
discovery  | Hydrostatic
equilibrium?

|
|
|
|
|
|
|

Name  | Designation | (km)  | (R☾)  | (1021 kg)  | (M☾)  | (g/cm3)  | (g)
Ganymede | Jupiter III |  | 2634.1±0.3 | 156.4%  | 148.2 | 201.8%  | 1.942±0.005 | 0.146  | 1610  |
Titan | Saturn VI |  | 2574.7±0.1 | 148.2%  | 134.5 | 183.2%  | 1.88±0.001 | 0.138  | 1655  | [21]
Callisto | Jupiter IV |  | 2410.3±1.5 | 138.8%  | 107.6 | 146.6%  | 1.834±0.003 | 0.126  | 1610  | [22]
Io | Jupiter I |  | 1821.6±0.5 | 104.9%  | 89.3 | 121.7%  | 3.528±0.006 | 0.183  | 1610  |
Moon (Luna) | Earth I |  | 1737.05 | 100%  | 73.4 | 100%  | 3.344±0.005 | 0.165  | Prehistoric  | [23]
Europa | Jupiter II |  | 1560.8±0.5 | 89.9%  | 48.0 | 65.4%  | 3.013±0.005 | 0.134  | 1610  |
Triton | Neptune I | Error creating thumbnail:  | 1353.4±0.9 | 79.9%  | 21.4 | 29.1%  | 2.059±0.005 | 0.080  | 1846  |
Titania | Uranus III |  | 788.9±1.8 | 45.4%  | 3.4550±0.0509[24] | 4.6%  | 1.683 | 0.040  | 1787  | [11]
Rhea | Saturn V |  | 764.3±1.0 | 44.0%  | 2.31 | 3.1%  | 1.237±0.005 | 0.027  | 1672  | [16]
Oberon | Uranus IV |  | 761.4±2.6 | 43.8%  | 3.1104±0.0749[24] | 4.2%  | 1.682 | 0.036  | 1787  | [11]
Iapetus | Saturn VIII |  | 735.6±1.5 | 42.3%  | 1.81 | 2.5%  | 1.089±0.007 | 0.022  | 1671  | [16]
Charon | Pluto I |  | 606±0.5 | 34.7%  | 1.58 | 2.1%  | 1.705±0.006 | 0.029  | 1978  | [14]
Umbriel | Uranus II |  | 584.7±2.8 | 33.7%  | 1.29±0.03 | 1.7%  | 1.538 | 0.023  | 1851  |
Ariel | Uranus I |  | 578.9±0.6 | 33.3%  | 1.23±0.02 | 1.7%  | 1.517 | 0.028  | 1851  |
Dione | Saturn IV |  | 561.4±0.4 | 32.3%  | 1.095 | 1.5%  | 1.476±0.004 | 0.024  | 1684  | [16]
Tethys | Saturn III |  | 533.0±0.7 | 30.7%  | 0.617 | 0.84%  | 0.984±0.004 | 0.015  | 1684  | [16]
Enceladus | Saturn II |  | 252.1±0.2 | 14.5%  | 0.108 | 0.15%  | 1.608±0.003 | 0.011  | 1789  | [16]
Miranda | Uranus V |  | 235.8±0.7 | 13.6%  | 0.063 | 0.09%  | 1.21±0.11 | 0.008  | 1948  |
Mimas | Saturn I |  | 198.2±0.4 | 11.4%  | 0.0375 | 0.05%  | 1.150±0.004 | 0.006  | 1789  | [16]

Methone, Pallene, and, with less certainty, Aegaeon are in hydrostatic equilibrium.[25] However, as they are not planetary-mass objects, these are not included as planetary-mass moons.

### Atmospheres

Titan has a denser atmosphere than Earth, with a surface pressure of 1.4 bar, while Triton has a relatively thinner atmosphere of 14 μbar; Titan and Triton are the only known moons to have atmospheres significant enough to drive weather and climate processes.[26] Io (1.9 ) and Callisto (26 {{Not a typo|pbar} tmospheres, but still enough to have collisions between molecules and thus to be gaseous. Other planetary-mass moons only have exospheres at most.[27] Exospheres have been detected around Earth's Moon, Europa, Ganymede,[27] Enceladus,[28] Dione,[29] and Rhea.[30] An exosphere around Titania is a possibility, though it has not been confirmed.[31]

## See also

* List of gravitationally rounded objects of the Solar System
* List of Solar System objects by size

## References

1. ↑ "Should Large Moons Be Called 'Satellite Planets'?". Discovery News. News.discovery.com. 2010-05-14. http://news.discovery.com/space/should-large-moons-be-called-satellite-planets.html#post-a-comment.
2. ↑ Galileo Galilei (1989). Siderius Nuncius. Albert van Helden. University of Chicago Press. p. 26.
3. ↑ Christiani Hugenii (Christiaan Huygens) (1659). Systema Saturnium: Sive de Causis Miradorum Saturni Phaenomenon, et comite ejus Planeta Novo. Adriani Vlacq. pp. 1–50.
4. ↑ Giovanni Cassini (1673). Decouverte de deux Nouvelles Planetes autour de Saturne. Sabastien Mabre-Craniusy. pp. 6–14.
5. ↑ Cassini, G. D. (1686–1692). "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". Philosophical Transactions of the Royal Society of London 16 (179–191): 79–85. doi:10.1098/rstl.1686.0013. Bibcode: 1686RSPT...16...79C.
6. ↑ William Herschel (1787). An Account of the Discovery of Two Satellites Around the Georgian Planet. Read at the Royal Society. J. Nichols. pp. 1–4.
7. ↑ See primary citations in Timeline of discovery of Solar System planets and their moons
8. ↑ Smith, Asa (1868). Smith's Illustrated Astronomy. Nichols & Hall. p. 23. https://archive.org/details/smithsillustrat00smitgoog. "secondary planet Herschel."
9. ↑ "Should Large Moons Be Called 'Satellite Planets'?". Discovery News. News.discovery.com. May 14, 2010. http://news.discovery.com/space/should-large-moons-be-called-satellite-planets.html.
10. ↑ Basri, Gibor; Brown, Michael E. (2006). "Planetesimals to Brown Dwarfs: What is a Planet?". Annual Review of Earth and Planetary Sciences 34: 193–216. doi:10.1146/annurev.earth.34.031405.125058. Bibcode: 2006AREPS..34..193B. http://siba.unipv.it/fisica/articoli/A/Annual Review Earth Planetary Sciences_vol.34_2006_pp.193-216.pdf.
11. ↑ 11.0 11.1 11.2 11.3 11.4 Hussmann, Hauke; Sohl, Frank; Spohn, Tilman (November 2006). "Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-Neptunian objects". Icarus 185 (1): 258–273. doi:10.1016/j.icarus.2006.06.005. Bibcode: 2006Icar..185..258H. https://www.researchgate.net/publication/225019299.
12. ↑ Emily Lakdawalla et al., What Is A Planet? The Planetary Society, 21 April 2020
13. ↑ Williams, Matt. "A geophysical planet definition". Phys.org. https://phys.org/news/2017-02-geophysical-planet-definition.html.
14. ↑ 14.0 14.1 14.2 Nimmo, Francis (2017). "Mean radius and shape of Pluto and Charon from New Horizons images". Icarus 287: 12–29. doi:10.1016/j.icarus.2016.06.027. Bibcode: 2017Icar..287...12N.
15. ↑ Planetary Science Decadal Survey Community White Paper, Ganymede science questions and future exploration
16. ↑ 16.0 16.1 16.2 16.3 16.4 16.5 16.6 16.7 P.C. Thomas (2010) 'Sizes, shapes, and derived properties of the Saturnian satellites after the Cassini nominal mission' , Icarus 208: 395–401
17. ↑ Leliwa-Kopystyński, J.; Kossacki, K. J. (2000). "Evolution of porosity in small icy bodies". Planetary and Space Science 48 (7–8): 727–745. doi:10.1016/S0032-0633(00)00038-6. Bibcode: 2000P&SS...48..727L.
18. ↑ Schenk, Paul; Buratti, Bonnie; Clark, Roger; Byrne, Paul; McKinnon, William; Matsuyama, Isamu; Nimmo, Francis; Scipioni, Francesca (2022). "Red Streaks on Tethys: Evidence for Recent Activity". European Planetary Science Congress (Europlanet Science Congress 2022). doi:10.5194/epsc2022-732. Bibcode: 2022EPSC...16..732S. https://meetingorganizer.copernicus.org/EPSC2022/EPSC2022-732.html. Retrieved 20 November 2022.
19. ↑ Grundy, W.M.; Noll, K.S.; Buie, M.W.; Benecchi, S.D.; Ragozzine, D.; Roe, H.G. (2019). "The Mutual Orbit, Mass, and Density of Transneptunian Binary Gǃkúnǁʼhòmdímà ((229762) 2007 UK126)". Icarus 334: 30–38. doi:10.1016/j.icarus.2018.12.037. Bibcode: 2019Icar..334...30G. http://www2.lowell.edu/users/grundy/abstracts/2019.G-G.html.(229762)+2007+UK126)&rft.jtitle=Icarus&rft.aulast=Grundy&rft.aufirst=W.M.&rft.au=Grundy, W.M.&rft.au=Noll, K.S.&rft.au=Buie, M.W.&rft.au=Benecchi, S.D.&rft.au=Ragozzine, D.&rft.au=Roe, H.G.&rft.date=2019&rft.volume=334&rft.pages=30–38&rft_id=info:doi/10.1016/j.icarus.2018.12.037&rft_id=info:bibcode/2019Icar..334...30G&rft_id=http://www2.lowell.edu/users/grundy/abstracts/2019.G-G.html&rfr_id=info:sid/en.wikibooks.org:Astronomy:Planetary-mass_moon">
20. ↑ Most figures are from the NASA/JPL list of Planetary Satellite Physical Parameters , apart from the masses of the Uranian moons, which are from Jacobson (2014).
21. ↑ Durante, Daniele; Hemingway, D. J.; Racioppa, P.; Iess, L.; Stevenson, D. J. (2019). "Titan's gravity field and interior structure after Cassini". Icarus 326: 123–132. doi:10.1016/j.icarus.2019.03.003. Bibcode: 2019Icar..326..123D. https://douglashemingway.com/publications/Durante2019.pdf. Retrieved 3 April 2022.
22. ↑ Castillo-Rogez, J. C. (2011). "How differentiated is Callisto". 42nd Lunar and Planetary Science Conference (1608): 2580. Bibcode: 2011LPI....42.2580C. https://www.lpi.usra.edu/meetings/lpsc2011/pdf/2580.pdf. Retrieved 2 January 2020.
23. ↑ Garrick-Bethell, I.; Wisdom, J; Zuber, MT (4 August 2006). "Evidence for a Past High-Eccentricity Lunar Orbit". Science 313 (5787): 652–655. doi:10.1126/science.1128237. PMID 16888135. Bibcode: 2006Sci...313..652G.
24. ↑ 24.0 24.1 French, Richard G.; Hedman, Matthew M.; Nicholson, Philip D.; Longaretti, Pierre-Yves; McGhee-French, Colleen A. (2024-03-15). "The Uranus system from occultation observations (1977–2006): Rings, pole direction, gravity field, and masses of Cressida, Cordelia, and Ophelia". Icarus 411. doi:10.1016/j.icarus.2024.115957. ISSN 0019-1035. Bibcode: 2024Icar..41115957F. https://www.sciencedirect.com/science/article/pii/S0019103524000150.
25. ↑ Thomas, P. C.; Burns, J. A.; Tiscareno, M. S.; Hedman, M. M.; Helfenstein, P. (March 2013). "Saturn's Mysterious Arc-Embedded Moons: Recycled Fluff?". 44th Lunar and Planetary Science Conference. The Woodlands, Texas. Bibcode: 2013LPI....44.1598T. https://www.lpi.usra.edu/meetings/lpsc2013/pdf/1598.pdf.
26. ↑ Ingersoll, Andrew P. (1990). "Dynamics of Triton's atmosphere". Nature 344 (6264): 315–317. doi:10.1038/344315a0. Bibcode: 1990Natur.344..315I.
27. ↑ 27.0 27.1 A Moon with Atmosphere , Emily Lakdwalla, The Planetary Society (8 April 2015)
28. ↑ Dougherty, M. K. et al. (2006). "Identification of a Dynamic Atmosphere at Enceladus with the Cassini Magnetometer". Science 311 (5766): 1406–9. doi:10.1126/science.1120985. PMID 16527966. Bibcode: 2006Sci...311.1406D.
29. ↑ Ghosh, Pallab (2 March 2012). "Oxygen envelops Saturn's icy moon". BBC News. https://www.bbc.co.uk/news/science-environment-17225127.
30. ↑ Teolis, B. D.; Jones, G. H.; Miles, P. F.; Tokar, R. L.; Magee, B. A.; Waite, J. H.; Roussos, E.; Young, D. T. et al. (2010). "Cassini Finds an Oxygen-Carbon Dioxide Atmosphere at Saturn's Icy Moon Rhea". Science 330 (6012): 1813–1815. doi:10.1126/science.1198366. PMID 21109635. Bibcode: 2010Sci...330.1813T.
31. ↑ Widemann, T.; Sicardy, B.; Dusser, R.; Martinez, C.; Beisker, W.; Bredner, E.; Dunham, D.; Maley, P. et al. (February 2009). "Titania's radius and an upper limit on its atmosphere from the September 8, 2001 stellar occultation". Icarus 199 (2): 458–476. doi:10.1016/j.icarus.2008.09.011. Bibcode: 2009Icar..199..458W. http://www.lesia.obspm.fr/perso/thomas-widemann/eprint/Widemann_etal2009.pdf.

## Further reading

* 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

* v
* t
* e

Natural satellites of the Solar System

Planetary
satellites|

* Terrestrial
* Martian
* Jovian
* Saturnian
* Uranian
* Neptunian

Dwarf-planet
satellites|

* Plutonian
* Haumean
* Makemakean
* Eridian
* OR10
* Quaoaran
* Orcean

Minor-planet moons|

* Main belt binaries:
* Antiope
* Frostia
* Berna
* Tokai
* Ostro
* Balam
* Doppler
* Pauling
* Iwamoto
* Wolff
* Bettig
* Christophedumas
* Trojans:
* Patroclus–Menoetius
* Hektor–Skamandrios
* Iphthime
* Eurydamas
* TNOs:
* Huya
* Typhon–Echidna
* Lempo–Paha–Hiisi
* 2002 UX25
* Logos–Zoe
* Ceto–Phorcys
* Borasisi–Pabu
* Sila–Nunam
* Teharonhiawako–Sawiskera
* Salacia–Actaea
* 2002 WC19
* Altjira
* Varda–Ilmarë
* 2003 AZ84
* Gǃkúnǁʼhòmdímà–Gǃòʼé ǃHú
* Mors–Somnus
* Manwë–Thorondor
* ǂKá̦gára–ǃHãunu
* 2013 FY27

Ranked by size|

* Planetary-mass moon
* Ganymede
* largest: 5268 km / 0.413 Earths
* Titan
* Callisto
* Io
* Moon
* Europa
* Triton
* Titania
* Rhea
* Oberon
* Iapetus
* Charon
* Umbriel
* Ariel
* Dione
* Tethys
* Dysnomia
* Enceladus
* Miranda
* Vanth
* Proteus
* Mimas
* Ilmarë
* Nereid
* Hiʻiaka
* Actaea
* Hyperion
* ...

* Discovery timeline
* Inner moons
* Irregular moons
* List
* Planetary-mass moons
* Naming
* Subsatellite
* Regular moons
* Trojan moons

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