Planetary-mass moon


A planetary-mass moon is a planetary-mass object that is also a natural satellite. They are large and ellipsoidal (sometimes spherical) in shape. Two moons in the Solar System are larger than the planet Mercury (though less massive): Ganymede and Titan, and seven are larger and more massive than the dwarf planet Pluto.

The concept of satellite planets – the idea that planetary-mass objects, including planetary-mass moons, are planets – is used 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 where it orbits (planetary dynamics).[1] 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). In addition, the IAU definition of 'hydrostatic equilibrium' is quite restrictive – that the object's mass be sufficient for gravity to overcome rigid-body forces to become plastic – whereas planetary-mass moons may be in hydrostatic equilibrium due to tidal or radiogenic heating, in some cases forming a subsurface ocean.

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 reports of natural satellite discoveries used the term "satellite" exclusively,[7] though the 1868 book Smith's Illustrated Astronomy referred to satellites as "secondary planets".[8]

Modern concept

In the modern era, Alan Stern considers satellite planets to be one of three categories of planet, along with dwarf planets and classical planets.[9] The term planemo ("planetary-mass object") covers all three populations.[10] Both Stern's and the IAU's definition of 'planet' depends on hydrostatic equilibrium – on the mass of the body being sufficient to render it plastic, so that it relaxes into an ellipsoid under its own gravity. The IAU definition specifies that the mass be 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. It is possible that all the larger icy moons have subsurface oceans.[11]

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. However, there are seven large moons that are more massive than the dwarf planets Eris and Pluto, which are universally believed (though not yet actually demonstrated) to be in equilibrium. 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). All of these moons are ellipsoidal in shape. 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.

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 moon, which is entirely rocky, solidified out of equilibrium billions of years ago,[12] but most of the other six moons larger than Pluto, five of which are 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 fluid movement of electrically conducting material in its interior, though whether that fluid is a metallic core or a subsurface ocean is unknown.[13] One of the mid-sized moons of Saturn (Rhea) may also be in equilibrium,[14][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.[14] The situation for Uranus's three smaller ellipsoidal moons (Umbriel, Ariel and Miranda) is unclear, as is that of Pluto's moon Charon.[12]

Not included are Eris's moon Dysnomia (diameter 700±115 km, albedo ~0.04) and Orcus' moon Vanth (diameter 442.5±10.2 km, albedo ~0.12). Dysnomia is larger than the three smallest ellipsoidal moons of Saturn and Uranus (Enceladus, Miranda and Mimas); Vanth is larger than Mimas, but may be smaller than non-ellipsoidal Proteus (Neptune VIII, the second-largest moon of Neptune). Thus they might be ellipsoidal. However Grundy et al. argue that objects in the size range of 400–1000 km, with albedos less than ≈0.2 and densities of ≈1.2 g/cm3 or less, have likely never compressed into fully solid bodies, let alone differentiated.[15]


Yes – believed to be in equilibrium
No – confirmed to not be in equilibrium
Maybe – uncertain evidence
List of ellipsoidal moons[16]
Moon Image Radius Mass Density Year of
Name Designation (km) (R) (1021 kg) (M) (g/cm3)
Ganymede Jupiter III
Ganymede g1 true-edit1.jpg
2634.1±0.3 156.4% 148.2 201.8% 1.942±0.005 1610 Yes
Titan Saturn VI
Titan in true color.jpg
2574.7±0.1 148.2% 134.5 183.2% 1.882±0.001 1655 Yes
Callisto Jupiter IV
2410.3±1.5 138.8% 107.6 146.6% 1.834±0.003 1610 Maybe[17]
Io Jupiter I
Io highest resolution true color.jpg
1821.6±0.5 104.9% 89.3 121.7% 3.528±0.006 1610 Yes
Luna Earth I
1737.05 100% 73.4 100% 3.344±0.005 No[18]
Europa Jupiter II
1560.8±0.5 89.9% 48.0 65.4% 3.013±0.005 1610 Yes
Triton Neptune I
Triton moon mosaic Voyager 2 (large).jpg
1353.4±0.9 79.9% 21.4 29.1% 2.059±0.005 1846 Yes
Titania Uranus III
Titania (moon) color cropped.jpg
788.9±1.8 45.4% 3.40±0.06 4.6% 1.66±0.04 1787 Maybe[11]
Rhea Saturn V
PIA07763 Rhea full globe5.jpg
764.3±1.0 44.0% 2.31 3.1% 1.233±0.005 1672 Maybe[14]
Oberon Uranus IV
Voyager 2 picture of Oberon.jpg
761.4±2.6 43.8% 3.08±0.09 4.2% 1.56±0.06 1787 Maybe[11]
Iapetus Saturn VIII
Iapetus as seen by the Cassini probe - 20071008.jpg
735.6±1.5 42.3% 1.81 2.5% 1.083±0.007 1671 No[14]
Charon Pluto I
Charon in True Color - High-Res.jpg
603.6±1.4 34.7% 1.53 2.1% 1.664±0.012 1978 Maybe[12]
Umbriel Uranus II
PIA00040 Umbrielx2.47.jpg
584.7±2.8 33.7% 1.28±0.03 1.7% 1.46±0.09 1851
Ariel Uranus I
Ariel (moon).jpg
578.9±0.6 33.3% 1.25±0.02 1.7% 1.59±0.09 1851
Dione Saturn IV
Dione in natural light.jpg
561.4±0.4 32.3% 1.10 1.5% 1.476±0.004 1684 No[14]
Tethys Saturn III
533.0±0.7 30.7% 0.617 0.84% 0.973±0.004 1684 No[14]
Enceladus Saturn II
252.1±0.2 14.5% 0.108 0.15% 1.608±0.003 1789 No[14]
Miranda Uranus V
235.8±0.7 13.6% 0.064±0.003 0.09% 1.21±0.11 1948
Mimas Saturn I
Mimas Cassini.jpg
198.2±0.4 11.4% 0.038 0.05% 1.150±0.004 1789 No[14]

(Saturn VII is Hyperion, which is not gravitationally rounded; it is smaller than Mimas.)

See also


  1. ^ "Should Large Moons Be Called 'Satellite Planets'?". 2010-05-14. Archived from the original on 2014-10-25.
  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. Of April 22 st. N. 1686. Giving an Account of Two New Satellites of Saturn, Discovered Lately by Mr. Cassini at the Royal Observatory at Paris". Philosophical Transactions of the Royal Society of London. 16 (179–191): 79–85. Bibcode:1686RSPT...16...79C. doi:10.1098/rstl.1686.0013. JSTOR 101844.
  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. secondary planet Herschel.
  9. ^ "Should Large Moons Be Called 'Satellite Planets'?". May 14, 2010. Retrieved November 4, 2011.
  10. ^ Basri, Gibor; Brown, Michael E. (2006). "Planetesimals to Brown Dwarfs: What is a Planet?" (PDF). Annual Review of Earth and Planetary Sciences. 34: 193–216. arXiv:astro-ph/0608417. Bibcode:2006AREPS..34..193B. doi:10.1146/ S2CID 119338327. Archived from the original (PDF) on July 31, 2013.
  11. ^ a b c d e 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. Bibcode:2006Icar..185..258H. doi:10.1016/j.icarus.2006.06.005.
  12. ^ a b c Nimmo, Francis; et al. (2017). "Mean radius and shape of Pluto and Charon from New Horizons images". Icarus. 287: 12–29. arXiv:1603.00821. Bibcode:2017Icar..287...12N. doi:10.1016/j.icarus.2016.06.027. S2CID 44935431.
  13. ^ Planetary Science Decadal Survey Community White Paper, Ganymede science questions and future exploration
  14. ^ a b c d e f g h P.C. Thomas (2010) 'Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission', Icarus 208: 395–401
  15. ^ W.M. Grundy, K.S. Noll, M.W. Buie, S.D. Benecchi, D. Ragozzine & H.G. Roe, 'The Mutual Orbit, Mass, and Density of Transneptunian Binary Gǃkúnǁʼhòmdímà ((229762) 2007 UK126)', Icarus (forthcoming, available online 30 March 2019) Archived 7 April 2019 at the Wayback Machine DOI: 10.1016/j.icarus.2018.12.037,
  16. ^ 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).
  17. ^ Castillo-Rogez, J. C.; et al. (2011). "How differentiated is Callisto" (PDF). 42nd Lunar and Planetary Science Conference: 2580. Retrieved 2 January 2020.
  18. ^ 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. S2CID 317360.