Planetary-mass moon

Last updated

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 seven largest moons are 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). Ganymede and Titan are additionally larger than the planet Mercury, and Callisto is almost as large. All of these moons are ellipsoidal in shape. 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.

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, diameter 420±14 km). Thus they might be ellipsoidal. However Grundy et al. argue that trans-Neptunian 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 check.svg – believed to be in equilibrium
X mark.svg – confirmed to not be in equilibrium
Commons-emblem-query.svg – uncertain evidence
List of ellipsoidal moons [16]
MoonImageRadiusMassDensityYear of
NameDesignation(km)(R)(1021  kg)(M)(g/cm3)
Ganymede Jupiter III
Ganymede g1 true-edit1.jpg
2634.1±0.3156.4%148.2201.8%1.942±0.0051610 Yes check.svg
Titan Saturn VI
Titan in true color.jpg
2574.7±0.1148.2%134.5183.2%1.882±0.0011655 Yes check.svg
Callisto Jupiter IV
2410.3±1.5138.8%107.6146.6%1.834±0.0031610 Commons-emblem-query.svg [17]
Io Jupiter I
Io highest resolution true color.jpg
1821.6±0.5104.9%89.3121.7%3.528±0.0061610 Yes check.svg
Luna Earth I
1737.05100%73.4100%3.344±0.005 X mark.svg [18]
Europa Jupiter II
1560.8±0.589.9%48.065.4%3.013±0.0051610 Yes check.svg
Triton Neptune I
Triton moon mosaic Voyager 2 (large).jpg
1353.4±0.979.9%21.429.1%2.059±0.0051846 Yes check.svg
Titania Uranus III
Titania (moon) color cropped.jpg
788.9±1.845.4%3.40±0.064.6%1.66±0.041787 Commons-emblem-query.svg [11]
Rhea Saturn V
PIA07763 Rhea full globe5.jpg
764.3±1.044.0%2.313.1%1.233±0.0051672 Commons-emblem-query.svg [14]
Oberon Uranus IV
Voyager 2 picture of Oberon.jpg
761.4±2.643.8%3.08±0.094.2%1.56±0.061787 Commons-emblem-query.svg [11]
Iapetus Saturn VIII
Iapetus as seen by the Cassini probe - 20071008.jpg
735.6±1.542.3%1.812.5%1.083±0.0071671 X mark.svg [14]
Charon Pluto I
Charon in True Color - High-Res.jpg
603.6±1.434.7%1.532.1%1.664±0.0121978 Commons-emblem-query.svg [12]
Umbriel Uranus II
PIA00040 Umbrielx2.47.jpg
Ariel Uranus I
Ariel (moon).jpg
Dione Saturn IV
Dione in natural light.jpg
561.4±0.432.3%1.101.5%1.476±0.0041684 X mark.svg [14]
Tethys Saturn III
533.0±0.730.7%0.6170.84%0.973±0.0041684 X mark.svg [14]
Enceladus Saturn II
252.1±0.214.5%0.1080.15%1.608±0.0031789 X mark.svg [14]
Miranda Uranus V
Mimas Saturn I
Mimas Cassini.jpg
198.2±0.411.4%0.0380.05%1.150±0.0041789 X mark.svg [14]

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

See also

Related Research Articles

Planet Class of astronomical body directly orbiting a star or stellar remnant

A planet is an astronomical body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and – according to the International Astronomical Union but not all planetary scientists – has cleared its neighbouring region of planetesimals.

Callisto (moon) Second largest Galilean moon of Jupiter and third largest in the solar system

Callisto, or Jupiter IV, is the second-largest moon of Jupiter, after Ganymede. It is the third-largest moon in the Solar System after Ganymede and Saturn's largest moon Titan, and the largest object in the Solar System that may not be properly differentiated. Callisto was discovered in 1610 by Galileo Galilei. At 4821 km in diameter, Callisto has about 99% the diameter of the planet Mercury but only about a third of its mass. It is the fourth Galilean moon of Jupiter by distance, with an orbital radius of about 1883000 km. It is not in an orbital resonance like the three other Galilean satellites—Io, Europa, and Ganymede—and is thus not appreciably tidally heated. Callisto's rotation is tidally locked to its orbit around Jupiter, so that the same hemisphere always faces inward. Because of this, there is a sub-Jovian point on Callisto's surface, from which Jupiter would appear to hang directly overhead. It is less affected by Jupiter's magnetosphere than the other inner satellites because of its more remote orbit, located just outside Jupiter's main radiation belt.

Natural satellite Astronomical body that orbits a planet

A natural satellite is in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small solar system body. While natural satellites are often colloquially referred to as moons, there is only the Moon of Earth.

Timeline of Solar System astronomy Timeline of the history of Solar System astronomy

The following is a timeline of Solar System astronomy.

Rhea (moon) Moon of Saturn

Rhea is the second-largest moon of Saturn and the ninth-largest moon in the Solar System. It is the smallest body in the Solar System for which precise measurements have confirmed a shape consistent with hydrostatic equilibrium. It was discovered in 1672 by Giovanni Domenico Cassini.

Tethys (moon) Moon of Saturn

Tethys, or Saturn III, is a mid-sized moon of Saturn about 1,060 km (660 mi) across. It was discovered by G. D. Cassini in 1684 and is named after the titan Tethys of Greek mythology.

Icy moons are a class of natural satellites with surfaces composed mostly of ice. An icy moon may harbor an ocean underneath the surface, and possibly include a rocky core of silicate or metallic rocks. It is thought that they may be composed of ice II or other polymorph of water ice. The prime example of this class of object is Europa.

Hyperion (moon) Moon of Saturn

Hyperion, also known as Saturn VII (7), is a moon of Saturn discovered by William Cranch Bond, his son George Phillips Bond and William Lassell in 1848. It is distinguished by its irregular shape, its chaotic rotation, and its unexplained sponge-like appearance. It was the first non-round moon to be discovered.

Moons of Saturn Natural satellites of the planet Saturn

The moons of Saturn are numerous and diverse, ranging from tiny moonlets only tens of meters across to enormous Titan, which is larger than the planet Mercury. Saturn has 83 moons with confirmed orbits that are not embedded in its rings – of which only 13 have diameters greater than 50 kilometers – as well as dense rings that contain millions of embedded moonlets and innumerable smaller ring particles. Seven Saturnian moons are large enough to have collapsed into a relaxed, ellipsoidal shape, though only one or two of those, Titan and possibly Rhea, are currently in hydrostatic equilibrium. Particularly notable among Saturn's moons are Titan, the second-largest moon in the Solar System, with a nitrogen-rich Earth-like atmosphere and a landscape featuring dry river networks and hydrocarbon lakes, Enceladus, which emits jets of gas and dust from its south-polar region, and Iapetus, with its contrasting black and white hemispheres.

The naming of moons has been the responsibility of the International Astronomical Union's committee for Planetary System Nomenclature since 1973. That committee is known today as the Working Group for Planetary System Nomenclature (WGPSN).

Definition of <i>planet</i> History of the word "planet" and its definition

The definition of planet, since the word was coined by the ancient Greeks, has included within its scope a wide range of celestial bodies. Greek astronomers employed the term asteres planetai, "wandering stars", for star-like objects which apparently moved over the sky. Over the millennia, the term has included a variety of different objects, from the Sun and the Moon to satellites and asteroids.

Many parts of the outer Solar System have been considered for possible future colonization. Most of the larger moons of the outer planets contain water ice, liquid water, and organic compounds that might be useful for sustaining human life.

Dwarf planet Planetary-mass object

A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun – something smaller than any of the eight classical planets, but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to planetary geologists is that, being possibly differentiated and geologically active bodies, they may display planetary geology, an expectation borne out by the Dawn mission to Ceres and the New Horizons mission to Pluto in 2015.

IAU definition of <i>planet</i> Formal definition of a planet in the context of the Solar System as ratified by the IAU in 2006

The International Astronomical Union (IAU) defined in August 2006 that, in the Solar System, a planet is a celestial body that:

  1. is in orbit around the Sun,
  2. has sufficient mass to assume hydrostatic equilibrium, and
  3. has "cleared the neighbourhood" around its orbit.

Planetary oceanography also called exo-oceanography is the study of oceans on planets and moons other than Earth. Unlike other planetary sciences like astrobiology, astrochemistry and planetary geology, it only began after the discovery of underground oceans in Saturn's Titan and Jupiter's Europa. This field remains speculative until further missions reach the oceans beneath the rock or ice layer of the moons. There are many theories about oceans or even ocean worlds of celestial bodies in the Solar System, from oceans made of diamond in Neptune to a gigantic ocean of liquid hydrogen that may exist underneath Jupiter's surface.

Under a geophysical definition, any planetary-mass object is a planet. There are various definitions in use among professional planetary scientists and astronomers.


  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. 1 2 3 4 5 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. 1 2 3 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. 1 2 3 4 5 6 7 8 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.