Planetary-mass moon

Last updated September 13, 2024

A planetary-mass moon is a planetary-mass object that is also a 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, Ganymede and Titan, are larger than the planet Mercury, and a third, Callisto, is just slightly smaller than it, although all three are less massive. Additionally, seven – Ganymede, Titan, Callisto, Io, Earth's Moon, Europa, and Triton – are larger and more massive than the dwarf planets Pluto and Eris.

The concept of satellite planets – the idea that all planetary-mass objects, including 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 its solar or non-solar orbit (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).

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

Comparative masses of the seven largest moons. Values are ×10²¹ kg. The moons smaller than Triton would be barely visible at this scale.

The masses of the mid-sized moons, compared to Triton. Values are ×10²¹ 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.

In the modern era, 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.

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,^[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
Satellite 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	Image	(km)	(R_☾)	(10²¹ kg)	(M_☾)	(g/cm³)	(g)	Year of discovery	Hydrostatic equilibrium?
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%	1.882±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		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%	3.40±0.06	4.6%	1.66±0.04	0.040	1787	^[11]
Rhea	Saturn V		764.3±1.0	44.0%	2.31	3.1%	1.233±0.005	0.027	1672	^[16]
Oberon	Uranus IV		761.4±2.6	43.8%	3.08±0.09	4.2%	1.56±0.06	0.036	1787	^[11]
Iapetus	Saturn VIII		735.6±1.5	42.3%	1.81	2.5%	1.083±0.007	0.022	1671	^[16]
Charon	Pluto I		603.6±1.4	34.7%	1.53	2.1%	1.664±0.012	0.029	1978	^[14]
Umbriel	Uranus II		584.7±2.8	33.7%	1.28±0.03	1.7%	1.46±0.09	0.023	1851
Ariel	Uranus I		578.9±0.6	33.3%	1.25±0.02	1.7%	1.59±0.09	0.028	1851
Dione	Saturn IV		561.4±0.4	32.3%	1.10	1.5%	1.476±0.004	0.024	1684	^[16]
Tethys	Saturn III		533.0±0.7	30.7%	0.617	0.84%	0.973±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%	0.064±0.003	0.09%	1.21±0.11	0.008	1948
Mimas	Saturn I		198.2±0.4	11.4%	0.038	0.05%	1.150±0.004	0.006	1789	^[16]

Methone, Pallene, and, with less certainty, Aegaeon are in hydrostatic equilibrium.^[24] 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.^[25] Io (1.9 nbar) and Callisto (26 pbar) have very thin atmospheres, but still enough to have collisions between atmospheric molecules. Other planetary-mass moons only have exospheres at most.^[26] Exospheres have been detected around Earth's Moon, Europa, Ganymede,^[26] Enceladus,^[27] Dione,^[28] and Rhea.^[29] An exosphere around Titania is a possibility, though it has not been confirmed.^[30]

Related Research Articles

A planet is a large, rounded astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets by the most restrictive definition of the term: the terrestrial planets Mercury, Venus, Earth, and Mars, and the giant planets Jupiter, Saturn, Uranus, and Neptune. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a young protostar orbited by a protoplanetary disk. Planets grow in this disk by the gradual accumulation of material driven by gravity, a process called accretion.

Callisto, or Jupiter IV, is the second-largest moon of Jupiter, after Ganymede. In the Solar System it is the third-largest moon after Ganymede and Saturn's largest moon Titan, and nearly as large as the smallest planet Mercury. Callisto is, with a diameter of 4,821 km, roughly a third larger than Earth's Moon and orbits Jupiter on average at a distance of 1,883,000 km, which is about six times further out than the Moon orbiting Earth. It is the outermost of the four large Galilean moons of Jupiter, which were discovered in 1610 with one of the first telescopes, being visible from Earth with common binoculars.

<span class="mw-page-title-main">Triton (moon)</span> Largest moon of Neptune

Triton is the largest natural satellite of the planet Neptune. It is the only moon of Neptune massive enough to be rounded under its own gravity and hosts a thin but well-structured atmosphere. Triton orbits Neptune in a retrograde orbit— revolving in the opposite direction to the parent planet's rotation — the only large moon in the Solar System to do so. Triton is thought to have once been a dwarf planet from the Kuiper belt, captured into Neptune's orbit by the latter's gravity.

<span class="mw-page-title-main">Natural satellite</span> 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. Natural satellites are colloquially referred to as moons, a derivation from the Moon of Earth.

Rhea is the second-largest moon of Saturn and the ninth-largest moon in the Solar System, with a surface area that is comparable to the area of Australia. It is the smallest body in the Solar System for which precise measurements have confirmed a shape consistent with hydrostatic equilibrium. Rhea has a nearly circular orbit around Saturn, but it is also tidally locked, like Saturn's other major moons; that is, it rotates with the same period it revolves (orbits), so one hemisphere always faces towards the planet.

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, also known as Saturn VII, is the eighth-largest moon of Saturn. It distinguished by its highly irregular shape, chaotic rotation, low density, and its unusual sponge-like appearance. It was the first non-rounded moon to be discovered.

The definition of the term planet has changed several times since the word was coined by the ancient Greeks. Greek astronomers employed the term ἀστέρες πλανῆται, 'wandering stars', for star-like objects which apparently moved over the sky. Over the millennia, the term has included a variety of different celestial bodies, from the Sun and the Moon to satellites and asteroids.

A cryovolcano is a type of volcano that erupts gases and volatile material such as liquid water, ammonia, and hydrocarbons. The erupted material is collectively referred to as cryolava; it originates from a reservoir of subsurface cryomagma. Cryovolcanic eruptions can take many forms, such as fissure and curtain eruptions, effusive cryolava flows, and large-scale resurfacing, and can vary greatly in output volumes. Immediately after an eruption, cryolava quickly freezes, constructing geological features and altering the surface.

A dwarf planet is a small planetary-mass object that is in direct orbit around the Sun, massive enough to be gravitationally rounded, but insufficient to achieve orbital dominance like the eight classical planets of the Solar System. The prototypical dwarf planet is Pluto, which for decades was regarded as a planet before the "dwarf" concept was adopted in 2006.

An ocean world, ocean planet or water world is a type of planet that contains a substantial amount of water in the form of oceans, as part of its hydrosphere, either beneath the surface, as subsurface oceans, or on the surface, potentially submerging all dry land. The term ocean world is also used sometimes for astronomical bodies with an ocean composed of a different fluid or thalassogen, such as lava, ammonia or hydrocarbons. The study of extraterrestrial oceans is referred to as planetary oceanography.

In astronomy, a regular moon or a regular satellite is a natural satellite following a relatively close, stable, and circular orbit which is generally aligned to its primary's equator. They form within discs of debris and gas that once surrounded their primary, usually the aftermath of a large collision or leftover material accumulated from the protoplanetary disc. Young regular moons then begin to accumulate material within the circumplanetary disc in a process similar to planetary accretion, as opposed to irregular moons, which formed independently before being captured into orbit around the primary.

A planetary surface is where the solid or liquid material of certain types of astronomical objects contacts the atmosphere or outer space. Planetary surfaces are found on solid objects of planetary mass, including terrestrial planets, dwarf planets, natural satellites, planetesimals and many other small Solar System bodies (SSSBs). The study of planetary surfaces is a field of planetary geology known as surface geology, but also a focus on a number of fields including planetary cartography, topography, geomorphology, atmospheric sciences, and astronomy. Land is the term given to non-liquid planetary surfaces. The term landing is used to describe the collision of an object with a planetary surface and is usually at a velocity in which the object can remain intact and remain attached.

Planetary oceanography, also called astro-oceanography or 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 moon Titan and Jupiter's moon 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.

A satellite system is a set of gravitationally bound objects in orbit around a planetary mass object or minor planet, or its barycenter. Generally speaking, it is a set of natural satellites (moons), although such systems may also consist of bodies such as circumplanetary disks, ring systems, moonlets, minor-planet moons and artificial satellites any of which may themselves have satellite systems of their own. Some bodies also possess quasi-satellites that have orbits gravitationally influenced by their primary, but are generally not considered to be part of a satellite system. Satellite systems can have complex interactions including magnetic, tidal, atmospheric and orbital interactions such as orbital resonances and libration. Individually major satellite objects are designated in Roman numerals. Satellite systems are referred to either by the possessive adjectives of their primary, or less commonly by the name of their primary. Where only one satellite is known, or it is a binary with a common centre of gravity, it may be referred to using the hyphenated names of the primary and major satellite.

Chemical cycling describes systems of repeated circulation of chemicals between other compounds, states and materials, and back to their original state, that occurs in space, and on many objects in space including the Earth. Active chemical cycling is known to occur in stars, many planets and natural satellites.

The International Union of Geological Sciences (IUGS) is the internationally recognized body charged with fostering agreement on nomenclature and classification across geoscientific disciplines. However, they have yet to create a formal definition of the term "planet". As a result, there are various geophysical definitions in use among professional geophysicists, planetary scientists, and other professionals in the geosciences. Many professionals opt to use one of several of these geophysical definitions instead of the definition voted on by the International Astronomical Union, the dominant organization for setting planetary nomenclature.

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v t e Natural satellites of the Solar System
Planetary satellites of	Earth Mars Jupiter Saturn Uranus Neptune
Dwarf planet satellites of	Orcus Pluto Haumea Quaoar Makemake Gonggong Eris
Minor-planet moons	Near-Earth Florence Didymos Dimorphos Moshup Squannit 1994 CC 2001 SN₂₆₃ Main belt Kalliope Linus Euphrosyne Daphne Peneius Eugenia Petit-Prince Sylvia Romulus Remus Minerva Aegis Gorgoneion Camilla Elektra Kleopatra Alexhelios Cleoselene Ida Dactyl Roxane Olympias Pulcova Balam Dinkinesh (Selam) Jupiter trojans Patroclus Menoetius Hektor Skamandrios Eurybates Queta TNOs Lempo Hiisi Paha 2002 UX₂₅ Sila–Nunam Salacia Actaea Varda Ilmarë Gǃkúnǁʼhòmdímà Gǃòʼé ǃHú 2013 FY₂₇
Ranked by size	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 Phoebe ...
Discovery timeline Inner moons Irregular moons List Planetary-mass moons Naming Subsatellite Regular moons Trojan moons