A planetary-mass moon is a

planetary-mass object A planetary-mass object (PMO), planemo, or planetary body (sometimes referred to as a world) is, by geophysical definition of celestial objects, any celestial object massive enough to achieve hydrostatic equilibrium, but not enough to sustain c ...

. They are large and ellipsoidal (sometimes spherical) in shape. Moons may be in

hydrostatic equilibrium In fluid mechanics, hydrostatic equilibrium, also called hydrostatic balance and hydrostasy, is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force. I ...

due to tidal or radiogenic heating, in some cases forming a

subsurface ocean 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 afte ...

. Two moons in the Solar System, Ganymede and

Titan Titan most often refers to: * Titan (moon), the largest moon of Saturn * Titans, a race of deities in Greek mythology Titan or Titans may also refer to: Arts and entertainment Fictional entities Fictional locations * Titan in fiction, fictiona ...

, are larger than the planet Mercury, and a third,

Callisto CALLISTO (''Cooperative Action Leading to Launcher Innovation in Stage Toss-back Operations'') is a reusable VTVL Prototype, demonstrator propelled by a small 40 kN Japanese LOX-LH2 rocket engine. It is being developed jointly by the CNES, French ...

, is just slightly smaller than it, although all three are less massive. Additionally, seven – Ganymede, Titan, Callisto, Io,

Luna Luna commonly refers to: * Earth's Moon, named "Luna" in Latin, Spanish and other languages * Luna (goddess) In Sabine and ancient Roman religion and myth, Luna is the divine embodiment of the Moon (Latin ''Lūna'' ). She is often presented as t ...

, Europa, and

Triton Triton commonly refers to: * Triton (mythology), a Greek god * Triton (moon), a satellite of Neptune Triton may also refer to: Biology * Triton cockatoo, a parrot * Triton (gastropod), a group of sea snails * ''Triton'', a synonym of ''Triturus' ...

– are larger and more massive than the dwarf planets and . The concept of ''satellite planets'' – the idea that all planetary-mass objects, including moons, are

planet A planet is a large, Hydrostatic equilibrium, rounded Astronomical object, 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 b ...

s – is used by some planetary scientists, such as

Alan Stern Sol Alan Stern (born November 22, 1957) is an American engineer, planetary scientist and space tourist. He is the principal investigator of the ''New Horizons'' mission to Pluto and the Chief Scientist at Moon Express. Stern has been involved ...

, who are more concerned with whether a celestial body has

planetary geology Planetary geology, alternatively known as astrogeology or exogeology, is a planetary science discipline concerned with the geology of celestial bodies such as planets and their moons, asteroids, comets, and meteorites. Although the geo- prefix t ...

(that is, whether it is a planetary body) than its solar or non-solar orbit ( planetary dynamics). This conceptualization of planets as three classes of objects (classical planets,

dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit around the Sun, massive enough to be hydrostatic equilibrium, gravitationally rounded, but insufficient to achieve clearing the neighbourhood, orbital dominance like the ...

s and satellite planets) has not been accepted by the

International Astronomical Union The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...

(the IAU).

Early history

The distinction between a satellite and a classical planet was not recognized until after the

heliocentric model Heliocentrism (also known as the heliocentric model) is a superseded astronomical model in which the Earth and planets orbit around the Sun at the center of the universe. Historically, heliocentrism was opposed to geocentrism, which placed th ...

of the Solar System was established. When in 1610

Galileo Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642), commonly referred to as Galileo Galilei ( , , ) or mononymously as Galileo, was an Italian astronomer, physicist and engineer, sometimes described as a poly ...

discovered the first satellites of another planet (the four

Galilean moons The Galilean moons (), or Galilean satellites, are the four largest moons of Jupiter. They are, in descending-size order, Ganymede (moon), Ganymede, Callisto (moon), Callisto, Io (moon), Io, and Europa (moon), Europa. They are the most apparent m ...

of Jupiter), he referred to them as "four planets flying around the star of Jupiter at unequal intervals and periods with wonderful swiftness." Similarly,

Christiaan Huygens Christiaan Huygens, Halen, Lord of Zeelhem, ( , ; ; also spelled Huyghens; ; 14 April 1629 – 8 July 1695) was a Dutch mathematician, physicist, engineer, astronomer, and inventor who is regarded as a key figure in the Scientific Revolution ...

, upon discovering Saturn's largest moon

in 1655, employed the terms "planeta" (planet), "Stella" (star), "luna" (moon), and the more modern "satellite" (attendant) to describe it.

Giovanni Cassini Giovanni Domenico Cassini (8 June 1625 – 14 September 1712) was an Italian-French mathematician, astronomer, astrologer and engineer. Cassini was born in Perinaldo, near Imperia, at that time in the County of Nice, part of the Savoyard sta ...

, in announcing his discovery of Saturn's moons

Iapetus In Greek mythology, Iapetus (; ; ), also Japetus, is a Titan, the son of Uranus and Gaia and father of Atlas, Prometheus, Epimetheus, and Menoetius. He was also called the father of Buphagus and Anchiale in other sources. Iapetus was linked ...

and Rhea in 1671 and 1672, described them as ''Nouvelles Planetes autour de Saturne'' ("New planets around Saturn"). 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". When William Herschel announced his discovery of two objects in orbit around Uranus ( Titania and

Oberon Oberon () is a king of the fairy, fairies in Middle Ages, medieval and Renaissance literature. He is best known as a character in William Shakespeare's play ''A Midsummer Night's Dream'', in which he is King of the Fairies and spouse of Titania ...

) in 1787, he referred to them as "satellites" and "secondary planets". All subsequent reports of natural satellite discoveries used the term "satellite" exclusively, though the 1868 book ''Smith's Illustrated Astronomy'' referred to satellites as "secondary planets".

Modern concept

In the modern era, Alan Stern considers satellite planets to be one of three categories of planets, along with

s and classical planets. The term ''

planemo A planetary-mass object (PMO), planemo, or planetary body (sometimes referred to as a world) is, by geophysical definition of celestial objects, any celestial object massive enough to achieve hydrostatic equilibrium, but not enough to sustain ...

'' ("planetary-mass object") covers all three populations. Stern's and the IAU's definition of 'planet' depends on

– 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. The seven largest moons are more massive than the dwarf planet

Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of Trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Su ...

, which is known to be in hydrostatic equilibrium. (They are also known to be more massive than , a dwarf planet even more massive than Pluto.) These seven are Earth's

Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

, the four Galilean moons of Jupiter ( Io, Europa, Ganymede and

), and the largest moons of Saturn (

) and of Neptune (

). 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 In Greek mythology, Proteus ( ; ) is an early prophetic sea god or god of rivers and oceanic bodies of water, one of several deities whom Homer calls the "Old Man of the Sea" (''hálios gérôn''). Some who ascribe a specific domain to Prote ...

(Neptune VIII) has occasionally been included by authors discussing or advocating geophysical conceptions of the 'planet'.Emily Lakdawalla et al.
What Is A Planet?
The Planetary Society, 21 April 2020 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, 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. One of the mid-sized moons of Saturn ( Rhea) may also be in equilibrium,P.C. Thomas (2010
'Sizes, shapes, and derived properties of the Saturnian satellites after the Cassini nominal mission'
, ''Icarus'' 208: 395–401 as may a couple of the moons of Uranus ( Titania and

). However, the other ellipsoidal moons of Saturn ( Mimas,

Enceladus Enceladus is the sixth-largest moon of Saturn and the 18th-largest in the Solar System. It is about in diameter, about a tenth of that of Saturn's largest moon, Titan. It is covered by clean, freshly deposited snow hundreds of meters thick, ...

, Tethys, Dione and

) are no longer in equilibrium. 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, in which case they would not be satellite planets. The situation for Uranus's three smaller ellipsoidal moons (

Umbriel Umbriel () is the third-largest moon of Uranus. It was discovered on October 24, 1851, by William Lassell at the same time as neighboring moon Ariel. It was named after a character in Alexander Pope's 1712 poem '' The Rape of the Lock''. Umb ...

, Ariel and Miranda) is unclear, as is that of Pluto's moon

Charon In Greek mythology, Charon or Kharon ( ; ) is a psychopomp, the ferryman of the Greek underworld. He carries the souls of those who have been given funeral rites across the rivers Acheron and Styx, which separate the worlds of the living and ...

. The TNO moons Eris I Dysnomia, Orcus I

Vanth Vanth is a chthonic figure in Etruscan mythology shown in a variety of forms of funerary art, such as in tomb paintings and on sarcophagi. Background Vanth is a female entity in the Etruscan underworld that is often accompanied either by additio ...

, 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.

List

: – believed to be in equilibrium : – confirmed not to be in equilibrium : – uncertain evidence Methone, Pallene, and, with less certainty, Aegaeon are in hydrostatic equilibrium. 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. Io (1.9 ) and Callisto (26 ) have very thin atmospheres, but still enough to have collisions between atmospheric molecules. Other planetary-mass moons only have

exosphere The exosphere is a thin, atmosphere-like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body, but where the density is so low that the molecules are essentially collision-less. In the case of ...

s at most.A Moon with Atmosphere
, Emily Lakdwalla, ''The Planetary Society'' (8 April 2015) Exospheres have been detected around Earth's Moon, Europa, Ganymede, Enceladus, Dione, and Rhea. An exosphere around Titania is a possibility, though it has not been confirmed.

Early history

Modern concept

Current equilibrium moons

List

Atmospheres

See also

References

Further reading