Planetary-mass moon

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Planetary-mass moons larger than Pluto, the largest Solar dwarf planet. Large Moons (4089199369).jpg
Planetary-mass moons larger than Pluto, the largest Solar dwarf planet.

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.

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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 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). 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

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 centre 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 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. 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 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. Dysnomia's shape is not known, but it appears to be dense enough that it must have collapsed to form a solid body.

Neptune's second-largest moon Proteus has occasionally been included by authors discussing or advocating geophysical conceptions of '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 moon, which is entirely rocky, 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 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] Eris' moon Dysnomia is larger than the three smallest ellipsoidal moons of Saturn and Uranus (Enceladus, Miranda and Mimas), and must be quite massive to have tidally locked its parent; thus it has been included. [19]

Orcus' moon Vanth has been included as a possibility; it is larger than Mimas, but is about the same size as non-ellipsoidal Proteus (Neptune VIII, the second-largest moon of Neptune, diameter 420±14 km). Also included is Varda's moon Ilmarë, which to within current uncertainties might be about the same size as Mimas.

List

Yes check.svg – believed to be in equilibrium
X mark.svg – confirmed not to be in equilibrium
Commons-emblem-query.svg – uncertain evidence
List of ellipsoidal moons, along with trans-Neptunian moons as large as Mimas [20]
MoonImageRadiusMassDensitySurface gravityYear of
discovery
Hydrostatic
equilibrium?
NameDesignation(km)(R)(1021  kg)(M)(g/cm3)(g)
Ganymede Jupiter III
Ganymede g1 true-edit1.jpg
2634.1±0.3156.4%148.2201.8%1.942±0.0050.1461610 Yes check.svg
Titan Saturn VI
Titan in true color.jpg
2574.7±0.1148.2%134.5183.2%1.882±0.0010.1381655 Commons-emblem-query.svg [21]
Callisto Jupiter IV
Callisto.jpg
2410.3±1.5138.8%107.6146.6%1.834±0.0030.1261610 Commons-emblem-query.svg [22]
Io Jupiter I
Io highest resolution true color.jpg
1821.6±0.5104.9%89.3121.7%3.528±0.0060.1831610 Yes check.svg
Luna Earth I
FullMoon2010.jpg
1737.05100%73.4100%3.344±0.0050.165 X mark.svg [23]
Europa Jupiter II
Europa-moon.jpg
1560.8±0.589.9%48.065.4%3.013±0.0050.1341610 Yes check.svg
Triton Neptune I
Triton moon mosaic Voyager 2 (large).jpg
1353.4±0.979.9%21.429.1%2.059±0.0050.0801846 Yes check.svg
Titania Uranus III
Titania (moon) color cropped.jpg
788.9±1.845.4%3.40±0.064.6%1.66±0.040.0401787 Commons-emblem-query.svg [11]
Rhea Saturn V
PIA07763 Rhea full globe5.jpg
764.3±1.044.0%2.313.1%1.233±0.0050.0271672 Commons-emblem-query.svg [16]
Oberon Uranus IV
Voyager 2 picture of Oberon.jpg
761.4±2.643.8%3.08±0.094.2%1.56±0.060.0361787 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±0.0070.0221671 X mark.svg [16]
Charon Pluto I
Charon in True Color - High-Res.jpg
603.6±1.434.7%1.532.1%1.664±0.0120.0291978 Commons-emblem-query.svg [14]
Umbriel Uranus II
PIA00040 Umbrielx2.47.jpg
584.7±2.833.7%1.28±0.031.7%1.46±0.090.0231851
Ariel Uranus I
Ariel (moon).jpg
578.9±0.633.3%1.25±0.021.7%1.59±0.090.0281851
Dione Saturn IV
Dione in natural light.jpg
561.4±0.432.3%1.101.5%1.476±0.0040.0241684 X mark.svg [16]
Tethys Saturn III
PIA18317-SaturnMoon-Tethys-Cassini-20150411.jpg
533.0±0.730.7%0.6170.84%0.973±0.0040.0151684 X mark.svg [16]
Dysnomia Eris I
Eris and dysnomia2.jpg
350±5820.1% ± 3.3%0.3–0.5 [19] 0.4%–0.7%1.8–2.4 [19] 0.016–0.0282005 Commons-emblem-query.svg [24]
Enceladus Saturn II
PIA17202-SaturnMoon-Enceladus-ApproachingFlyby-20151028.jpg
252.1±0.214.5%0.1080.15%1.608±0.0030.0111789 X mark.svg [16]
Miranda Uranus V
Miranda.jpg
235.8±0.713.6%0.064±0.0030.09%1.21±0.110.0081948
Vanth Orcus I
Orcus-vanth hst2.jpg
221±512.7% ± 0.3%0.02–0.060.03%–0.08%≈0.80.003–0.0082005
Mimas Saturn I
Mimas Cassini.jpg
198.2±0.411.4%0.0380.05%1.150±0.0040.0061789 X mark.svg [16]
Ilmarë Varda I
Varda-ilmare hst.jpg
163+19
−17
[25]
10.4% ± 1.2%ca. 0.02? [26] ca. 0.03%1.24+0.50
−0.35
(for system)
0.004–0.0062009

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

Titan has a denser atmosphere (1.4 bar) than Earth; it is the only known moon with a significant atmosphere. Triton (14 μbar), Io (1.9 nbar), Callisto (26 pbar), and Ganymede (1.2 nbar) have very thin atmospheres, but still enough to have collisions between atmospheric molecules. Other planetary-mass moons only have exospheres at most. [27] Exospheres have been detected around Earth's Moon, Europa, [27] Enceladus, [28] Dione, [29] and Rhea. [30] An exosphere around Titania is a possibility, though it has not been confirmed. [31]

See also

Related Research Articles

<span class="mw-page-title-main">Planet</span> Astronomical object

A planet is a large, rounded astronomical body that is neither a star nor its remnant. 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. The Solar System has at least eight planets: the terrestrial planets Mercury, Venus, Earth and Mars, and the giant planets Jupiter, Saturn, Uranus and Neptune. These planets each rotate around an axis tilted with respect to its orbital pole. All of them possess an atmosphere, although that of Mercury is tenuous, and some share such features as ice caps, seasons, volcanism, hurricanes, tectonics, and even hydrology. Apart from Venus and Mars, the Solar System planets generate magnetic fields, and all except Venus and Mercury have natural satellites. The giant planets bear planetary rings, the most prominent being those of Saturn.

<span class="mw-page-title-main">Callisto (moon)</span> 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. With a diameter of 4821 km, Callisto is 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.

<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 often colloquially referred to as moons, a derivation from the Moon of Earth.

<span class="mw-page-title-main">Rhea (moon)</span> 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.

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.

<span class="mw-page-title-main">Hyperion (moon)</span> Moon of Saturn

Hyperion, also known as Saturn VII, 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.

<span class="mw-page-title-main">Moons of Saturn</span> 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 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 objects, from the Sun and the Moon to satellites and asteroids.

<span class="mw-page-title-main">Dwarf planet</span> Small planetary-mass object

A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, 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, since they are possibly differentiated and geologically active bodies, they may display planetary geology, an expectation that was borne out by the Dawn mission to Ceres and the New Horizons mission to Pluto, both in 2015.

<span class="mw-page-title-main">Habitability of natural satellites</span> Measure of the potential of natural satellites to have environments hospitable to life

The habitability of natural satellites is a measure of their potential to sustain life in favorable circumstances. Habitable environments do not necessarily harbor life. Natural satellite habitability is a new area that is significant to astrobiology for various reasons, the most important of which being that natural satellites are expected to outnumber planets by a large margin, and it is projected that habitability parameters will be comparable to those of planets. There are, nevertheless, significant environmental variables that affect moons as prospective alien life locations. The strongest candidates for natural satellite habitability are currently icy satellites such as those of Jupiter and Saturn—Europa and Enceladus respectively, although if life exists in either place, it would probably be confined to subsurface habitats. Historically, life on Earth was thought to be strictly a surface phenomenon, but recent studies have shown that up to half of Earth's biomass could live below the surface. Europa and Enceladus exist outside the circumstellar habitable zone which has historically defined the limits of life within the Solar System as the zone in which water can exist as liquid at the surface. In the Solar System's habitable zone, there are only three natural satellites—the Moon, and Mars's moons Phobos and Deimos —none of which sustain an atmosphere or water in liquid form. Tidal forces are likely to play as significant a role providing heat as stellar radiation in the potential habitability of natural satellites.

In astronomy, planetary mass is a measure of the mass of a planet-like astronomical object. Within the Solar System, planets are usually measured in the astronomical system of units, where the unit of mass is the solar mass (M), the mass of the Sun. In the study of extrasolar planets, the unit of measure is typically the mass of Jupiter (MJ) for large gas giant planets, and the mass of Earth (MEarth) for smaller rocky terrestrial planets.

<span class="mw-page-title-main">Planetary-mass object</span> Size-based definition of celestial objects

A planetary-mass object (PMO), planemo, or planetary body is by geophysical definition of celestial objects any celestial object massive enough to achieve hydrostatic equilibrium, but not enough to sustain core fusion like a star.

<span class="mw-page-title-main">Ice planet</span> Planet with an icy surface

An ice planet or icy planet is a type of planet with an icy surface of volatiles such as water, ammonia, and methane. Ice planets consist of a global cryosphere.

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 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.

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. As such, many professionals do not use the definition voted on by the International Astronomical Union.

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