Natural satellite

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Charon (left) is a natural satellite of Pluto (right), orbiting around a point Pluto charon 150709 color final.png
Charon (left) is a natural satellite of Pluto (right), orbiting around a point

A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as moons, a derivation from the Moon of Earth.

Contents

In the Solar System, there are six planetary satellite systems containing 207 known natural satellites altogether. Seven objects commonly considered dwarf planets by astronomers are also known to have natural satellites: Orcus, Pluto, Haumea, Quaoar, Makemake, Gonggong, and Eris. [1] As of November 2021, there are 442 other minor planets known to have natural satellites. [2]

A planet usually has at least around 10,000 times the mass of any natural satellites that orbit it, with a correspondingly much larger diameter. [3] The Earth–Moon system is a unique exception in the Solar System; at 3,474 km (2,158 miles) across, the Moon is 0.273 times the diameter of Earth and about 180 of its mass. [4] The next largest ratios are the NeptuneTriton system at 0.055 (with a mass ratio of about 1 to 5000), the SaturnTitan system at 0.044 (with the second mass ratio next to the Earth–Moon system, 1 to 4250), the JupiterGanymede system at 0.038, and the UranusTitania system at 0.031. For the category of dwarf planets, Charon has the largest ratio, being 0.52 the diameter of Pluto.

Terminology

The first known natural satellite was the Moon, but it was considered a "planet" until Copernicus' introduction of De revolutionibus orbium coelestium in 1543. Until the discovery of the Galilean satellites in 1610 there was no opportunity for referring to such objects as a class. Galileo chose to refer to his discoveries as Planetæ ("planets"), but later discoverers chose other terms to distinguish them from the objects they orbited.[ citation needed ]

The first to use the term satellite to describe orbiting bodies was the German astronomer Johannes Kepler in his pamphlet Narratio de Observatis a se quatuor Iouis satellitibus erronibus ("Narration About Four Satellites of Jupiter Observed") in 1610. He derived the term from the Latin word satelles, meaning "guard", "attendant", or "companion", because the satellites accompanied their primary planet in their journey through the heavens. [5]

The term satellite thus became the normal one for referring to an object orbiting a planet, as it avoided the ambiguity of "moon". In 1957, however, the launching of the artificial object Sputnik created a need for new terminology. [5] The terms man-made satellite and artificial moon were very quickly abandoned in favor of the simpler satellite, and as a consequence, the term has become linked primarily with artificial objects flown in space – including, sometimes, even those not in orbit around a planet.[ dubious ][ citation needed ]

Because of this shift in meaning, the term moon, which had continued to be used in a generic sense in works of popular science and in fiction, has regained respectability and is now used interchangeably with natural satellite, even in scientific articles. When it is necessary to avoid both the ambiguity of confusion with Earth's natural satellite the Moon and the natural satellites of the other planets on the one hand, and artificial satellites on the other, the term natural satellite (using "natural" in a sense opposed to "artificial") is used. To further avoid ambiguity, the convention is to capitalize the word Moon when referring to Earth's natural satellite (a proper noun), but not when referring to other natural satellites (common nouns).

Many authors define "satellite" or "natural satellite" as orbiting some planet or minor planet, synonymous with "moon" – by such a definition all natural satellites are moons, but Earth and other planets are not satellites. [6] [7] [8] A few recent authors define "moon" as "a satellite of a planet or minor planet", and "planet" as "a satellite of a star" – such authors consider Earth as a "natural satellite of the Sun". [9] [10] [11]

Definition of a moon

Size comparison of Earth and the Moon Moon, Earth size comparison.jpg
Size comparison of Earth and the Moon

There is no established lower limit on what is considered a "moon". Every natural celestial body with an identified orbit around a planet of the Solar System, some as small as a kilometer across, has been considered a moon, though objects a tenth that size within Saturn's rings, which have not been directly observed, have been called moonlets . Small asteroid moons (natural satellites of asteroids), such as Dactyl, have also been called moonlets. [12]

The upper limit is also vague. Two orbiting bodies are sometimes described as a double planet rather than primary and satellite. Asteroids such as 90 Antiope are considered double asteroids, but they have not forced a clear definition of what constitutes a moon. Some authors consider the Pluto–Charon system to be a double (dwarf) planet. The most common[ citation needed ] dividing line on what is considered a moon rests upon whether the barycentre is below the surface of the larger body, though this is somewhat arbitrary, because it depends on distance as well as relative mass.

Origin and orbital characteristics

The natural satellites orbiting relatively close to the planet on prograde, uninclined circular orbits (regular satellites) are generally thought to have been formed out of the same collapsing region of the protoplanetary disk that created its primary. [13] [14] In contrast, irregular satellites (generally orbiting on distant, inclined, eccentric and/or retrograde orbits) are thought to be captured asteroids possibly further fragmented by collisions. Most of the major natural satellites of the Solar System have regular orbits, while most of the small natural satellites have irregular orbits. [15] The Moon [16] and possibly Charon [17] are exceptions among large bodies in that they are thought to have originated by the collision of two large proto-planetary objects (see the giant impact hypothesis). The material that would have been placed in orbit around the central body is predicted to have reaccreted to form one or more orbiting natural satellites. As opposed to planetary-sized bodies, asteroid moons are thought to commonly form by this process. Triton is another exception; although large and in a close, circular orbit, its motion is retrograde and it is thought to be a captured dwarf planet.

Temporary satellites

The capture of an asteroid from a heliocentric orbit is not always permanent. According to simulations, temporary satellites should be a common phenomenon. [18] [19] The only observed examples are 1991 VG , 2006 RH120 , 2020 CD3 .

2006 RH120 was a temporary satellite of Earth for nine months in 2006 and 2007. [20] [21]

Tidal locking

Most regular moons (natural satellites following relatively close and prograde orbits with small orbital inclination and eccentricity) in the Solar System are tidally locked to their respective primaries, meaning that the same side of the natural satellite always faces its planet. This phenomenon comes about through a loss of energy due to tidal forces raised by the planet, slowing the rotation of the satellite until it is negligible. [22] The only known exception is Saturn's natural satellite Hyperion, which rotates chaotically because of the gravitational influence of Titan.

In contrast, the outer natural satellites of the giant planets (irregular satellites) are too far away to have become locked. For example, Jupiter's Himalia, Saturn's Phoebe, and Neptune's Nereid have rotation periods in the range of ten hours, whereas their orbital periods are hundreds of days.

Satellites of satellites

Artist impression of Rhea's proposed rings Rhean rings PIA10246 Full res.jpg
Artist impression of Rhea's proposed rings

No "moons of moons" or subsatellites (natural satellites that orbit a natural satellite of a planet) are currently known. In most cases, the tidal effects of the planet would make such a system unstable.

However, calculations performed after the 2008 detection [23] of a possible ring system around Saturn's moon Rhea indicate that satellites orbiting Rhea could have stable orbits. Furthermore, the suspected rings are thought to be narrow, [24] a phenomenon normally associated with shepherd moons. However, targeted images taken by the Cassini spacecraft failed to detect rings around Rhea. [25]

It has also been proposed that Saturn's moon Iapetus had a satellite in the past; this is one of several hypotheses that have been put forward to account for its equatorial ridge. [26]

Trojan satellites

Two natural satellites are known to have small companions at both their L4 and L5 Lagrangian points, sixty degrees ahead and behind the body in its orbit. These companions are called trojan moons, as their orbits are analogous to the trojan asteroids of Jupiter. The trojan moons are Telesto and Calypso, which are the leading and following companions, respectively, of the Saturnian moon Tethys; and Helene and Polydeuces, the leading and following companions of the Saturnian moon Dione.

Asteroid satellites

The discovery of 243 Ida's natural satellite Dactyl in the early 1990s confirmed that some asteroids have natural satellites; indeed, 87 Sylvia has two. Some, such as 90 Antiope, are double asteroids with two comparably sized components.

Shape

The relative masses of the natural satellites of the Solar System. Mimas, Enceladus, and Miranda are too small to be visible at this scale. All the irregularly shaped natural satellites, even added together, would also be too small to be visible. Masses of all moons.png
The relative masses of the natural satellites of the Solar System. Mimas, Enceladus, and Miranda are too small to be visible at this scale. All the irregularly shaped natural satellites, even added together, would also be too small to be visible.

Neptune's moon Proteus is the largest irregularly shaped natural satellite; the shapes of Eris' moon Dysnomia and Orcus' moon Vanth are unknown. All other known natural satellites that are at least the size of Uranus's Miranda have lapsed into rounded ellipsoids under hydrostatic equilibrium, i.e. are "round/rounded satellites" and are sometimes categorized as planetary-mass moons. The larger natural satellites, being tidally locked, tend toward ovoid (egg-like) shapes: squat at their poles and with longer equatorial axes in the direction of their primaries (their planets) than in the direction of their motion. Saturn's moon Mimas, for example, has a major axis 9% greater than its polar axis and 5% greater than its other equatorial axis. Methone, another of Saturn's moons, is only around 3 km in diameter and visibly egg-shaped. The effect is smaller on the largest natural satellites, where their own gravity is greater relative to the effects of tidal distortion, especially those that orbit less massive planets or, as in the case of the Moon, at greater distances.

NameSatellite ofDifference in axes
km
% of mean
diameter
Mimas Saturn 33.4 (20.4 /13.0)8.4 (5.1 /3.3)
Enceladus Saturn16.63.3
Miranda Uranus 14.23.0
Tethys Saturn25.82.4
Io Jupiter 29.40.8
The Moon Earth 4.30.1

Geological activity

Of the nineteen known natural satellites in the Solar System that are large enough to have lapsed into hydrostatic equilibrium, several remain geologically active today. Io is the most volcanically active body in the Solar System, while Europa, Enceladus, Titan and Triton display evidence of ongoing tectonic activity and cryovolcanism. In the first three cases, the geological activity is powered by the tidal heating resulting from having eccentric orbits close to their giant-planet primaries. (This mechanism would have also operated on Triton in the past, before its orbit was circularized.) Many other natural satellites, such as Earth's Moon, Ganymede, Tethys and Miranda, show evidence of past geological activity, resulting from energy sources such as the decay of their primordial radioisotopes, greater past orbital eccentricities (due in some cases to past orbital resonances), or the differentiation or freezing of their interiors. Enceladus and Triton both have active features resembling geysers, although in the case of Triton solar heating appears to provide the energy. Titan and Triton have significant atmospheres; Titan also has hydrocarbon lakes. Also Io and Callisto have atmospheres, even if they are extremely thin. [27] Four of the largest natural satellites, Europa, Ganymede, Callisto, and Titan, are thought to have subsurface oceans of liquid water, while smaller Enceladus may have localized subsurface liquid water.

Occurrence in the Solar System

Euler diagram showing the types of bodies in the Solar System. Euler diagram of solar system bodies.svg
Euler diagram showing the types of bodies in the Solar System.

Beside planets and dwarf planets objects within our Solar System known to have natural satellites are 76 in the asteroid belt (five with two each), four Jupiter trojans, 39 near-Earth objects (two with two satellites each), and 14 Mars-crossers. [2] There are also 84 known natural satellites of trans-Neptunian objects. [2] Some 150 additional small bodies have been observed within the rings of Saturn, but only a few were tracked long enough to establish orbits. Planets around other stars are likely to have satellites as well, and although numerous candidates have been detected to date, none have yet been confirmed.

Of the inner planets, Mercury and Venus have no natural satellites; Earth has one large natural satellite, known as the Moon; and Mars has two tiny natural satellites, Phobos and Deimos. The giant planets have extensive systems of natural satellites, including half a dozen comparable in size to Earth's Moon: the four Galilean moons, Saturn's Titan, and Neptune's Triton. Saturn has an additional six mid-sized natural satellites massive enough to have achieved hydrostatic equilibrium, and Uranus has five. It has been suggested that some satellites may potentially harbour life. [28]

Among the objects generally agreed by astronomers to be dwarf planets, Ceres and Sedna have no known natural satellites. Pluto has the relatively large natural satellite Charon and four smaller natural satellites; Styx, Nix, Kerberos, and Hydra. [29] Haumea has two natural satellites; Orcus, Quaoar, Makemake, Gonggong, and Eris have one each. The Pluto–Charon system is unusual in that the center of mass lies in open space between the two, a characteristic sometimes associated with a double-planet system.

The seven largest natural satellites in the Solar System (those bigger than 2,500 km across) are Jupiter's Galilean moons (Ganymede, Callisto, Io, and Europa), Saturn's moon Titan, Earth's moon, and Neptune's captured natural satellite Triton. Triton, the smallest of these, has more mass than all smaller natural satellites together. Similarly in the next size group of nine mid-sized natural satellites, between 1,000 km and 1,600 km across, Titania, Oberon, Rhea, Iapetus, Charon, Ariel, Umbriel, Dione, and Tethys, the smallest, Tethys, has more mass than all smaller natural satellites together. As well as the natural satellites of the various planets, there are also over 80 known natural satellites of the dwarf planets, minor planets and other small Solar System bodies. Some studies estimate that up to 15% of all trans-Neptunian objects could have satellites.

The following is a comparative table classifying the natural satellites in the Solar System by diameter. The column on the right includes some notable planets, dwarf planets, asteroids, and trans-Neptunian objects for comparison. The natural satellites of the planets are named after mythological figures. These are predominantly Greek, except for the Uranian natural satellites, which are named after Shakespearean characters. The nineteen bodies massive enough to have achieved hydrostatic equilibrium are in bold in the table below. Minor planets and satellites suspected but not proven to have achieved a hydrostatic equilibrium are italicized in the table below.

Mean
diameter
(km)
Satellites of planetsSatellites of dwarf planetsSatellites of
other
minor planets
Non-satellites
for comparison
EarthMarsJupiterSaturnUranusNeptuneOrcusPlutoHaumeaQuaoarMakemakeGonggongEris
12,000–13,000 Earth
Venus
6,000–7,000 Mars
4,000–6,000 Ganymede
Callisto
Titan Mercury
3,000–4,000 Moon Io
Europa
2,000–3,000 Triton Eris
Pluto
1,000–2,000 Rhea
Iapetus
Dione
Tethys
Titania
Oberon
Umbriel
Ariel
Charon Makemake
Haumea
Gonggong,
Quaoar
500–1,000 Enceladus Dysnomia Sedna , Ceres ,
Salacia, Orcus,
Pallas, Vesta
many more TNOs
250–500 Mimas
Hyperion
Miranda Proteus
Nereid
Vanth Hiʻiaka Varda I Ilmarë
Salacia I Actaea
Lempo II Hiisi
10 Hygiea
704 Interamnia
87 Sylvia
47171 Lempo
and many others
100–250 Amalthea
Himalia
Thebe
Phoebe
Janus
Epimetheus
Sycorax
Puck
Portia
Larissa
Galatea
Despina
Namaka S/2015 (136472) 1 S/2005 (82075)  1
Sila–Nunam  I
Ceto  I Phorcys
Patroclus  I Menoetius
Lempo I Paha
~20 more moons of TNOs
3 Juno
15760 Albion
5 Astraea
617 Patroclus
42355 Typhon
and many others
50–100 Elara
Pasiphae
Prometheus
Pandora
Caliban
Juliet
Belinda
Cressida
Rosalind
Desdemona
Bianca
Thalassa
Halimede
Neso
Naiad
Weywot Xiangliu (probably) 90 Antiope  I
Typhon  I Echidna
Logos  I Zoe
5 more moons of TNOs
90 Antiope
58534 Logos
253 Mathilde
and many others
25–50 Carme
Metis
Sinope
Lysithea
Ananke
Siarnaq
Helene
Albiorix
Atlas
Pan
Ophelia
Cordelia
Setebos
Prospero
Perdita
Stephano
Sao
Laomedeia
Psamathe
Hippocamp
Hydra
Nix [30]
Kalliope I Linus 1036 Ganymed
243 Ida
and many others
10–25 Phobos
Deimos
Leda
Adrastea
Telesto
Paaliaq
Calypso
Ymir
Kiviuq
Tarvos
Ijiraq
Erriapus
Mab
Cupid
Francisco
Ferdinand
Margaret
Trinculo
Kerberos
Styx
762 Pulcova I
Sylvia I Romulus
624 Hektor  I Skamandrios
Eugenia I Petit-Prince
121 Hermione  I
283 Emma  I
1313 Berna  I
107 Camilla I
433 Eros
1313 Berna
and many others
< 10 64 moons 57 moons Sylvia II Remus
Ida I Dactyl
and many others
many

Visual summary

Solar System moons we have seen clearly
Ganymede - Perijove 34 Composite.png
Titan in true color.jpg
Callisto.jpg
Io highest resolution true color.jpg
FullMoon2010.jpg
Europa-moon-with-margins.jpg
Triton moon mosaic Voyager 2 (large).jpg
Ganymede
(moon of Jupiter)
Titan
(moon of Saturn)
Callisto
(moon of Jupiter)
Io
(moon of Jupiter)
Moon
(moon of Earth)
Europa
(moon of Jupiter)
Triton
(moon of Neptune)
Titania (moon) color cropped.jpg
PIA07763 Rhea full globe5.jpg
Voyager 2 picture of Oberon.jpg
Iapetus as seen by the Cassini probe - 20071008.jpg
Charon in True Color - High-Res.jpg
PIA00040 Umbrielx2.47.jpg
Ariel (moon).jpg
Titania
(moon of Uranus)
Rhea
(moon of Saturn)
Oberon
(moon of Uranus)
Iapetus
(moon of Saturn)
Charon
(moon of Pluto)
Umbriel
(moon of Uranus)
Ariel
(moon of Uranus)
Dione in natural light.jpg
PIA18317-SaturnMoon-Tethys-Cassini-20150411.jpg
PIA17202 - Approaching Enceladus.jpg
PIA18185 Miranda's Icy Face.jpg
Proteus (Voyager 2).jpg
Mimas Cassini.jpg
Hyperion true.jpg
Dione
(moon of Saturn)
Tethys
(moon of Saturn)
Enceladus
(moon of Saturn)
Miranda
(moon of Uranus)
Proteus
(moon of Neptune)
Mimas
(moon of Saturn)
Hyperion
(moon of Saturn)
Phoebe cassini.jpg
Larissa 1.jpg
PIA12714 Janus crop.jpg
Amalthea (moon).png
Puck.png
PIA09813 Epimetheus S. polar region.jpg
Thebe.jpg
Phoebe
(moon of Saturn)
Larissa
(moon of Neptune)
Janus
(moon of Saturn)
Amalthea
(moon of Jupiter)
Puck
(moon of Uranus)
Epimetheus
(moon of Saturn)
Thebe
(moon of Jupiter)
Prometheus 12-26-09a.jpg
PIA21055 - Pandora Up Close.jpg
Hydra Enhanced Color.jpg
Nix best view.jpg
Leading hemisphere of Helene - 20110618.jpg
Atlas (NASA).jpg
Pan by Cassini, March 2017.jpg
Prometheus
(moon of Saturn)
Pandora
(moon of Saturn)
Hydra
(moon of Pluto)
Nix
(moon of Pluto)
Helene
(moon of Saturn)
Atlas
(moon of Saturn)
Pan
(moon of Saturn)
Telesto cassini closeup.jpg
Calypso N1644755236 1.jpg
Phobos colour 2008.jpg
Deimos-MRO.jpg
Daphnis (Saturn's Moon).jpg
Methone PIA14633.jpg
Dactyl-HiRes.jpg
Telesto
(moon of Saturn)
Calypso
(moon of Saturn)
Phobos
(moon of Mars)
Deimos
(moon of Mars)
Daphnis
(moon of Saturn)
Methone
(moon of Saturn)
Dactyl
(moon of Ida)
Largest moons to scale with their parent planets and dwarf planet. Moons of the Solar System To Scale (43564841545).png
Largest moons to scale with their parent planets and dwarf planet.

See also

Moons of planets

Moons of dwarf planets and small Solar System bodies

Related Research Articles

Double planet A binary system where two planetary-mass objects share an orbital axis external to both

In astronomy, a double planet is a binary satellite system where both objects are planets, or planetary-mass objects, that share an orbital axis external to both planetary bodies.

Orbital resonance Regular and periodic gravitational influence by two orbiting celestial bodies exerted on each other

In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationship is found between a pair of objects. The physical principle behind orbital resonance is similar in concept to pushing a child on a swing, whereby the orbit and the swing both have a natural frequency, and the body doing the "pushing" will act in periodic repetition to have a cumulative effect on the motion. Orbital resonances greatly enhance the mutual gravitational influence of the bodies. In most cases, this results in an unstable interaction, in which the bodies exchange momentum and shift orbits until the resonance no longer exists. Under some circumstances, a resonant system can be self-correcting and thus stable. Examples are the 1:2:4 resonance of Jupiter's moons Ganymede, Europa and Io, and the 2:3 resonance between Pluto and Neptune. Unstable resonances with Saturn's inner moons give rise to gaps in the rings of Saturn. The special case of 1:1 resonance between bodies with similar orbital radii causes large solar system bodies to eject most other bodies sharing their orbits; this is part of the much more extensive process of clearing the neighbourhood, an effect that is used in the current definition of a planet.

Planet 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.

Solar System The Sun, its planets and their moons

The Solar System is the gravitationally bound system of the Sun and the objects that orbit it. It formed 4.6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud. The vast majority (99.86%) of the system's mass is in the Sun, with most of the remaining mass contained in the planet Jupiter. The four inner system planets—Mercury, Venus, Earth and Mars—are terrestrial planets, being composed primarily of rock and metal. The four giant planets of the outer system are substantially larger and more massive than the terrestrials. The two largest, Jupiter and Saturn, are gas giants, being composed mainly of hydrogen and helium; the next two, Uranus and Neptune, are ice giants, being composed mostly of volatile substances with relatively high melting points compared with hydrogen and helium, such as water, ammonia, and methane. All eight planets have nearly circular orbits that lie near the plane of Earth's orbit, called the ecliptic.

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.

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.

Formation and evolution of the Solar System Modelling its structure and composition

The formation of the Solar System began about 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the center, forming the Sun, while the rest flattened into a protoplanetary disk out of which the planets, moons, asteroids, and other small Solar System bodies formed.

Dwarf planet 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 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.

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.

Planetary-mass object

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.

Planetary-mass moon Moons comparable in size to small planets

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

The following outline is provided as an overview of and topical guide to the Solar System:

Satellite system (astronomy) Set of gravitationally bound objects in orbit

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.

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.

References

  1. "Planet and Satellite Names and Discoverers". International Astronomical Union (IAU) Working Group for Planetary System Nomenclature (WGPSN). Retrieved 27 January 2012.
  2. 1 2 3 Wm. Robert Johnston (30 September 2018). "Asteroids with Satellites". Johnston's Archive. Retrieved 22 October 2018.
  3. Canup, Robin M.; Ward, William R. (June 2006). "A common mass scaling for satellite systems of gaseous planets". Nature. 441 (7095): 834–839. Bibcode:2006Natur.441..834C. doi:10.1038/nature04860. ISSN   1476-4687. PMID   16778883. S2CID   4327454.
  4. Glenday, Craig (2014). Guinness World Records 2014. p.  186. ISBN   978-1-908843-15-9.
  5. 1 2 "Early History – First Satellites". www.jpl.nasa.gov. Retrieved 8 February 2018.
  6. Kenneth R. Lang. "The Cambridge Guide to the Solar System". 2011. p. 15. quote: "Any object that orbits a planet is now called a satellite, and a natural satellite is also now called a moon."
  7. Therese Encrenaz, et al. "The Solar System". 2004. p. 30.
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All moons

Jupiter's moons

Saturn's moons