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A natural satellite is in the most common usage, an
astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science Natural science is a branch of science Science (from the Latin word ''scientia'', meaning "knowled ...
that
orbit In celestial mechanics, an orbit is the curved trajectory of an physical body, object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an satellite, artificial satellite around an object or po ...

orbit
s a
planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and ...

planet
,
dwarf planet 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 d ...
, or
small Solar System body A small Solar System body (SSSB) is an object in the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the n ...
(or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'', a derivation from the
Moon The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...

Moon
of
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
. In the
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

Solar System
, there are six planetary satellite systems containing 207 known natural satellites altogether. Seven objects commonly considered
dwarf planet 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 d ...
s by astronomers are also known to have natural satellites: ,
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 was the first and the largest Kuiper belt object to be discovered. After Pluto wa ...

Pluto
,
Haumea Haumea (minor-planet designation A formal minor planet designation is, in its final form, a number–name combination given to a minor planet (asteroid, centaur (minor planet), centaur, trans-Neptunian object and dwarf planet but not comet). S ...
, ,
Makemake Makemake ( minor-planet designation 136472 Makemake) is a likely dwarf planet A dwarf planet is a planetary-mass object that does not dominate its region of space (as a planet A planet is an astronomical body orbiting a star or Stellar ...

Makemake
, , and Eris. , there are 442 other minor planets known to have natural satellites. 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. The
Earth–Moon system The Moon is Earth's only Claimed moons of Earth, proper natural satellite. At one-quarter the diameter of Earth (comparable to the width of Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...
is a unique exception in the Solar System; at 3,474 km (2,158 miles) across, the Moon is 0.273 times the
diameter In geometry Geometry (from the grc, γεωμετρία; ' "earth", ' "measurement") is, with , one of the oldest branches of . It is concerned with properties of space that are related with distance, shape, size, and relative position ...

diameter
of Earth and about 1/80th of its mass. The next largest ratios are the
Neptune Neptune is the eighth and farthest-known Solar planet from the Sun. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times the mass of Earth, slightly mo ...

Neptune
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'', ...

Triton
system at 0.055 (with a mass ratio of about 1 to 5000), the
Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It only has one-eighth the average density of Earth; how ...

Saturn
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, fictional ...
system at 0.044 (with the second mass ratio next to the Earth-Moon system, 1 to 4250), the
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

Jupiter
system at 0.038, and the
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus, who, according to Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and ...

Uranus
Titania Titania may refer to: Astronomy * Titania (moon) Titania (), also designated Uranus III, is the largest of the moons of Uranus and the List of natural satellites by diameter, eighth largest moon in the Solar System at a diameter of . Discovere ...
system at 0.031. For the category of
dwarf planet 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 d ...
s,
Charon In Greek mythology Greek mythology is the body of s originally told by the , and a of . These stories concern the and , the lives and activities of , , and , and the origins and significance of the ancient Greeks' own and practices. M ...

Charon
has the largest ratio, being 0.52 the diameter of
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 was the first and the largest Kuiper belt object to be discovered. After Pluto wa ...

Pluto
.


Terminology

The first known natural satellite was the
Moon The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...

Moon
, but it was considered a "planet" until
Copernicus Nicolaus Copernicus (; pl, Mikołaj Kopernik; gml, link=no, Niclas Koppernigk, modern: ''Nikolaus Kopernikus''; 19 February 1473 – 24 May 1543) was a Renaissance polymath, active as a mathematician, astronomer, and Catholic Church, C ...

Copernicus
' introduction of ''
De revolutionibus orbium coelestium ''De revolutionibus orbium coelestium'' (; English translation: ''On the Revolutions of the Heavenly Spheres'') is the seminal work on the heliocentric theory Heliocentrism is the astronomical model in which the Earth Earth is the third ...

De revolutionibus orbium coelestium
'' in 1543. Until the discovery of the
Galilean satellites 's four Galilean moons, in a composite image depicting part of Jupiter and their relative sizes (positions are illustrative, not actual). From top to bottom: Io (moon), Io, Europa (moon), Europa, Ganymede (moon), Ganymede, Callisto (moon), Callisto. ...
in 1610 there was no opportunity for referring to such objects as a class.
Galileo Galileo di Vincenzo Bonaiuti de' Galilei ( , ; 15 February 1564 – 8 January 1642), commonly referred to as Galileo, was an astronomer An astronomer is a in the field of who focuses their studies on a specific question or field o ...

Galileo
chose to refer to his discoveries as ''Planetæ'' ("planets"), but later discoverers chose other terms to distinguish them from the objects they orbited. The first to use the term ''satellite'' to describe orbiting bodies was the German astronomer
Johannes Kepler Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer An astronomer is a in the field of who focuses their studies on a specific question or field outside the scope of . They observe s such as s, s, , s and ...

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 Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republic, it became ...

Latin
word ''satelles'', meaning "guard", "attendant", or "companion", because the ''satellites'' accompanied their primary planet in their journey through the heavens. 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 Sputnik 1 (; see § Etymology) was the first artificial Earth satellite. It was launched into an elliptical low Earth orbit A low Earth orbit (LEO) is an Earth-centered orbit near the planet, often specified as having a period Period may ...

Sputnik
created a need for new terminology. 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. 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 The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...

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, but not when referring to other natural satellites. 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. 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".


Definition of a 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 The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

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 ''
moonlet A moonlet, minor moon, minor natural satellite, or minor satellite is a particularly small natural satellite orbiting a planet, dwarf planet, or other minor planet. Up until 1995, moonlets were only hypothetical components of Saturn's F-ring ...
s''. Small asteroid moons (natural satellites of asteroids), such as
Dactyl Dactyl may refer to: * Dactyl (mythology) In Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of Ancient Greek folklore. These stories concern the Cosmogony, origin and Co ...
, have also been called moonlets. The upper limit is also vague. Two orbiting bodies are sometimes described as a
double planet In astronomy, a double planet (also binary planet) is a binary Binary may refer to: Science and technology Mathematics * Binary number In mathematics and digital electronics, a binary number is a number expressed in the base-2 numer ...
rather than primary and satellite.
Asteroid An asteroid is a minor planet of the Solar System#Inner solar system, inner Solar System. Historically, these terms have been applied to any astronomical object orbiting the Sun that did not resolve into a disc in a telescope and was not observ ...

Asteroid
s such as
90 Antiope Antiope (minor planet designation A formal minor-planet designation is, in its final form, a number–name combination given to a minor planet A minor planet is an astronomical object in direct orbit around the Sun (or more broadly, any s ...
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 dividing line on what is considered a moon rests upon whether the
barycentre In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses ...
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 A protoplanetary disk is a rotating circumstellar disc A circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accumulation of matter composed of gas, Cosmic dust, dust, planetesimals, asteroids, or collision fragments i ...

protoplanetary disk
that created its primary. In contrast,
irregular satellite In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses mat ...
s (generally orbiting on distant, inclined, eccentric and/or retrograde orbits) are thought to be captured
asteroid An asteroid is a minor planet of the Solar System#Inner solar system, inner Solar System. Historically, these terms have been applied to any astronomical object orbiting the Sun that did not resolve into a disc in a telescope and was not observ ...

asteroid
s possibly further fragmented by collisions. Most of the major natural satellites of the
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

Solar System
have regular orbits, while most of the small natural satellites have irregular orbits. The
Moon The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...

Moon
and possibly
Charon In Greek mythology Greek mythology is the body of s originally told by the , and a of . These stories concern the and , the lives and activities of , , and , and the origins and significance of the ancient Greeks' own and practices. M ...

Charon
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 giant-impact hypothesis, sometimes called the Big Splash, or the Theia Impact, suggests that the Moon The Moon is Earth's only proper natural satellite. At one-quarter the diameter of Earth (comparable to the width of Australia ...
). 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 moon A minor-planet moon is an astronomical object that orbits a minor planet as its natural satellite. , there are #List, 424 minor planets known or suspected to have moons. Discoveries of minor-planet moons (and binary objects, in general) are impor ...
s are thought to commonly form by this process.
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'', ...

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 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 d ...
.


Temporary satellites

The capture of an asteroid from a heliocentric orbit is not always permanent. According to simulations,
temporary satelliteA temporary satellite is an object which has been captured by the gravitational field of a planet and thus has become the planet's natural satellite, but, unlike irregular moons of the larger outer planets of the Solar System The Solar System ...
s should be a common phenomenon. The only observed examples are , , . was a temporary satellite of
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
for nine months in 2006 and 2007.


Tidal locking

Most
regular moon In astronomy, a regular moon is a natural satellite A natural satellite, or moon, is, in the most common usage, an astronomical body that orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, ...
s (natural satellites following relatively close and prograde orbits with small orbital inclination and eccentricity) in the
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

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. The only known exception is
Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It only has one-eighth the average density of Earth; how ...

Saturn
's natural satellite Hyperion, which rotates chaotically because of the gravitational influence of
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, fictional ...
. 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
PhoebePhoebe may refer to: __NOTOC__ Personal name * Phoebe (given name), a list of people, mythological, biblical and fictional characters * Anna Phoebe (born 1981), violinist *Phoebe (mythological characters), Phoebe (mythology) * Phoebe, an epithet of S ...
, and Neptune's
Nereid In Greek mythology Greek mythology is the body of s originally told by the , and a of . These stories concern the and , the lives and activities of , , and , and the origins and significance of the ancient Greeks' own and practices. ...
have rotation periods in the range of ten hours, whereas their orbital periods are hundreds of days.


Satellites of satellites

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 of a possible
ring system A ring system is a disc or ring, orbiting an astronomical object In , an astronomical object or celestial object is a naturally occurring , association, or structure that exists in the . In , the terms ''object'' and ''body'' are often u ...
around Saturn's moon indicate that satellites orbiting Rhea could have stable orbits. Furthermore, the suspected rings are thought to be narrow, a phenomenon normally associated with
shepherd moon A shepherd moon (also herder moon or watcher moon) is a small natural satellite that clears a gap in planetary-ring material or keeps particles within a ring contained. The name is a result of the fact they limit the "herd" of the ring particles ...
s. However, targeted images taken by the '' Cassini'' spacecraft failed to detect rings around Rhea. It has also been proposed that Saturn's moon
Iapetus In Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of Ancient Greek folklore. These stories concern the Cosmogony, origin and Cosmology#Metaphysical cosmology, nature of ...
had a satellite in the past; this is one of several hypotheses that have been put forward to account for its equatorial ridge.


Trojan satellites

Two natural satellites are known to have small companions at both their and
Lagrangian point In celestial mechanics, the Lagrange points (also Lagrangian points, L-points, or libration points) are points of equilibrium for small-mass objects under the influence of two massive orbit, orbiting bodies. Mathematically, this involves th ...
s, sixty degrees ahead and behind the body in its orbit. These companions are called
trojan moonIn astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses mathem ...
s, as their orbits are analogous to the
trojan asteroid In astronomy, a trojan is a small celestial body (mostly asteroids) that shares the orbit of a larger one, remaining in a stable orbit approximately 60° ahead or behind the main body near one of its Lagrangian points and . Trojans can share the ...
s of
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

Jupiter
. The trojan moons are and
Calypso Calypso refers to: *Calypso (mythology), a nymph who, famously in Homer's ''Odyssey'', kept Odysseus with her on her island of Ogygia for seven years. *Calypso (nymphs), other nymphs called Calypso. Calypso may also refer to: Places *Calypso ...
, which are the leading and following companions, respectively, of the Saturnian moon Tethys; and Helene and , the leading and following companions of the Saturnian moon Dione.


Asteroid satellites

The discovery of
243 Ida Ida, minor planet designation A formal minor-planet designation is, in its final form, a number–name combination given to a minor planet A minor planet is an astronomical object in direct orbit around the Sun (or more broadly, any sta ...

243 Ida
's natural satellite
Dactyl Dactyl may refer to: * Dactyl (mythology) In Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of Ancient Greek folklore. These stories concern the Cosmogony, origin and Co ...
in the early 1990s confirmed that some
asteroid An asteroid is a minor planet of the Solar System#Inner solar system, inner Solar System. Historically, these terms have been applied to any astronomical object orbiting the Sun that did not resolve into a disc in a telescope and was not observ ...

asteroid
s have natural satellites; indeed, 87 Sylvia has two. Some, such as
90 Antiope Antiope (minor planet designation A formal minor-planet designation is, in its final form, a number–name combination given to a minor planet A minor planet is an astronomical object in direct orbit around the Sun (or more broadly, any s ...
, are double asteroids with two comparably sized components.


Shape

Neptune's moon
Proteus Image:Proteus-Alciato.gif, Illustration of Proteus by Andrea Alciato from ''The Book of Emblems'' (1531) In Greek mythology, Proteus (; Ancient Greek: Πρωτεύς, ''Prōteus'') is an early prophetic sea-god or god of rivers and oceanic bod ...

Proteus
is the largest irregularly shaped natural satellite; the shapes of Eris' moon Dysnomia and Orcus' moon
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. Vanth is a female demon in the Etruscan underworld that is often accompanied either by additional Vanth fig ...
are unknown. All other known natural satellites that are at least the size of Uranus's Miranda have lapsed into rounded
ellipsoid An ellipsoid is a surface that may be obtained from a sphere by deforming it by means of directional Scaling (geometry), scalings, or more generally, of an affine transformation. An ellipsoid is a quadric surface;  that is, a Surface (mathemat ...

ellipsoid
s under
hydrostatic equilibrium In fluid mechanics, hydrostatic equilibrium (hydrostatic balance, 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. In the planetary ph ...

hydrostatic equilibrium
, i.e. are "round/rounded satellites". The larger natural satellites, being tidally locked, tend toward
ovoid An oval (from Latin ''ovum'', "egg") is a closed curve in a plane which resembles the outline of an egg. The term is not very specific, but in some areas ( projective geometry, technical drawing, etc.) it is given a more precise definition, wh ...

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 The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...

Moon
, at greater distances.


Geological activity

Of the nineteen known natural satellites in the
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

Solar System
that are large enough to have lapsed into
hydrostatic equilibrium In fluid mechanics, hydrostatic equilibrium (hydrostatic balance, 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. In the planetary ph ...

hydrostatic equilibrium
, several remain geologically active today. Io is the most volcanically active body in the Solar System, while
Europa Europa may refer to: Places *Europe Europe is a continent A continent is one of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven regions are commonly regard ...
,
Enceladus Enceladus (; Greek: Εγκέλαδος) is the sixth-largest Moons of Saturn, moon of Saturn. It is about in diameter, about a tenth of that of Saturn, Saturn's largest moon, Titan (moon), Titan. Enceladus is mostly covered by fresh, clean ice ...

Enceladus
,
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, fictional ...
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'', ...

Triton
display evidence of ongoing tectonic activity and
cryovolcanism , one of the most reliably identified cryovolcanoes on Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of ...
. In the first three cases, the geological activity is powered by the
tidal heating Tidal heating (also known as tidal working or tidal flexing) occurs through the tidal friction processes: orbital and rotational energy is dissipated as heat in either (or both) the surface ocean or interior of a planet or satellite. When an object ...
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 The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...

Moon
, , Tethys and Miranda, show evidence of past geological activity, resulting from energy sources such as the of their primordial
radioisotopes A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by ...
, greater past orbital eccentricities (due in some cases to past
orbital resonance . Conjunctions ''Conjunctions'' is a biannual American literature, American literary journal based at Bard College. It was founded in 1981 and is currently edited by Bradford Morrow. Morrow received the PEN/Nora Magid Award for Magazine Editing ...
s), or the
differentiation Differentiation may refer to: Business * Differentiation (economics), the process of making a product different from other similar products * Product differentiation, in marketing * Differentiated service, a service that varies with the identity o ...
or freezing of their interiors. Enceladus 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'', ...

Triton
both have active features resembling
geysers A geyser (, ) is a spring (hydrosphere), spring characterized by an intermittent discharge of water ejected turbulently and accompanied by steam. As a fairly rare phenomenon, the formation of geysers is due to particular hydrogeological conditio ...

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.A moon with atmosphere , The Planetary Society
/ref> Four of the largest natural satellites,
Europa Europa may refer to: Places *Europe Europe is a continent A continent is one of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven regions are commonly regard ...
, , Callisto, 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, fictional ...
, are thought to have subsurface oceans of liquid water, while smaller Enceladus may have localized subsurface liquid water.


Natural satellites of the Solar System

Of the objects within our Solar System known to have natural satellites, there are 76 in the
asteroid belt The asteroid belt is a torus-shaped region in the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizin ...

asteroid belt
(five with two each), four
Jupiter trojan The Jupiter trojans, commonly called trojan asteroids or simply trojans, are a large group of asteroid An asteroid is a minor planet of the Solar System#Inner solar system, inner Solar System. Historically, these terms have been applied to ...
s, 39
near-Earth object A near-Earth object (NEO) is any small Solar System body A small Solar System body (SSSB) is an object in the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body rega ...
s (two with two satellites each), and 14 Mars-crossers. There are also 84 known natural satellites of
trans-Neptunian objects A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet A minor planet is an astronomical object in direct orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, objec ...
. Some 150 additional small bodies have been observed within the
rings of Saturn The rings of Saturn are the most extensive ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometers to meters, that orbit around Saturn. The ring particles are made almost entirely ...

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 Mercury usually refers to: * Mercury (planet) Mercury is the smallest planet in the Solar System and the closest to the Sun. Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. It is named after the Roman g ...

Mercury
and
Venus Venus is the second planet from the Sun. It is named after the Venus (mythology), Roman goddess of love and beauty. As List of brightest natural objects in the sky, the brightest natural object in Earth's night sky after the Moon, Venus can ...

Venus
have no natural satellites; Earth has one large natural satellite, known as the Moon; and
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, being larger than only Mercury (planet), Mercury. In English, Mars carries the name of the Mars (mythology), Roman god of war and is often referred to ...

Mars
has two tiny natural satellites, Phobos and Deimos. The
giant planet The giant planets constitute a diverse type of planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, ...
s have extensive systems of natural satellites, including half a dozen comparable in size to Earth's Moon: the four
Galilean moon 's four Galilean moons, in a composite image depicting part of Jupiter and their relative sizes (positions are illustrative, not actual). From top to bottom: Io (moon), Io, Europa (moon), Europa, Ganymede (moon), Ganymede, Callisto (moon), Callisto. ...
s,
Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It only has one-eighth the average density of Earth; how ...

Saturn
's
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, fictional ...
, and
Neptune Neptune is the eighth and farthest-known Solar planet from the Sun. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times the mass of Earth, slightly mo ...

Neptune
's
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'', ...

Triton
. Saturn has an additional six mid-sized natural satellites massive enough to have achieved
hydrostatic equilibrium In fluid mechanics, hydrostatic equilibrium (hydrostatic balance, 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. In the planetary ph ...

hydrostatic equilibrium
, and
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus, who, according to Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and ...

Uranus
has five. It has been suggested that some satellites may potentially harbour life. Among the objects generally agreed by astronomers to be dwarf planets,
Ceres Ceres most commonly refers to: * Ceres (dwarf planet) Ceres (; minor-planet designation: 1 Ceres) is the smallest recognized dwarf planet, the closest dwarf planet to the Sun, and the List of notable asteroids, largest object in the main astero ...
and have no known natural satellites. Pluto has the relatively large natural satellite Charon and four smaller natural satellites;
Styx In Greek mythology, Styx (; grc, Στύξ ) is a deity and a river that forms the boundary between Gaia, Earth (Gaia) and the Greek underworld, Underworld. The rivers Acheron, Cocytus, Lethe, Phlegethon, and Styx all converge at the centre of the ...

Styx
, Nix, , and .
Haumea Haumea (minor-planet designation A formal minor planet designation is, in its final form, a number–name combination given to a minor planet (asteroid, centaur (minor planet), centaur, trans-Neptunian object and dwarf planet but not comet). S ...

Haumea
has two natural satellites; , ,
Makemake Makemake ( minor-planet designation 136472 Makemake) is a likely dwarf planet A dwarf planet is a planetary-mass object that does not dominate its region of space (as a planet A planet is an astronomical body orbiting a star or Stellar ...

Makemake
, , and have one each. The Pluto–Charon system is unusual in that the
center of mass In physics, the center of mass of a distribution of mass Mass is the quantity Quantity is a property that can exist as a multitude or magnitude, which illustrate discontinuity and continuity. Quantities can be compared in terms of " ...
lies in open space between the two, a characteristic sometimes associated with a double-planet system. The seven largest
natural satellites A natural satellite is in the most common usage, an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science Natural science is a branch ...
in the
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

Solar System
(those bigger than 2,500 km across) are
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

Jupiter
's
Galilean moon 's four Galilean moons, in a composite image depicting part of Jupiter and their relative sizes (positions are illustrative, not actual). From top to bottom: Io (moon), Io, Europa (moon), Europa, Ganymede (moon), Ganymede, Callisto (moon), Callisto. ...
s (, Callisto, Io, and
Europa Europa may refer to: Places *Europe Europe is a continent A continent is one of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven regions are commonly regard ...
), Saturn's moon Titan, Earth's
moon The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...

moon
, and Neptune's captured natural satellite
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'', ...

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 Titania may refer to: Astronomy * Titania (moon) Titania (), also designated Uranus III, is the largest of the moons of Uranus and the List of natural satellites by diameter, eighth largest moon in the Solar System at a diameter of . Discovere ...
,
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, ...
, ,
Iapetus In Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of Ancient Greek folklore. These stories concern the Cosmogony, origin and Cosmology#Metaphysical cosmology, nature of ...
,
Charon In Greek mythology Greek mythology is the body of s originally told by the , and a of . These stories concern the and , the lives and activities of , , and , and the origins and significance of the ancient Greeks' own and practices. M ...

Charon
,
Ariel The Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL), is a space telescope A space telescope or space observatory is a telescope in outer space used to observe astronomical objects. Suggested by Lyman Spitzer in 1946, the ...

Ariel
, , 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 planet 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 d ...
s,
minor planet A minor planet is an astronomical object in direct orbit around the Sun (or more broadly, any star with a planetary system) that is neither a planet nor exclusively classified as a comet. Before 2006, the International Astronomical Union (IAU) ...
s and other
small Solar System bodies A small Solar System body (SSSB) is an object in the Solar System that is neither a planet, a dwarf planet, nor a natural satellite. The term was first IAU definition of planet, defined in 2006 by the International Astronomical Union (IAU) as foll ...
. 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 , 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.


Visual summary


History


See also


Moons of planets


Moons of dwarf planets and small Solar System bodies


References


External links


All moons


Natural Satellite Physical Parameters
(JPL-NASA, with refs – last updated July 2006)

(The Planetary Society, as of March 2009)
The JPL's Solar System Dynamics page
*
Planetary Names: Planet and Satellite Names and Discoverers

"Upper size limit for moons explained"
Kelly Young. ''Nature'' (vol 441, p. 834) 14 June 2006




Album
of moon images by Kevin M. Gill
The Atlas of Moons
by the National Geographic Society


Jupiter's moons

*

Scott S. Sheppard

Scott S. Sheppard


Saturn's moons


Satellite-hunters find four new moons of the planet Saturn
David Brand , 26 October 2000

Scott S. Sheppard {{DEFAULTSORT:Natural Satellite