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A satellite system is a set of gravitationally bound objects in orbit around a
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 (to be rounded under its own gravity), but not enough to sustain ...
(incl.
sub-brown dwarf A sub-brown dwarf or planetary-mass brown dwarf is an astronomical object that formed in the same manner as stars and brown dwarfs (i.e. through the collapse of a gas cloud) but that has a planetary mass, therefore by definition below the limi ...
s and
rogue planet A rogue planet (also termed a free-floating planet (FFP), interstellar, nomad, orphan, starless, unbound or wandering planet) is an interstellar object of planetary-mass, therefore smaller than fusors (stars and brown dwarfs) and without a h ...
s) or
minor planet According to the International Astronomical Union (IAU), a minor planet is an astronomical object in direct orbit around the Sun that is exclusively classified as neither a planet nor a comet. Before 2006, the IAU officially used the term ''minor ...
, or its
barycenter In astronomy, the barycenter (or barycentre; ) is the center of mass of two or more bodies that orbit one another and is the point about which the bodies orbit. A barycenter is a dynamical point, not a physical object. It is an important con ...
. Generally speaking, it is a set of
natural satellites 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 ''moo ...
(moons), although such systems may also consist of bodies such as circumplanetary disks, ring systems,
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,
minor-planet moon A minor-planet moon is an astronomical object that orbits a minor planet as its natural satellite. , there are 457 minor planets known or suspected to have moons. Discoveries of minor-planet moons (and binary objects, in general) are importan ...
s and
artificial satellites A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisotope ...
any of which may themselves have satellite systems of their own (see Subsatellites). 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 resonance 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 relationsh ...
s and
libration In lunar astronomy, libration is the wagging or wavering of the Moon perceived by Earth-bound observers and caused by changes in their perspective. It permits an observer to see slightly different hemispheres of the surface at different tim ...
. Individually major satellite objects are designated in Roman numerals. Satellite systems are referred to either by the possessive adjectives of their primary (e.g. " Jovian system"), or less commonly by the name of their primary (e.g. "Jupiter system"). 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 (e.g. the "
Earth-Moon system The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
"). Many Solar System objects are known to possess satellite systems, though their origin is still unclear. Notable examples include the largest satellite system, the Jovian system, with 82 known moons (including the large
Galilean moons The Galilean moons (), or Galilean satellites, are the four largest moons of Jupiter: Io, Europa, Ganymede, and Callisto. They were first seen by Galileo Galilei in December 1609 or January 1610, and recognized by him as satellites of Jupite ...
) and the Saturnian System with 83 known moons (and the most visible ring system in the Solar System). Both satellite systems are large and diverse. In fact all of the giant planets of the Solar System possess large satellite systems as well as planetary rings, and it is inferred that this is a general pattern. Several objects farther from the Sun also have satellite systems consisting of multiple moons, including the complex Plutonian system where multiple objects orbit a common
center of mass In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force may ...
, as well as many asteroids and plutinos. Apart from the Earth-Moon system and Mars' system of two tiny natural satellites, the other terrestrial planets are generally not considered satellite systems, although some have been orbited by artificial satellites originating from Earth. Little is known of satellite systems beyond the Solar System, although it is inferred that natural satellites are common. J1407b is an example of an extrasolar satellite system. It is also theorised that
Rogue planet A rogue planet (also termed a free-floating planet (FFP), interstellar, nomad, orphan, starless, unbound or wandering planet) is an interstellar object of planetary-mass, therefore smaller than fusors (stars and brown dwarfs) and without a h ...
s ejected from their planetary system could retain a system of satellites.


Natural formation and evolution

Satellite systems, like planetary systems, are the product of gravitational attraction, but are also sustained through
fictitious force A fictitious force is a force that appears to act on a mass whose motion is described using a non-inertial frame of reference, such as a linearly accelerating or rotating reference frame. It is related to Newton's second law of motion, which trea ...
s. While the general consensus is that most planetary systems are formed from an accretionary disks, the formation of satellite systems is less clear. The origin of many moons are investigated on a case by case basis, and the larger systems are thought to have formed through a combination of one or more processes.


System stability

The
Hill sphere The Hill sphere of an astronomical body is the region in which it dominates the attraction of satellites. To be retained by a planet, a moon must have an orbit that lies within the planet's Hill sphere. That moon would, in turn, have a Hill sph ...
is the region in which an astronomical body dominates the attraction of satellites. Of the Solar System planets, Neptune and Uranus have the largest Hill spheres, due to the lessened gravitational influence of the Sun at their far orbits, however all of the giant planets have Hill spheres in the vicinity of 100 million kilometres in radius. By contrast, the Hill spheres of Mercury and Ceres, being closer to the Sun are quite small. Outside of the Hill sphere, the Sun dominates the gravitational influence, with the exception of the
Lagrangian point In celestial mechanics, the Lagrange points (; also Lagrangian points or libration points) are points of equilibrium for small-mass objects under the influence of two massive orbiting bodies. Mathematically, this involves the solution of t ...
s. Satellites are stable at the and Lagrangian points. These lie at the third corners of the two
equilateral triangle In geometry, an equilateral triangle is a triangle in which all three sides have the same length. In the familiar Euclidean geometry, an equilateral triangle is also equiangular; that is, all three internal angles are also congruent to each oth ...
s in the plane of orbit whose common base is the line between the centers of the two masses, such that the point lies behind () or ahead () of the smaller mass with regard to its orbit around the larger mass. The triangular points ( and ) are stable equilibria, provided that the ratio of M1/M2 is nearly 24.96.More precisely, \tfrac ≈ 24.9599357944
The Lagrange Points
', Neil J. Cornish with input from Jeremy Goodman
When a body at these points is perturbed, it moves away from the point, but the factor opposite of that which is increased or decreased by the perturbation (either gravity or angular momentum-induced speed) will also increase or decrease, bending the object's path into a stable, kidney-bean-shaped orbit around the point (as seen in the corotating frame of reference). It is generally thought that natural satellites should orbit in the same direction as the planet is rotating (known as prograde orbit). As such, the terminology regular moon is used for these orbit. However a retrograde orbit (the opposite direction to the planet) is also possible, the terminology
irregular moon In astronomy, an irregular moon, irregular satellite or irregular natural satellite is a natural satellite following a distant, inclined, and often eccentric and retrograde orbit. They have been captured by their parent planet, unlike regular ...
is used to describe known exceptions to the rule, it is believed that irregular moons have been inserted into orbit through gravitational capture.


Accretion theories

Accretion disks around giant planets may occur in a similar way to the occurrence of disks around stars, out of which planets form (for example, this is one of the theories for the formations of the satellite systems of Uranus, Saturn, and Jupiter). This early cloud of gas is a type of circumplanetary disk known as a proto-satellite disk (in the case of the Earth-Moon system, the proto-lunar disk). Models of gas during the formation of planets coincide with a general rule for planet-to-satellite(s) mass ratio of 10,000:1 (a notable exception is Neptune). Accretion is also proposed by some as a theory for the origin of the Earth-Moon system, however the angular momentum of system and the Moon's smaller iron core can not easily be explained by this.


Debris disks

Another proposed mechanism for satellite system formation is accretion from debris. Scientists theorise that the Galilean moons are thought by some to be a more recent generation of moons formed from the disintegration of earlier generations of accreted moons. Ring systems are a type of circumplanetary disk that can be the result of satellites disintegrated near the Roche limit. Such disks could, over time, coalesce to form natural satellites.


Collision theories

Collision is one of the leading theories for the formation of satellite systems, particularly those of the Earth and Pluto. Objects in such a system may be part of a
collisional family In astronomy, a collisional family is a group of objects that are thought to have a common origin in an impact (collision). They have similar compositions and most share similar orbital elements. Numerous asteroid families, most of the irregular ...
and this origin may be verified comparing their
orbital elements Orbital elements are the parameters required to uniquely identify a specific orbit. In celestial mechanics these elements are considered in two-body systems using a Kepler orbit. There are many different ways to mathematically describe the same ...
and composition. Computer simulations have been used to demonstrate that giant impacts could have been the
origin of the Moon The origin of the Moon is usually explained by a Mars-sized body striking the Earth, making a debris ring that eventually collected into a single natural satellite, the Moon, but there are a number of variations on this giant-impact hypothesis, a ...
. It is thought that early Earth had multiple moons resulting from the giant impact. Similar models have been used to explain the creation of the Plutonian system as well as those of other
Kuiper belt object The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 time ...
s and asteroids. This is also a prevailing theory for the origin of the moons of Mars. Both sets of findings support an origin of Phobos from material ejected by an impact on Mars that reaccreted in Martian orbit. Collision is also used to explain peculiarities in the Uranian system. Models developed in 2018 explain the planet's unusual spin support an oblique collision with an object twice the size of Earth which likely to have re-coalesced to form the system's icy moons.


Gravitational capture theories

Some theories suggest that gravitational capture is the origin of Neptune's major moon Triton, the moons of Mars, and Saturn's moon Phoebe. Some scientists have put forward extended atmospheres around young planets as a mechanism for slowing the movement of a passing objects to aid in capture. The hypothesis has been put forward to explain the irregular satellite orbits of
Jupiter Jupiter is the fifth planet from the Sun and the 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 slightly less than one-thousand ...
and
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 has only one-eighth the average density of Earth; h ...
, for example. A tell-tale sign of capture is a retrograde orbit, which can result from an object approaching the side of the planet which it is rotating towards. Capture has even been proposed as the origin of Earth's Moon. In the case of the latter, however, virtually identical
isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers ( mass num ...
ratios found in samples of the Earth and Moon cannot be explained easily by this theory.


Temporary capture

Evidence for the natural process of satellite capture has been found in direct observation of objects captured by Jupiter. Five such captures have been observed, the longest being for approximately twelve years. Based on computer modelling, the future capture of comet 111P/Helin-Roman-Crockett for 18 years is predicted to begin in 2068. However temporary captured orbits have highly irregular and unstable, the theorised processes behind stable capture may be exceptionally rare.


Controversial theories

Some controversial early theories, for example
Spaceship Moon Theory The Hollow Moon hypothesis and the closely related Spaceship Moon hypothesis propose that Earth's Moon is either wholly hollow or otherwise contains a substantial interior space. No scientific evidence exists to support the idea; seismic observat ...
and Shklovsky's "Hollow Phobos" hypothesis have suggested that moons were not formed naturally at all. These theories tend to fail
Occam's razor Occam's razor, Ockham's razor, or Ocham's razor ( la, novacula Occami), also known as the principle of parsimony or the law of parsimony ( la, lex parsimoniae), is the problem-solving principle that "entities should not be multiplied beyond neces ...
. While artificial satellites are now a common occurrence in the Solar System, the largest, the
International Space Station The International Space Station (ISS) is the largest Modular design, modular space station currently in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (United States), Roscosmos ( ...
is 108.5 metres at its widest, is tiny compared to the several kilometres of the smallest natural satellites.


Notable satellite systems

Known satellite systems of the Solar System consisting of multiple objects or around planetary mass objects, in order of perihelion:


Planetary Mass


Small Solar System body


Features and interactions

Natural satellite systems, particularly those involving multiple planetary mass objects can have complex interactions which can have effects on multiple bodies or across the wider system.


Ring systems

Ring systems are collections of
dust Dust is made of fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian process), volcanic eruptions, and pollution. Dust in ...
,
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, or other small objects. The most notable examples are those around Saturn, but the other three
gas giant A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are also called failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" ...
s (
Jupiter Jupiter is the fifth planet from the Sun and the 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 slightly less than one-thousand ...
,
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus ( Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars), grandfather of Zeus (Jupiter) and father of ...
and
Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 time ...
) also have ring systems. Studies of exoplanets indicate that they may be common around giant planets. The 90 million km (0.6 AU) circumplanetary ring system discovered around J1407b has been described as "
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 has only one-eighth the average density of Earth; h ...
on steroids" or “
Super Saturn A ring system is a disc or ring, orbiting an astronomical object, that is composed of solid material such as dust and moonlets, and is a common component of satellite systems around giant planets. A ring system around a planet is also known as ...
Luminosity studies suggest that an even larger disk exists in the
PDS 110 PDS 110 is a young 11th magnitude star located approximately away in the constellation Orion. In 2017, it was discovered that the star is orbited by an exoplanet or brown dwarf with a disk of dust around it. Description PDS 110 is a y ...
system. Other objects have also been found to possess rings.
Haumea , discoverer = , discovered = , earliest_precovery_date = March 22, 1955 , mpc_name = (136108) Haumea , pronounced = , adjectives = Haumean , note = yes , alt_names = , named_after = Haumea , mp_category = , orbit_ref = , epoc ...
was the first dwarf planet and Trans-Neptunian object found to possess a ring system.
Centaur A centaur ( ; grc, κένταυρος, kéntauros; ), or occasionally hippocentaur, is a creature from Greek mythology with the upper body of a human and the lower body and legs of a horse. Centaurs are thought of in many Greek myths as bein ...
10199 Chariklo 10199 Chariklo is the largest confirmed centaur (small body of the outer Solar System). It orbits the Sun between Saturn and Uranus, grazing the orbit of Uranus. On 26 March 2014, astronomers announced the discovery of two rings (nickname ...
, with a diameter of about , is the smallest object with rings ever discovered consisting of two narrow and dense bands, 6–7 km (4 mi) and 2–4 km (2 mi) wide, separated by a gap of . 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 has only one-eighth the average density of Earth; h ...
ian moon Rhea may have a tenuous
ring system A ring system is a disc or ring, orbiting an astronomical object, that is composed of solid material such as dust and moonlets, and is a common component of satellite systems around giant planets. A ring system around a planet is also known as ...
consisting of three narrow, relatively dense bands within a particulate disk, the first predicted around a
moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
. Most rings were thought to be unstable and to dissipate over the course of tens or hundreds of millions of years. Studies of Saturn's rings however indicate that they may date to the early days of the Solar System. Current theories suggest that some ring systems may form in repeating cycles, accreting into natural satellites that break up as soon as they reach the Roche limit. This theory has been used to explain the longevity of Saturn's rings as well the moons of Mars.


Gravitational interactions


Orbital configurations

Cassini's laws describe the motion of satellites within a system with their precessions defined by the
Laplace plane The Laplace plane or Laplacian plane of a planetary satellite, named after its discoverer Pierre-Simon Laplace (1749–1827), is a mean or reference plane about whose axis the instantaneous orbital plane of that satellite precesses. Laplace's ...
. Most satellite systems are found orbiting the ecliptic plane of the primary. An exception is Earth's moon, which orbits in to the planet's
equatorial plane The celestial equator is the great circle of the imaginary celestial sphere on the same plane as the equator of Earth. This plane of reference bases the equatorial coordinate system. In other words, the celestial equator is an abstract projecti ...
. When orbiting bodies exert a regular, periodic gravitational influence on each other is known as orbital resonance. Orbital resonances are present in several satellite systems: * 2:4 TethysMimas (Saturn's moons) * 1:2 Dione
Enceladus Enceladus is the sixth-largest moon of Saturn (19th largest in the Solar System). It is about in diameter, about a tenth of that of Saturn's largest moon, Titan. Enceladus is mostly covered by fresh, clean ice, making it one of the most refle ...
(Saturn's moons) * 3:4 Hyperion
Titan Titan most often refers to: * Titan (moon), the largest moon of Saturn * Titans, a race of deities in Greek mythology Titan or Titans may also refer to: Arts and entertainment Fictional entities Fictional locations * Titan in fiction, fictiona ...
(Saturn's moons) * 1:2:4 Ganymede
Europa Europa may refer to: Places * Europe * Europa (Roman province), a province within the Diocese of Thrace * Europa (Seville Metro), Seville, Spain; a station on the Seville Metro * Europa City, Paris, France; a planned development * Europa Clif ...
Io (Jupiter's moons) * 1:3:4:5:6 near resonances -
Styx In Greek mythology, Styx (; grc, Στύξ ) is a river that forms the boundary between Earth (Gaia) and the Underworld. The rivers Acheron, Cocytus, Lethe, Phlegethon, and Styx all converge at the centre of the underworld on a great marsh, ...
,
Nix Nix or NIX may refer to: Places * Nix, Alabama, an unincorporated community, United States * Nix, Texas, a ghost town in southwestern Lampasas County, Texas, United States * Nix (moon), a moon of Pluto People * Nix (surname), listing people wit ...
, Kerberos, and Hydra (Pluto's moons) (Styx approximately 5.4% from resonance, Nix approximately 2.7%, Kerberos approximately 0.6%, and Hydra approximately 0.3%). Other possible orbital interactions include libration and co-orbital configuration. The Saturnian moons Janus and Epimetheus share their orbits, the difference in semi-major axes being less than either's mean diameter. Libration is a perceived oscillating motion of orbiting bodies relative to each other. The Earth-moon satellite system is known to produce this effect. Several systems are known to orbit a common centre of mass and are known as binary companions. The most notable system is the Plutonian system, which is also dwarf planet binary. Several minor planets also share this configuration, including "true binaries" with near equal mass, such as
90 Antiope Antiope (minor planet designation: 90 Antiope) is a double asteroid in the outer asteroid belt. It was discovered on October 1, 1866, by Robert Luther. In 2000, it was found to consist of two almost-equally-sized bodies orbiting each other. At ...
and
(66063) 1998 RO1 is a stony near-Earth object of the Aten group on a highly-eccentric orbit. The synchronous binary system measures approximately in diameter. It was discovered by astronomers of the Lincoln Near-Earth Asteroid Research at the Lincoln Laborat ...
. Some orbital interactions and binary configurations have been found to cause smaller moons to take non-spherical forms and "tumble" chaotically rather than rotate, as in the case of Nix, Hydra (moons of Pluto) and Hyperion (moon of Saturn).


Tidal interaction

Tidal energy including tidal acceleration can have effects on both the primary and satellites. The Moon's tidal forces deform the Earth and hydrosphere, similarly heat generated from tidal friction on the moons of other planets is found to be responsible for their geologically active features. Another extreme example of physical deformity is the massive
equatorial ridge Equatorial ridges are a feature of at least three of Saturn's moons: the large moon Iapetus and the tiny moons Atlas and Pan. They are ridges that closely follow the moons' equators. They appear to be unique to the Saturnian system, but it is ...
of the near-Earth asteroid
66391 Moshup 66391 Moshup , provisional designation , is a binary asteroid, classified as a near-Earth object and potentially hazardous asteroid of the Aten group, approximately 1.3 kilometers in diameter. It was discovered on 20 May 1999, by Lincoln Near- ...
created by the tidal forces of its moon, such deformities may be common among near-Earth asteroids. Tidal interactions also cause stable orbits to change over time. For instance, Triton's orbit around Neptune is decaying and 3.6 billion years from now, it is predicted that this will cause Triton to pass within Neptune's Roche limit resulting in either a collision with Neptune's atmosphere or the breakup of Triton, forming a large ring similar to that found around Saturn. A similar process is drawing Phobos closer to Mars, and it is predicted that in 50 million years it will either collide with the planet or break up into a
planetary ring A ring system is a disc or ring, orbiting an astronomical object, that is composed of solid material such as dust and moonlets, and is a common component of satellite systems around giant planets. A ring system around a planet is also known ...
.
Tidal acceleration Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite (e.g. the Moon) and the primary planet that it orbits (e.g. Earth). The acceleration causes a gradual recession of a satellite in a prograde orbit away f ...
, on the other hand, gradually moves the Moon away from Earth, such that it may eventually be released from its gravitational bounding and exit the system.


Perturbation and instability

While tidal forces from the primary are common on satellites, most satellite systems remain stable.
Perturbation Perturbation or perturb may refer to: * Perturbation theory, mathematical methods that give approximate solutions to problems that cannot be solved exactly * Perturbation (geology), changes in the nature of alluvial deposits over time * Perturbat ...
between satellites can occur, particularly in the early formation, as the gravity of satellites affect each other, and can result in ejection from the system or collisions between satellites or with the primary. Simulations show that such interactions cause the orbits of the inner moons of the Uranus system to be chaotic and possibly unstable. Some of Io's active can be explained by perturbation from Europa's gravity as their orbits resonate. Perturbation has been suggested as a reason that Neptune does not follow the 10,000:1 ratio of mass between the parent planet and collective moons as seen in all other known giant planets. One theory of the Earth-Moon system suggest that a second companion which formed at the same time as the Moon, was perturbed by the Moon early in the system's history, causing it to impact with the Moon.


Atmospheric and magnetic interaction

Some satellite systems have been known to have gas interactions between objects. Notable examples include the Jupiter, Saturn and Pluto systems. The
Io plasma torus The magnetosphere of Jupiter is the cavity created in the solar wind by the planet's magnetic field. Extending up to seven million kilometers in the Sun's direction and almost to the orbit of Saturn in the opposite direction, Jupiter's magnetosp ...
is a transfer of oxygen and sulfur from the tenuous atmosphere of Jupiter's volcanic moon, Io and other objects including Jupiter and Europa. A torus of oxygen and hydrogen produced by Saturn's moon,
Enceladus Enceladus is the sixth-largest moon of Saturn (19th largest in the Solar System). It is about in diameter, about a tenth of that of Saturn's largest moon, Titan. Enceladus is mostly covered by fresh, clean ice, making it one of the most refle ...
forms part of the E ring around Saturn. Nitrogen gas transfer between Pluto and Charon has also been modelled and is expected to be observable by the
New Horizons ''New Horizons'' is an interplanetary space probe that was launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research Institute (SwRI), with a ...
space probe. Similar tori produced by Saturn's moon
Titan Titan most often refers to: * Titan (moon), the largest moon of Saturn * Titans, a race of deities in Greek mythology Titan or Titans may also refer to: Arts and entertainment Fictional entities Fictional locations * Titan in fiction, fictiona ...
(nitrogen) and Neptune's moon
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'' ...
(hydrogen) is predicted. Complex magnetic interactions have been observed in satellite systems. Most notably, the interaction of Jupiter's strong magnetic field with those of Ganymede and Io. Observations suggest that such interactions can cause the stripping of atmospheres from moons and the generation of spectacular auroras.


History

The notion of satellite systems pre-dates history. The Moon was known by the earliest humans. The earliest models of astronomy were based around celestial bodies (or a "celestial sphere") orbiting the Earth. This idea was known as
geocentrism In astronomy, the geocentric model (also known as geocentrism, often exemplified specifically by the Ptolemaic system) is a superseded description of the Universe with Earth at the center. Under most geocentric models, the Sun, Moon, stars, a ...
(where the Earth is the centre of the universe). However the geocentric model did not generally accommodate the possibility of celestial objects orbiting other observed planets, such as Venus or Mars.
Seleucus of Seleucia Seleucus of Seleucia ( el, Σέλευκος ''Seleukos''; born c. 190 BC; fl. c. 150 BC) was a Hellenistic astronomer and philosopher. Coming from Seleucia on the Tigris, Mesopotamia, the capital of the Seleucid Empire, or, alternatively, Seleuk ...
(b. 190 BCE) made observations which may have included the phenomenon of
tide Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables ...
s, which he supposedly theorized to be caused by the attraction to the
Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
and by the revolution of the
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...
around an
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...
-
Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
'center of mass'. As
heliocentrism Heliocentrism (also known as the Heliocentric model) is the astronomical model in which the Earth and planets revolve around the Sun at the center of the universe. Historically, heliocentrism was opposed to geocentrism, which placed the Earth ...
(the doctrine that the Sun is the centre of the universe) began to gain in popularity in the 16th century, the focus shifted to planets and the idea of systems of planetary satellites fell out of general favour. Nevertheless, in some of these models, the Sun and Moon would have been satellites of the Earth. Nicholas Copernicus published a model in which the Moon orbited around the Earth in the '' Dē revolutionibus orbium coelestium'' (''On the Revolutions of the Celestial Spheres''), in the year of his death, 1543. It was not until the discovery of the Galilean moons in either 1609 or 1610 by
Galileo Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...
, that the first definitive proof was found for celestial bodies orbiting planets. The first suggestion of a ring system was in 1655, when
Christiaan Huygens Christiaan Huygens, Lord of Zeelhem, ( , , ; also spelled Huyghens; la, Hugenius; 14 April 1629 – 8 July 1695) was a Dutch mathematician, physicist, engineer, astronomer, and inventor, who is regarded as one of the greatest scientists o ...
thought that Saturn was surrounded by rings. The first probe to explore a satellite system other than Earth was Mariner 7 in 1969, which observed Phobos. The twin probes ''
Voyager 1 ''Voyager 1'' is a space probe launched by NASA on September 5, 1977, as part of the Voyager program to study the outer Solar System and interstellar space beyond the Sun's heliosphere. Launched 16 days after its twin '' Voyager 2'', ''V ...
'' and ''
Voyager 2 ''Voyager 2'' is a space probe launched by NASA on August 20, 1977, to study the outer planets and interstellar space beyond the Sun's heliosphere. As a part of the Voyager program, it was launched 16 days before its twin, '' Voyager 1'', ...
'' were the first to explore the Jovian system in 1979.


Zones and habitability

Based on tidal heating models, scientists have defined zones in satellite systems similarly to those of planetary systems. One such zone is the circumplanetary habitable zone (or "habitable edge"). According to this theory, moons closer to their planet than the habitable edge cannot support liquid water at their surface. When effects of eclipses as well as constraints from a satellite's orbital stability are included into this concept, one finds that — depending on a moon's orbital eccentricity — there is a minimum mass of roughly 0.2 solar masses for stars to host habitable moons within the stellar HZ. The magnetic environment of exomoons, which is critically triggered by the intrinsic magnetic field of the host planet, has been identified as another effect on exomoon habitability. Most notably, it was found that moons at distances between about 5 and 20 planetary radii from a giant planet can be habitable from an illumination and tidal heating point of view, but still the planetary magnetosphere would critically influence their habitability.


See also

*
Minor-planet moon A minor-planet moon is an astronomical object that orbits a minor planet as its natural satellite. , there are 457 minor planets known or suspected to have moons. Discoveries of minor-planet moons (and binary objects, in general) are importan ...
* Subsatellite *
Double planet In astronomy, a double planet (also binary 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. Although up to a third of the star syst ...
* Quasi-satellite *
List of extraterrestrial orbiters List of extraterrestrial orbiters is a listing of spacecraft that achieved an extraterrestrial orbit. Sun First artificial object on heliocentric orbit was Luna 1 (1959). Mercury Venus Moon Mars Jupiter Saturn Minor planets and c ...


Notes


References

{{Portal bar, Astronomy, Stars, Spaceflight, Outer space, Solar System Planetary science