A satellite system is a set of gravitationally bound objects in orbit around a planetary mass object (incl. sub-brown dwarfs and rogue planets) 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 (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 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 (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").
Many Solar System objects are known to possess satellite systems, though their origin is still unclear. Notable examples include the Jovian system, with 95 known moons [1] (including the large Galilean moons) and the largest overall, the Saturnian System, with 146 known moons (including Titan and the most visible rings in the Solar System alongside). 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, 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. [2] It is also theorised that Rogue planets ejected from their planetary system could retain a system of satellites. [3]
Satellite systems, like planetary systems, are the product of gravitational attraction, but are also sustained through fictitious forces. 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.
The Hill sphere 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 points.
Satellites are stable at the L4 and L5 Lagrangian points. These lie at the third corners of the two equilateral triangles in the plane of orbit whose common base is the line between the centers of the two masses, such that the point lies behind (L5) or ahead (L4) of the smaller mass with regard to its orbit around the larger mass. The triangular points (L4 and L5) are stable equilibria, provided that the ratio of M1/M2 is nearly 24.96. [note 1] [4] 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 is used to describe known exceptions to the rule, it is believed that irregular moons have been inserted into orbit through gravitational capture. [5]
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, [6] Saturn, and Jupiter). This early cloud of gas is a type of circumplanetary disk [7] [8] 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 [9] (a notable exception is Neptune). Accretion is also proposed by some as a theory for the origin of the Earth-Moon system, [10] however the angular momentum of system and the Moon's smaller iron core can not easily be explained by this. [10]
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. [11] 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 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 and this origin may be verified comparing their orbital elements and composition. Computer simulations have been used to demonstrate that giant impacts could have been the origin of the Moon. 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 objects and asteroids. This is also a prevailing theory for the origin of the moons of Mars. [12] Both sets of findings support an origin of Phobos from material ejected by an impact on Mars that reaccreted in Martian orbit. [13] Collision is also used to explain peculiarities in the Uranian system. [14] [15] 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. [16]
Some theories suggest that gravitational capture is the origin of Neptune's major moon Triton, [17] the moons of Mars, [18] and Saturn's moon Phoebe. [19] [20] 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 and Saturn, for example. [21] 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. [5] Capture has even been proposed as the origin of Earth's Moon. In the case of the latter, however, virtually identical isotope ratios found in samples of the Earth and Moon cannot be explained easily by this theory. [22]
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. [23] [24] However temporary captured orbits have highly irregular and unstable, the theorised processes behind stable capture may be exceptionally rare.
Some controversial early theories, for example Spaceship Moon Theory and Shklovsky's "Hollow Phobos" hypothesis have suggested that moons were not formed naturally at all. These theories tend to fail Occam's razor. While artificial satellites are now a common occurrence in the Solar System, the largest, the International Space Station is 108.5 metres at its widest, is tiny compared to the several kilometres of the smallest natural satellites.
Known satellite systems of the Solar System consisting of multiple objects or around planetary mass objects, in order of perihelion:
Object | Class | Perihelion (AU) | Natural satellites | Planetary-mass satellites | Artificial satellites | Ring/s groups | Note |
---|---|---|---|---|---|---|---|
Venus | Planet | 0.7184 | 1 | See Akatsuki (spacecraft) | |||
Earth | Planet | 0.9832687 | 1 | The Moon | 2,465* | See List of Earth observation satellites, List of satellites in geosynchronous orbit, List of space stations | |
The Moon | Natural satellite | 1.0102 | 10* | See Lunar Reconnaissance Orbiter, Lunar Orbiter program | |||
Mars | Planet | 1.3814 | 2 | 11* | *6 are derelict (see List of Mars orbiters) | ||
1 Ceres | Dwarf planet | 2.5577 | 1* | *Dawn | |||
Jupiter | Planet | 4.95029 | 95 [1] | Ganymede, Callisto, Io, Europa | 1 | 4 | With ring system and four large Galilean moons. Juno since 2017. See also Moons of Jupiter and Rings of Jupiter |
Saturn | Planet | 9.024 | 146 | Titan, Rhea, lapetus, Dione, Tethys, Enceladus, Mimas | 7 | ||
Uranus | Planet | 20.11 | 28 | Titania, Oberon | 13 | With ring system. See also Moons of Uranus | |
134340 Pluto-Charon | Dwarf planet (binary) | 29.658 | 5 | Charon (binary) | See also Moons of Pluto | ||
Neptune | Planet | 29.81 | 16 | Triton | 5 | With ring system. See also Moons of Neptune | |
90482 Orcus | Dwarf planet candidate | 30.866 | 1 | Vanth? | |||
225088 Gonggong | Dwarf planet | 33.781 | 1 | ||||
136108 Haumea | Dwarf planet | 34.952 | 2 | 1 | See also Moons of Haumea, ring system discovered 2017 | ||
(532037) 2013 FY27 | Dwarf planet candidate | 35.199 | 1 | ||||
120347 Salacia | Dwarf planet candidate | 37.697 | 1 | ||||
136199 Eris | Dwarf planet | 37.911 | 1 | Dysnomia | |||
136472 Makemake | Dwarf planet | 38.590 | 1 | ||||
174567 Varda | Dwarf planet candidate | 39.510 | 1 | Ilmarë? | |||
50000 Quaoar | Dwarf planet | 41.868 | 1 | 2 | Ring system discovered 2023 |
Object | Class | Perihelion (AU) | Natural satellites | Artificial satellites | Ring/s groups | Note |
---|---|---|---|---|---|---|
66391 Moshup | Mercury-crosser asteroid | 0.20009 | 1 | Binary system | ||
(66063) 1998 RO1 | Aten asteroid | 0.27733 | 1 | Binary system | ||
(136617) 1994 CC | near-Earth asteroid | 0.95490 | 2 | Trinary system | ||
(153591) 2001 SN263 | near-Earth asteroid | 1.03628119 | 2 | Trinary system | ||
(285263) 1998 QE2 | near-Earth asteroid | 1.0376 | 1 | Binary system | ||
67P/Churyumov–Gerasimenko | Comet | 1.2432 | 1* | *Rosetta, since August 2014 | ||
2577 Litva | Mars-crosser | 1.6423 | 2 | Binary system | ||
3749 Balam | Main-belt Asteroid | 1.9916 | 2 | Binary system | ||
41 Daphne | Main-belt Asteroid | 2.014 | 1 | Binary system | ||
216 Kleopatra | Main-belt Asteroid | 2.089 | 2 | |||
93 Minerva | Main-belt Asteroid | 2.3711 | 2 | |||
45 Eugenia | Main-belt Asteroid | 2.497 | 2 | |||
130 Elektra | Main-belt Asteroid | 2.47815 | 3 | |||
22 Kalliope | Main-belt Asteroid | 2.6139 | 1 | Binary: Linus | ||
90 Antiope | Main-belt Asteroid | 2.6606 | 1 | Binary: S/2000 (90) 1 | ||
87 Sylvia | Main-belt Asteroid | 3.213 | 2 | |||
107 Camilla | Cybele asteroid | 3.25843 | 2 | Trinary system | ||
617 Patroclus | Jupiter Trojan | 4.4947726 | 1 | Binary: Menoetius | ||
2060 Chiron | Centaur | 8.4181 | 2 | |||
10199 Chariklo | Centaur | 13.066 | 2 | First minor planet known to possess a ring system. see Rings of Chariklo | ||
47171 Lempo | Trans-Neptunian object | 30.555 | 2 | Trinary/Binary with companion | ||
(48639) 1995 TL8 | Kuiper belt object | 40.085 | 1 | Binary: S/2002 (48639) 1 | ||
1998 WW31 | Kuiper belt object | 40.847 | 1 | Binary: S/2000 (1998 WW31) 1 |
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 are collections of dust, moonlets, or other small objects. The most notable examples are those around Saturn, but the other three gas giants (Jupiter, Uranus and Neptune) 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 on steroids" [25] or “Super Saturn” [26] [2] Luminosity studies suggest that an even larger disk exists in the PDS 110 system. [27]
Other objects have also been found to possess rings. Haumea was the first dwarf planet and Trans-Neptunian object found to possess a ring system. [28] Centaur 10199 Chariklo, with a diameter of about 250 kilometres (160 mi), is the smallest object with rings ever discovered [29] consisting of two narrow and dense bands, 6–7 km (4 mi) and 2–4 km (2 mi) wide, separated by a gap of 9 kilometres (6 mi). [29] [30] The Saturnian moon Rhea may have a tenuous ring system consisting of three narrow, relatively dense bands within a particulate disk, the first predicted around a moon. [31]
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. [32] 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. [33] This theory has been used to explain the longevity of Saturn's rings as well the moons of Mars.
Cassini's laws describe the motion of satellites within a system [34] with their precessions defined by the Laplace plane. [35] 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. [34]
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:
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 and (66063) 1998 RO1. 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). [37]
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 of the near-Earth asteroid 66391 Moshup created by the tidal forces of its moon, such deformities may be common among near-Earth asteroids. [38]
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 [39] resulting in either a collision with Neptune's atmosphere or the breakup of Triton, forming a large ring similar to that found around Saturn. [39] 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. [40] Tidal acceleration, 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. [41]
While tidal forces from the primary are common on satellites, most satellite systems remain stable. Perturbation 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. [42] 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. [43] 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. [44]
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 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 forms part of the E ring around Saturn. Nitrogen gas transfer between Pluto and Charon has also been modelled [45] and is expected to be observable by the New Horizons space probe. Similar tori produced by Saturn's moon Titan (nitrogen) and Neptune's moon Triton (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.
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 (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 (b. 190 BCE) made observations which may have included the phenomenon of tides, [46] which he supposedly theorized to be caused by the attraction to the Moon and by the revolution of the Earth around an Earth-Moon 'center of mass'.
As heliocentrism (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, 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 thought that Saturn was surrounded by rings. [47]
The first probe to explore a satellite system other than Earth was Mariner 7 in 1969, which observed Phobos. The twin probes Voyager 1 and Voyager 2 were the first to explore the Jovian system in 1979.
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. [48]
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. [49] 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.
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.
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 Neptune and Pluto. 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 planetary 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.
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.
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 like Saturn. A ring system around a planet is also known as a planetary ring system.
The Solar System is the gravitationally bound system of the Sun and the objects that orbit it. The largest of these objects are the eight planets, which in order from the Sun are four terrestrial planets ; two gas giants ; and two ice giants. The Solar System developed 4.6 billion years ago when a dense region of a molecular cloud collapsed, forming the Sun and a protoplanetary disc.
Triton is the largest natural satellite of the planet Neptune, and was the first Neptunian moon to be discovered, on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, an orbit in the direction opposite to its planet's rotation. Because of its retrograde orbit and composition similar to Pluto, Triton is thought to have been a dwarf planet, captured from the Kuiper belt.
A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body. Natural satellites are colloquially referred to as moons, a derivation from the Moon of Earth.
Tidal locking between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where a tidally locked body possesses synchronous rotation, the object takes just as long to rotate around its own axis as it does to revolve around its partner. For example, the same side of the Moon always faces Earth, although there is some variability because the Moon's orbit is not perfectly circular. Usually, only the satellite is tidally locked to the larger body. However, if both the difference in mass between the two bodies and the distance between them are relatively small, each may be tidally locked to the other; this is the case for Pluto and Charon, as well as for Eris and Dysnomia. Alternative names for the tidal locking process are gravitational locking, captured rotation, and spin–orbit locking.
The planet Neptune has 16 known moons, which are named for minor water deities and a water creature in Greek mythology. By far the largest of them is Triton, discovered by William Lassell on October 10, 1846, 17 days after the discovery of Neptune itself. Over a century passed before the discovery of the second natural satellite, Nereid, in 1949, and another 40 years passed before Proteus, Neptune's second largest moon, was discovered in 1989. Neptune's outermost moon S/2021 N 1, which has an orbital period of about 27 Julian years, orbits farther from its planet than any other moon in the Solar System.
Planetary migration occurs when a planet or other body in orbit around a star interacts with a disk of gas or planetesimals, resulting in the alteration of its orbital parameters, especially its semi-major axis. Planetary migration is the most likely explanation for hot Jupiters. The generally accepted theory of planet formation from a protoplanetary disk predicts that such planets cannot form so close to their stars, as there is insufficient mass at such small radii and the temperature is too high to allow the formation of rocky or icy planetesimals.
An exomoon or extrasolar moon is a natural satellite that orbits an exoplanet or other non-stellar extrasolar body.
There is evidence that 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.
Extraterrestrial liquid water is water in its liquid state that naturally occurs outside Earth. It is a subject of wide interest because it is recognized as one of the key prerequisites for life as we know it and thus surmised as essential for extraterrestrial life.
The habitability of natural satellites is the potential of moons to provide habitats for life, though it is not an indicator that they harbor it. Natural satellites are expected to outnumber planets by a large margin and the study of their habitability is therefore important to astrobiology and the search for extraterrestrial life. There are, nevertheless, significant environmental variables specific to moons.
The Nicemodel is a scenario for the dynamical evolution of the Solar System. It is named for the location of the Côte d'Azur Observatory—where it was initially developed in 2005—in Nice, France. It proposes the migration of the giant planets from an initial compact configuration into their present positions, long after the dissipation of the initial protoplanetary disk. In this way, it differs from earlier models of the Solar System's formation. This planetary migration is used in dynamical simulations of the Solar System to explain historical events including the Late Heavy Bombardment of the inner Solar System, the formation of the Oort cloud, and the existence of populations of small Solar System bodies such as the Kuiper belt, the Neptune and Jupiter trojans, and the numerous resonant trans-Neptunian objects dominated by Neptune.
In astronomy, a regular moon or a regular satellite is a natural satellite following a relatively close, stable, and circular orbit which is generally aligned to its primary's equator. They form within discs of debris and gas that once surrounded their primary, usually the aftermath of a large collision or leftover material accumulated from the protoplanetary disc. Young regular moons then begin to accumulate material within the circumplanetary disc in a process similar to planetary accretion, as opposed to irregular moons, which formed independently before being captured into orbit around the primary.
Retrograde motion in astronomy is, in general, orbital or rotational motion of an object in the direction opposite the rotation of its primary, that is, the central object. It may also describe other motions such as precession or nutation of an object's rotational axis. Prograde or direct motion is more normal motion in the same direction as the primary rotates. However, "retrograde" and "prograde" can also refer to an object other than the primary if so described. The direction of rotation is determined by an inertial frame of reference, such as distant fixed stars.
The five-planet Nice model is a numerical model of the early Solar System that is a revised variation of the Nice model. It begins with five giant planets, the four that exist today plus an additional ice giant between Saturn and Uranus in a chain of mean-motion resonances.
The jumping-Jupiter scenario specifies an evolution of giant-planet migration described by the Nice model, in which an ice giant is scattered inward by Saturn and outward by Jupiter, causing their semi-major axes to jump, and thereby quickly separating their orbits. The jumping-Jupiter scenario was proposed by Ramon Brasser, Alessandro Morbidelli, Rodney Gomes, Kleomenis Tsiganis, and Harold Levison after their studies revealed that the smooth divergent migration of Jupiter and Saturn resulted in an inner Solar System significantly different from the current Solar System. During this migration secular resonances swept through the inner Solar System exciting the orbits of the terrestrial planets and the asteroids, leaving the planets' orbits too eccentric, and the asteroid belt with too many high-inclination objects. The jumps in the semi-major axes of Jupiter and Saturn described in the jumping-Jupiter scenario can allow these resonances to quickly cross the inner Solar System without altering orbits excessively, although the terrestrial planets remain sensitive to its passage.
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