Satellite system (astronomy)
A satellite system is a set of gravitationally bound objects in orbit around a
Many Solar System objects are known to possess satellite systems, though their origin is still unclear. Notable examples include the Jovian system, with
Little is known of satellite systems beyond the Solar System, although it is inferred that natural satellites are common.
Natural formation and evolution
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.
System stability
The
Satellites are stable at the L4 and L5 Lagrangian points. These lie at the third corners of the two
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 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,[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]
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.[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 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
Gravitational capture theories
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]
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
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
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
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 | 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 |
Small Solar System body
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
|
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
Other objects have also been found to possess rings.
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.
Gravitational interactions
Orbital configurations
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 Mimas(Saturn's moons)
- 1:2 Dione–Enceladus (Saturn's moons)
- 3:4 Hyperion–Titan (Saturn's moons)
- 1:2:4 Ganymede–Europa–Io (Jupiter's moons)
- 1:3:4:5:6 near resonances - Styx, Nix, 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%).[36]
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 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 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
Perturbation and instability
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]
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
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
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
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
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.[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.
See also
Notes
- ^ More precisely, ≈ 24.9599357944
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