The first known natural satellite was theMoon, but it was considered a "planet" untilCopernicus' introduction ofDe revolutionibus orbium coelestium in 1543. Until the discovery of theGalilean satellites in 1610 there was no opportunity for referring to such objects as a class.Galileo chose to refer to his discoveries asPlanetæ ("planets"), but later discoverers chose other terms to distinguish them from the objects they orbited.[3]
The first to use the termsatellite to describe orbiting bodies was the German astronomerJohannes Kepler in his pamphletNarratio de Observatis a se quatuor Iouis satellitibus erronibus ("Narration About Four Satellites of Jupiter Observed") in 1610. He derived the term from theLatin wordsatelles, meaning "guard", "attendant", or "companion", because thesatellites accompanied their primary planet in their journey through the heavens.[4]
The termsatellite 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 objectSputnik created a need for new terminology.[4] The termsman-made satellite andartificial moon were very quickly abandoned in favor of the simplersatellite. As a consequence, the term has become linked with artificial objects flown in space.
Because of this shift in meaning, the termmoon, which had continued to be used in a generic sense in works of popular science and fiction, has regained respectability and is now used interchangeably withnatural satellite, even in scientific articles. When it is necessary to avoid both the ambiguity of confusion with Earth's natural satellite the Moon and the natural satellites of the other planets on the one hand, and artificial satellites on the other, the termnatural 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 (aproper noun), but not when referring to other natural satellites (common nouns).
Many authors define "satellite" or "natural satellite" as orbiting some planet or minor planet, synonymous with "moon" – by such a definition all natural satellites are moons, but Earth and other planets are not satellites.[5][6][7] 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".[8][9][10]
There is no established lower limit on what is considered a "moon". Every natural celestial body with an identified orbit around a planet of theSolar System, some as small as a kilometer across, has been considered a moon,[11] though objects a tenth that size within Saturn's rings, which have not been directly observed, have been calledmoonlets. Smallasteroid moons (natural satellites of asteroids), such asDactyl, have also been called moonlets.[12]
The upper limit is also vague. Two orbiting bodies are sometimes described as adouble planet rather than a primary and satellite; the Earth-Moon system is used as an example.[13]Asteroids such as90 Antiope are considered double asteroids,[14] 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,[15] with one argument being that thebarycentre lies above the surface of the larger body.[16] In contrast, the barycenter of all planetary moons of the Solar System are located within the radius of their host planet.[17]
The natural satellites orbiting relatively close to the planet onprograde, uninclined circular orbits (regular satellites) are generally thought to have been formed out of the samecollapsing region of theprotoplanetary disk that created its primary.[18][19] In contrast,irregular satellites (generally orbiting on distant,inclined,eccentric and/orretrograde orbits) are thought to be capturedasteroids possibly further fragmented by collisions. Most of the major natural satellites of the Solar System have regular orbits, while most of the small natural satellites have irregular orbits.[20] TheMoon and theMoons of Pluto are exceptions among large bodies in that they are thought to have originated from the collision of two large protoplanetary objects early in the Solar System's history (see thegiant impact hypothesis).[21][22] The material that would have been placed in orbit around the central body is predicted to have reaccreted to form one or more orbiting natural satellites. As opposed to planetary-sized bodies,asteroid moons are thought to commonly form by this process.[23]Triton is another exception; although large and in a close, circular orbit, its motion is retrograde and it is thought to be a captureddwarf planet.[24]
Two natural satellites are known to have small companions at both theirL4 andL5Lagrangian points, sixty degrees ahead and behind the body in its orbit. These companions are calledtrojan moons, as their orbits are analogous to thetrojan asteroids ofJupiter. The trojan moons areTelesto andCalypso, which are the leading and following companions, respectively, of the Saturnian moonTethys; andHelene andPolydeuces, the leading and following companions of the Saturnian moonDione.[25]
The capture of an asteroid from a heliocentric orbit is not always permanent. According to simulations,temporary satellites should be a common phenomenon.[26][27] As of 2023[update], the observed minor bodies that have displayed transient co-orbital motion with Earth are:1991 VG,2006 RH120,2020 CD3, and2022 NX1.[28]
2006 RH120 was a temporary satellite of Earth for nine months in 2006 and 2007.[29][30]
Mostregular moons (natural satellites following relatively close and prograde orbits with small orbital inclination and eccentricity) in the Solar System are tidally locked to their respective primaries, meaning that the same side of the natural satellite always faces its planet. This phenomenon comes about through a loss of energy due to tidal forces raised by the planet, slowing the rotation of the satellite until it is negligible.[31] Exceptions are known; one such exception isSaturn's natural satelliteHyperion, which rotates chaotically because of the gravitational influence ofTitan.[32] Pluto's four, circumbinary small moons also rotate chaotically due to Charon's influence.[33]
In contrast, the outer natural satellites of the giant planets (irregular satellites) are too far away to have become locked. For example, Jupiter'sHimalia, Saturn'sPhoebe, and Neptune'sNereid have rotation periods in the range of ten hours, whereas their orbital periods are hundreds of days.[34]
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. Potential exceptions include large moons on wide orbits, including Titan, Iapetus, Callisto, and the Moon. However, other sources of dynamical instability may remove such submoons, such asmascons on the Moon.[35]
Calculations performed after the 2008 detection[36] of a possiblering system around Saturn's moonRhea indicate that satellites orbiting Rhea could have stable orbits. Furthermore, the suspected rings are thought to be narrow,[37] a phenomenon normally associated withshepherd moons. However, targeted images taken by theCassini spacecraft failed to detect rings around Rhea.[38]
It has also been proposed that Saturn's moonIapetus had a satellite in the past; this is one of several hypotheses that have been put forward to account for itsequatorial ridge.[39]
Light-curve analysis suggests that Saturn's irregular satelliteKiviuq is extremely prolate, and is likely acontact binary or even a binary moon.[40]
The relative masses of the natural satellites of theSolar System.Mimas,Enceladus, andMiranda are too small to be visible at this scale. All the irregularly shaped natural satellites, even added together, would also be too small to be visible.
Neptune's moonProteus is the largest irregularly shaped natural satellite, and is about as large as an icy moon can become before becomingrelaxed into aspheroidal shape.[41] The shapes of moons in synchronous orbit are expected toasymptotically change shape into roundedellipsoids underhydrostatic equilibrium, although this may not happen in the age of the Solar System. For example, the larger Saturnian moons are in equilibrium, whileIapetus, Mimas, and Enceladus are apparently not.[42][43]The "round/rounded satellites" are sometimes categorized asplanetary-mass moons.[citation needed] The shapes of Eris' moonDysnomia andOrcus' moonVanth are presently unknown, although Dysnomia's density is high enough that it is probably a solid ellipsoid as well.[citation needed]
The larger natural satellites, being tidally locked, tend towardovoid (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.[44]: 184–187 Saturn's moonMimas, for example, has a major axis 9% greater than its polar axis and 5% greater than its other equatorial axis.[45]Methone, another of Saturn's moons, is only around 3 km in diameter andvisibly egg-shaped.[46] The effect is smaller on the largest natural satellites, where their gravity is greater relative to the effects of tidal distortion, especially those that orbit less massive planets or, as in the case of the Moon, at greater distances.[44]: 167–170
Of the nineteen known natural satellites in the Solar System that are large enough to be gravitationally rounded, several remain geologically active today.Io is the most volcanically active body in the Solar System,[47] whileEuropa,[48]Enceladus,[49] andTriton display evidence of ongoingtectonic activity andcryovolcanism. In the first three cases, the geological activity is powered by thetidal heating resulting from havingeccentric orbits close to their giant-planet primaries.[47][48][49] (This mechanism would have also operated on Triton in the past before its orbit wascircularized.[50]) Many other natural satellites, such as Earth's Moon,Ganymede,Titan, Tethys, and Miranda show evidence of past geological activity, resulting from energy sources such as thedecay of theirprimordialradioisotopes, greater past orbital eccentricities (due in some cases to pastorbital resonances), or thedifferentiation or freezing of their interiors.[51] Enceladus and Triton both have active features resemblinggeysers, although in the case of Triton solar heating appears to provide the energy. Titan and Triton have significant atmospheres; Titan also hashydrocarbon lakes.[51] All four of the Galilean moons have atmospheres, though they are extremely thin.[52][53][54] Four of the largest natural satellites, Europa,[48] Ganymede,Callisto, and Titan, are thought to have subsurface oceans of liquid water,[51] while smaller Enceladus also supports a global subsurface ocean of liquid water.[49]
Largest moons to scale with their parent planets and dwarf planet
Of the inner planets,Mercury andVenus have no natural satellites; Earth has one large natural satellite, known as the Moon; andMars has two tiny natural satellites,Phobos andDeimos. Thegiant planets have extensive systems of natural satellites, including half a dozen comparable in size to Earth's Moon: the fourGalilean moons, Saturn's Titan, andNeptune's Triton.[55] Saturn has an additional six mid-sized natural satellites massive enough to have achievedhydrostatic equilibrium, andUranus has five. It has been suggested that somesatellites may potentially harbour life.[56]
Among the objects generally agreed by astronomers to be dwarf planets,Ceres andSedna have no known natural satellites. Pluto has the relatively large natural satellite Charon and four smaller natural satellites;Styx,Nix,Kerberos, andHydra.[57]Haumea has two natural satellites;Orcus,Quaoar,Makemake,Gonggong, andEris have one each.[58] The Pluto–Charon system is unusual in that thecenter of mass lies in open space between the two, a characteristic sometimes associated with adouble-planet system.[15]
Planets around other stars are likely to have satellites as well, and although numerous candidates have been detected to date, as of 2024[update] none have yet been confirmed.[59]
The discovery of243 Ida's natural satelliteDactyl in the early 1990s confirmed that someasteroids have natural satellites;[60] indeed,87 Sylvia has two.[61] Some, such as90 Antiope, are double asteroids with two comparably sized components.[62] For astronomers, a useful aspect of an asteroid satellite is that it can be used to determine the density of the primary asteroid, without the need for a spacecraft fly-by mission.[63]
Besides planets and dwarf planets objects within the Solar System known to have natural satellites are 76 in theasteroid belt (five with two each), fourJupiter trojans, 39near-Earth objects (two with two satellites each), and 14Mars-crossers.[2] There are also 84 known natural satellites oftrans-Neptunian objects.[2] Some 150 additional small bodies have been observed within therings of Saturn, but only a few were tracked long enough to establish orbits.
The seven largestnatural satellites in the Solar System (those bigger than 2,500 km across) are Jupiter'sGalilean moons (Ganymede,Callisto, Io, andEuropa), Saturn's moon Titan, Earth's moon, and Neptune's captured natural satellite Triton.[64] Of these, Ganymede and Titan are larger than the planet Mercury, while Callisto is about the same size.[65] The next size group of nine mid-sized natural satellites, between 1,000 km and 1,600 km across, consists ofTitania,Oberon,Rhea,Iapetus, Charon,Ariel,Umbriel,Dione, and Tethys, the smallest.[66] As well as the natural satellites of the various planets, there are hundreds of known natural satellites of thedwarf planets,minor planets and othersmall Solar System bodies.[2]
A planet usually has at least around 10,000 times the mass of any natural satellites that orbit it, with a correspondingly much larger diameter.[67][verification needed] TheEarth–Moon system is a unique exception in the Solar System; at 3,474 kilometres (2,158 miles) across, the Moon is 0.273 times thediameter of Earth and about1⁄80 of its mass.[68] The next largest ratios are theNeptune–Triton system at 0.055 (with a mass ratio of about 1 to 4790), theSaturn–Titan system at 0.044 (with the second mass ratio next to the Earth–Moon system, 1 to 4225), theJupiter–Ganymede system at 0.038 (with a mass ratio of 12810), and theUranus–Titania system at 0.031 (with a mass ratio of 25125). For the category ofdwarf planets,Charon has the largest ratio, being 0.52 the diameter and 12.2% the mass ofPluto.
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 theUranian natural satellites, which are named after Shakespearean characters. The twenty satellites massive enough to be round are in bold in the table below. Minor planets and satellites where there is disagreement in the literature on roundness are italicized in the table below.
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