Natural satellite
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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 colloquially referred to as moons, a derivation from the Moon of Earth.
In the Solar System, there are six planetary satellite systems, altogether comprising 419 natural satellites with confirmed orbits. Seven objects commonly considered dwarf planets by astronomers are also known to have natural satellites: Template:Dp, Pluto, Haumea, Template:Dp, Makemake, Template:Dp, and Eris.<ref name="WGPSN"/> As of January 2022, there are 447 other minor planets known to have natural satellites.<ref name="Johnston"/>
Terminology
The first known natural satellite was the Moon, but it was considered a "planet" until Copernicus' introduction of De revolutionibus orbium coelestium in 1543. Until the discovery of the Galilean satellites in 1610 there was no opportunity for referring to such objects as a class. Galileo chose to refer to his discoveries as Planetæ ("planets"), but later discoverers chose other terms to distinguish them from the objects they orbited.<ref name=metzger22>Template:Cite journal</ref>
The first to use the term satellite to describe orbiting bodies was the German astronomer Johannes Kepler in his pamphlet Narratio de Observatis a se quatuor Iouis satellitibus erronibus ("Narration About Four Satellites of Jupiter Observed") in 1610. He derived the term from the Latin word satelles, meaning "guard", "attendant", or "companion", because the satellites accompanied their primary planet in their journey through the heavens.<ref name="NASA-firstsatellites"/>
The term satellite thus became the normal one for referring to an object orbiting a planet, as it avoided the ambiguity of "moon". In 1957, however, the launching of the artificial object Sputnik created a need for new terminology.<ref name="NASA-firstsatellites"/> The terms man-made satellite and artificial moon were very quickly abandoned in favor of the simpler satellite. As a consequence, the term has become linked with artificial objects flown in space.
Because of this shift in meaning, the term moon, which had continued to be used in a generic sense in works of popular science and fiction, has regained respectability and is now used interchangeably with natural satellite, even in scientific articles. When it is necessary to avoid both the ambiguity of confusion with Earth's natural satellite the Moon and the natural satellites of the other planets on the one hand, and artificial satellites on the other, the term natural satellite (using "natural" in a sense opposed to "artificial") is used. To further avoid ambiguity, the convention is to capitalize the word Moon when referring to Earth's natural satellite (a proper 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.<ref>Template:Cite book</ref><ref>Template:Cite book</ref><ref>Template:Cite encyclopedia</ref> 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".<ref>Template:Cite book</ref><ref>Template:Cite web</ref><ref name="NASA-what-is-a-satellite"/>
Definition of a moon
There is no established lower limit on what is considered a "moon". Every natural celestial body with an identified orbit around a planet of the Solar System, some as small as a kilometer across, has been considered a moon,<ref>Template:Cite book</ref> though objects a tenth that size within Saturn's rings, which have not been directly observed, have been called moonlets. Small asteroid moons (natural satellites of asteroids), such as Dactyl, have also been called moonlets.<ref name="Marchis05">Template:Cite journal</ref>
The upper limit is also vague. Two orbiting bodies are sometimes described as a double planet rather than a primary and satellite; the Earth-Moon system is used as an example.<ref>Template:Cite journal</ref> Asteroids such as 90 Antiope are considered double asteroids,<ref>Template:Cite journal</ref> 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,<ref name=Ksanfomality_2016>Template:Cite journal</ref> with one argument being that the barycentre lies above the surface of the larger body.<ref name=Rijsdijk_2007>Template:Cite journal</ref> In contrast, the barycenter of all planetary moons of the Solar System are located within the radius of their host planet.<ref name=Hahn_2020>Template:Cite book</ref>
Origin and orbital characteristics
Template:Further The natural satellites orbiting relatively close to the planet on prograde, uninclined circular orbits (regular satellites) are generally thought to have been formed out of the same collapsing region of the protoplanetary disk that created its primary.<ref name="arxiv0812">Template:Cite book</ref><ref name=dangelo_podolak_2015>Template:Cite journal</ref> In contrast, irregular satellites (generally orbiting on distant, inclined, eccentric and/or retrograde orbits) are thought to be captured asteroids 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.<ref>Template:Cite book</ref> The Moon and the Moons 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 the giant impact hypothesis).<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> 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.<ref name=Durda_et_al_2004>Template:Cite journal</ref> Triton is another exception; although large and in a close, circular orbit, its motion is retrograde and it is thought to be a captured dwarf planet.<ref>Template:Cite journal</ref>
Trojan satellites
Two natural satellites are known to have small companions at both their Template:L4 and Template:L5 Lagrangian points, sixty degrees ahead and behind the body in its orbit. These companions are called trojan moons, as their orbits are analogous to the trojan asteroids of Jupiter. The trojan moons are Telesto and Calypso, which are the leading and following companions, respectively, of the Saturnian moon Tethys; and Helene and Polydeuces, the leading and following companions of the Saturnian moon Dione.<ref>Template:Cite journal</ref>
Temporary satellites
Template:Main The capture of an asteroid from a heliocentric orbit is not always permanent. According to simulations, temporary satellites should be a common phenomenon.<ref name="Carlisle">Template:Cite news</ref><ref name="Fedorets">Template:Cite journal</ref> Template:Asof, the observed minor bodies that have displayed transient co-orbital motion with Earth are: Template:Mpl, Template:Mpl, Template:Mpl, and Template:Mpl.<ref>Template:Cite journal</ref>
Template:Mpl was a temporary satellite of Earth for nine months in 2006 and 2007.<ref name="Shefford">Template:Cite web</ref><ref name="Sinott">Template:Cite news</ref>
Tidal locking
Most regular 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.<ref name=Barnes_2010>Template:Cite book</ref> Exceptions are known; one such exception is Saturn's natural satellite Hyperion, which rotates chaotically because of the gravitational influence of Titan.<ref>Template:Cite journal</ref> Pluto's four, circumbinary small moons also rotate chaotically due to Charon's influence.<ref name="NASA2015"/>
In contrast, the outer natural satellites of the giant planets (irregular satellites) are too far away to have become locked. For example, Jupiter's Himalia, Saturn's Phoebe, and Neptune's Nereid have rotation periods in the range of ten hours, whereas their orbital periods are hundreds of days.<ref>Template:Cite journal</ref>
Satellites of satellites
No "moons of moons" or subsatellites (natural satellites that orbit a natural satellite of a planet) are currently known. In most cases, the tidal effects of the planet would make such a system unstable. 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 as mascons on the Moon.<ref>Template:Cite journal</ref>
Calculations performed after the 2008 detection<ref> Template:Cite journal</ref> of a possible ring system around Saturn's moon Rhea indicate that satellites orbiting Rhea could have stable orbits. Furthermore, the suspected rings are thought to be narrow,<ref>Template:Cite web</ref> a phenomenon normally associated with shepherd moons. However, targeted images taken by the Cassini spacecraft failed to detect rings around Rhea.<ref>Template:Cite journal</ref>
It has also been proposed that Saturn's moon Iapetus had a satellite in the past; this is one of several hypotheses that have been put forward to account for its equatorial ridge.<ref>Template:Cite web</ref>
Light-curve analysis suggests that Saturn's irregular satellite Kiviuq is extremely prolate, and is likely a contact binary or even a binary moon.<ref name="LPSC2654">Template:Cite conference</ref>
Shape
Neptune's moon Proteus is the largest irregularly shaped natural satellite, and is about as large as an icy moon can become before becoming relaxed into a spheroidal shape.<ref name=Croft_402>Template:Cite journal</ref> The shapes of moons in synchronous orbit are expected to asymptotically change shape into rounded ellipsoids under hydrostatic equilibrium, although this may not happen in the age of the Solar System. For example, the larger Saturnian moons are in equilibrium, while Iapetus, Mimas, and Enceladus are apparently not.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Template:Citation needed span The shapes of Eris' moon Dysnomia and Template:Dp' moon Vanth are presently unknown, although Dysnomia's density is high enough that it is probably a solid ellipsoid as well.Template:Cn
The larger natural satellites, being tidally locked, tend toward ovoid (egg-like) shapes: squat at their poles and with longer equatorial axes in the direction of their primaries (their planets) than in the direction of their motion.<ref name=Rambaux_Castillo-Rogez_2012/>Template:Rp Saturn's moon Mimas, for example, has a major axis 9% greater than its polar axis and 5% greater than its other equatorial axis.<ref name=Thomas_2010>Template:Cite journal</ref> Methone, another of Saturn's moons, is only around 3 km in diameter and visibly egg-shaped.<ref>Template:Cite conference</ref> 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.<ref name=Rambaux_Castillo-Rogez_2012>Template:Cite book</ref>Template:Rp
| Name | Satellite of | Difference in axes | |
|---|---|---|---|
| km | Template:Longitem | ||
| Mimas | Saturn | 33.4<ref name=Thomas_2010/> Template:Smaller | 8.4 Template:Smaller |
| Enceladus | Saturn | 16.6<ref name=Thomas_2010/> | 3.3 |
| Miranda | Uranus | 14.2 | 3.0 |
| Tethys | Saturn | 25.8<ref name=Thomas_2010/> | 2.4 |
| Io | Jupiter | 29.4 | 0.8 |
| Luna | Earth | 4.3 | 0.1 |
Geological activity
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,<ref name=Lainey_et_al_2009>Template:Cite journal</ref> while Europa,<ref name=Cook_et_al_2013>Template:Cite web</ref> Enceladus,<ref name=Roberts_Nimmo_2008>Template:Cite journal</ref> and Triton display evidence of ongoing tectonic activity and cryovolcanism. In the first three cases, the geological activity is powered by the tidal heating resulting from having eccentric orbits close to their giant-planet primaries.<ref name=Lainey_et_al_2009/><ref name=Cook_et_al_2013/><ref name=Roberts_Nimmo_2008/> (This mechanism would have also operated on Triton in the past before its orbit was circularized.<ref>Template:Cite journal</ref>) 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 the decay of their primordial radioisotopes, greater past orbital eccentricities (due in some cases to past orbital resonances), or the differentiation or freezing of their interiors.<ref name=Nimmo_2025/> Enceladus and Triton both have active features resembling geysers, although in the case of Triton solar heating appears to provide the energy. Titan and Triton have significant atmospheres; Titan also has hydrocarbon lakes.<ref name=Nimmo_2025>Template:Cite journal</ref> All four of the Galilean moons have atmospheres, though they are extremely thin.<ref>Template:Cite web</ref><ref name="EuropaAtmosphere">Template:Cite web</ref><ref name="JPLAtmosphere">Template:Cite web</ref> Four of the largest natural satellites, Europa,<ref name=Cook_et_al_2013/> Ganymede, Callisto, and Titan, are thought to have subsurface oceans of liquid water,<ref name=Nimmo_2025/> while smaller Enceladus also supports a global subsurface ocean of liquid water.<ref name=Roberts_Nimmo_2008/>
Occurrence in the Solar System
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Of the inner planets, Mercury and Venus have no natural satellites; Earth has one large natural satellite, known as the Moon; and Mars has two tiny natural satellites, Phobos and Deimos. The giant planets have extensive systems of natural satellites, including half a dozen comparable in size to Earth's Moon: the four Galilean moons, Saturn's Titan, and Neptune's Triton.<ref name=Kane_et_al_2013>Template:Cite journal</ref> Saturn has an additional six mid-sized natural satellites massive enough to have achieved hydrostatic equilibrium, and Uranus has five. It has been suggested that some satellites may potentially harbour life.<ref name="WRD-20150127">Template:Cite journal</ref>
Among the objects generally agreed by astronomers to be dwarf planets, Ceres and Template:Dp have no known natural satellites. Pluto has the relatively large natural satellite Charon and four smaller natural satellites; Styx, Nix, Kerberos, and Hydra.<ref name="esahubble">Template:Cite news</ref> Haumea has two natural satellites; Template:Dp, Template:Dp, Makemake, Template:Dp, and Template:Dp have one each.<ref name=Upadhyay_2025>Template:Cite book</ref> The Pluto–Charon system is unusual in that the center of mass lies in open space between the two, a characteristic sometimes associated with a double-planet system.<ref name=Ksanfomality_2016/>
Planets around other stars are likely to have satellites as well, and although numerous candidates have been detected to date, Template:Asof none have yet been confirmed.<ref>Template:Cite journal</ref>
Non-planetary satellites
The discovery of 243 Ida's natural satellite Dactyl in the early 1990s confirmed that some asteroids have natural satellites;<ref>Template:Cite journal</ref> indeed, 87 Sylvia has two.<ref>Template:Cite journal</ref> Some, such as 90 Antiope, are double asteroids with two comparably sized components.<ref>Template:Cite journal</ref> 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.<ref>Template:Cite book</ref>
Besides planets and dwarf planets objects within the Solar System known to have natural satellites are 76 in the asteroid belt (five with two each), four Jupiter trojans, 39 near-Earth objects (two with two satellites each), and 14 Mars-crossers.<ref name="Johnston" /> There are also 84 known natural satellites of trans-Neptunian objects.<ref name="Johnston" /> Some 150 additional small bodies have been observed within the rings of Saturn, but only a few were tracked long enough to establish orbits.
Dimensions
The seven largest natural satellites in the Solar System (those bigger than 2,500 km across) are Jupiter's Galilean moons (Ganymede, Callisto, Io, and Europa), Saturn's moon Titan, Earth's moon, and Neptune's captured natural satellite Triton.<ref name=Burns_Matthews_1986>Template:Cite book</ref> Of these, Ganymede and Titan are larger than the planet Mercury, while Callisto is about the same size.<ref name=Koupelis_2010>Template:Cite book</ref> The next size group of nine mid-sized natural satellites, between 1,000 km and 1,600 km across, consists of Titania, Oberon, Rhea, Iapetus, Charon, Ariel, Umbriel, Dione, and Tethys, the smallest.<ref name=Hussman_et_al_2010>Template:Cite book</ref> As well as the natural satellites of the various planets, there are hundreds of known natural satellites of the dwarf planets, minor planets and other small Solar System bodies.<ref name="Johnston"/>
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.<ref name="2006Natur.441..834C"/>Template:Verify source The Earth–Moon system is a unique exception in the Solar System; at 3,474 kilometres (2,158 miles) across, the Moon is 0.273 times the diameter of Earth and about Template:Frac of its mass.<ref name="GuinnessWR2014"/> The next largest ratios are the Neptune–Triton system at 0.055 (with a mass ratio of about 1 to 4790), the Saturn–Titan system at 0.044 (with the second mass ratio next to the Earth–Moon system, 1 to 4225), the Jupiter–Ganymede system at 0.038 (with a mass ratio of 12810), and the Uranus–Titania system at 0.031 (with a mass ratio of 25125). For the category of dwarf planets, Charon has the largest ratio, being 0.52 the diameter and 12.2% the mass of Pluto.
The following is a comparative table classifying the natural satellites in the Solar System by diameter. The column on the right includes some notable planets, dwarf planets, asteroids, and trans-Neptunian objects for comparison. The natural satellites of the planets are named after mythological figures. These are predominantly Greek, except for the Uranian 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.
See also
Moons of planets
Moons of dwarf planets and small Solar System bodies
References
<references> <ref name="WGPSN">Template:Cite web</ref>
<ref name="Johnston">Template:Cite web</ref>
<ref name="2006Natur.441..834C"> Template:Cite journal</ref>
<ref name="NASA-what-is-a-satellite"> Template:Cite web </ref>
<ref name="NASA-firstsatellites">Template:Cite web</ref>
<ref name="NASA2015">Template:Cite web</ref>
<ref name="GuinnessWR2014"> Template:Cite book</ref> </references>
External links
All moons
- Natural Satellite Physical Parameters (JPL-NASA, with refs – last updated July 2006)
- Moons of the Solar System (The Planetary Society, as of March 2009)
- The JPL's Solar System Dynamics page
- Template:Cite web
- Planetary Names: Planet and Satellite Names and Discoverers
- "Upper size limit for moons explained" Kelly Young. Nature (vol 441, p. 834) 14 June 2006
- Images of planets and major moons (not to scale)
- The Planetary Society – Moon Montage(s)
- Album of moon images by Kevin M. Gill
- The Atlas of Moons by the National Geographic Society
Jupiter's moons
- Template:Cite web* Scott S. Sheppard
- Scott S. Sheppard
Saturn's moons
- Satellite-hunters find four new moons of the planet Saturn David Brand | 26 October 2000
- Saturn's New Satellite S/2003 S1 Scott S. Sheppard
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