In the Solar System there are six planetary satellite systems containing 205 known natural satellites. Four IAU-listed dwarf planets are also known to have natural satellites: Pluto, Haumea, Makemake, and Eris. As of September 2018, there are 334 other minor planets known to have moons.
The Earth–Moon system is unique among planetary systems in that the ratio of the mass of the Moon to the mass of Earth is much greater than that of any other natural-satellite–planet ratio in the Solar System. At 3,474 km (2,158 miles) across, the Moon is 0.273 times the diameter of Earth. This is five times greater than the next largest moon-to-planet diameter ratio (with Neptune's largest moon at 0.055, Saturn's at 0.044, Jupiter's at 0.038 and Uranus' as 0.031). For the category of planetoids, among the five that are known in the Solar System, Charon has the largest ratio, being half (0.52) the diameter of Pluto.
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.
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.
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. The terms man-made satellite and artificial moon were very quickly abandoned in favor of the simpler satellite,
and as a consequence, the term has become linked primarily with
artificial objects flown in space – including, sometimes, even those not
in orbit around a planet.
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
in 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, but not when referring to
other natural satellites.
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.
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".
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, 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.
The upper limit is also vague. Two orbiting bodies are sometimes described as a double planet rather than primary and satellite. Asteroids such as 90 Antiope
are considered double asteroids, 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. The most common dividing line on what is considered a moon rests upon whether the barycentre
is below the surface of the larger body, though this is somewhat
arbitrary, because it depends on distance as well as relative mass.
Origin and orbital characteristics
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. 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. The Moon and possibly Charon
are exceptions among large bodies in that they are thought to have
originated by the collision of two large proto-planetary objects.
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. 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.
Temporary satellites
The capture of an asteroid from a heliocentric orbit is not always permanent. According to simulations, temporary satellites should be a common phenomenon. The only observed example is 2006 RH120, which was a temporary satellite of Earth for nine months in 2006 and 2007.
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. The only known
exception is Saturn's natural satellite Hyperion, which rotates chaotically because of the gravitational influence of Titan.
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.
Satellites of satellites
Artist impression of Rhea's proposed rings
No "moons of moons" or subsatellites (natural satellites that orbit a natural satellite of a planet) are currently known as of 2020. In most cases, the tidal effects of the planet would make such a system unstable.
However, calculations performed after the recent detection 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, a phenomenon normally associated with shepherd moons. However, targeted images taken by the Cassini spacecraft failed to detect rings around Rhea.
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.
Trojan satellites
Two natural satellites are known to have small companions at both their L4 and 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.
Asteroid satellites
The discovery of 243 Ida's natural satellite Dactyl in the early 1990s confirmed that some asteroids have natural satellites; indeed, 87 Sylvia has two. Some, such as 90 Antiope, are double asteroids with two comparably sized components.
Shape
The relative masses of the natural satellites of the Solar System. Mimas, Enceladus, and Miranda
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 moon Proteus is the largest irregularly shaped natural satellite. All other known natural satellites that are at least the size of Uranus's Miranda have lapsed into rounded ellipsoids under hydrostatic equilibrium, i.e. are "round/rounded satellites". 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. Saturn's moon Mimas, for example, has a major axis 9% greater than its polar axis and 5% greater than its other equatorial axis. Methone, another of Saturn's moons, is only around 3 km in diameter and visibly egg-shaped.
The effect is smaller on the largest natural satellites, where their
own 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.
| Name | Satellite of | Difference in axes | |
|---|---|---|---|
| km |
% of mean
diameter | ||
| Mimas | Saturn | 33.4 (20.4 / 13.0) | 8.4 (5.1 / 3.3) |
| Enceladus | Saturn | 16.6 | 3.3 |
| Miranda | Uranus | 14.2 | 3.0 |
| Tethys | Saturn | 25.8 | 2.4 |
| Io | Jupiter | 29.4 | 0.8 |
| The Moon | Earth | 4.3 | 0.1 |
Geological activity
Of the nineteen known natural satellites in the Solar System that are large enough to have lapsed into hydrostatic equilibrium, several remain geologically active today. Io is the most volcanically active body in the Solar System, while Europa, Enceladus, Titan 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. (This mechanism would have also operated on Triton in the past, before its orbit was circularized.) Many other natural satellites, such as Earth's Moon, Ganymede, 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. 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. Also Io and Callisto have atmospheres, even if they are extremely thin. Four of the largest natural satellites, Europa, Ganymede, Callisto, and Titan, are thought to have subsurface oceans of liquid water, while smaller Enceladus may have localized subsurface liquid water.
Natural satellites of the Solar System
Euler diagram showing the types of bodies in the Solar System.
Of the objects within our Solar System known to have natural satellites, there are 76 in the asteroid belt (five with two each), four Jupiter trojans, 39 near-Earth objects (two with two satellites each), nd 14 Mars-crossers. There are also 84 known natural satellites of trans-Neptunian objects. Some 150 additional small bodies have been observed within the rings of Saturn,
but only a few were tracked long enough to establish orbits. Planets
around other stars are likely to have satellites as well, and although
numerous candidates have been detected to date, none have yet been
confirmed.
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. 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.
Among the identified dwarf planets, Ceres has no known natural satellites. Pluto has the relatively large natural satellite Charon and four smaller natural satellites; Styx, Nix, Kerberos, and Hydra. Haumea has two natural satellites, and Eris and Makemake have one each. 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.
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.
Triton, the smallest of these, has more mass than all smaller natural
satellites together. Similarly in the next size group of nine mid-sized
natural satellites, between 1,000 km and 1,600 km across, Titania, Oberon, Rhea, Iapetus, Charon, Ariel, Umbriel, Dione, and Tethys,
the smallest, Tethys, has more mass than all smaller natural satellites
together. As well as the natural satellites of the various planets,
there are also over 80 known natural satellites of the dwarf planets, minor planets and other small Solar System bodies. Some studies estimate that up to 15% of all trans-Neptunian objects could have satellites.
