Formally classified natural satellites, or moons, include 176 planetary satellites orbiting six of the eight planets,<ref name="bodies"/> and eight orbiting three of the five IAU-listed dwarf planets.<ref name="WGPSN"/> Template:As of, over 200 minor-planet moons have been discovered.<ref name="Johnston"/> There are 76 objects in the asteroid belt with satellites (five with two satellites 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 were observed within rings of Saturn, but they were not tracked long enough to establish orbits. Planets around other stars are likely to have natural satellites as well, although none have yet been observed.
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 large gas giants 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, though there is currently no direct evidence of life.
The Earth–Moon system is unique in that the ratio of the mass of the Moon to the mass of the Earth is much greater than that of any other natural satellite:planet ratio in the Solar System, and the Moon's orbit with respect to the Sun is always concave.<ref> "...if you were to draw a picture of the orbits of the Earth and Moon about the Sun exactly to scale, you would see that the Moon's orbit is everywhere concave toward the Sun. It is always "falling toward" the Sun. All the other satellites, without exception, "fall away" from the Sun through part of their orbits, caught as they are by the superior pull of their primary planets – but not the Moon." – Isaac Asimov [Asimov, Isaac (1975). "Just Mooning Around", collected in Of Time and Space, and other things (Avon)]</ref>
Among the dwarf planets, Ceres has no natural satellites. Pluto has the relatively large natural satellite Charon and four smaller natural satellites.<ref name="esahubble">Template:Cite news</ref> Haumea has two natural satellites, and Eris has one. 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.
Origin and orbital characteristics
The natural satellites orbiting relatively close to the planet on prograde, uninclined circular orbits (regular satellites) are generally believed 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.<ref>Encyclopedia of the Solar System, page 366, Academic Press, 2007, Lucy-Ann Adams McFadden, Paul Robert Weissman, Torrence V. Johnson</ref> The Earth's Moon<ref>Template:Cite journal</ref> and possibly Charon<ref>Template:Cite journal</ref> are exceptions among large bodies in that they are believed to have originated by the collision of two large proto-planetary objects (see the giant impact hypothesis). 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.
Template:Main 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 gas giants (irregular satellites) are too far away to have become locked. For example, Jupiter's natural satellite Himalia, Saturn's natural satellite Phoebe, and Neptune's natural satellite Nereid have rotation periods in the range of ten hours, while their orbital periods are hundreds of days.
Satellites of satellites
No "moons of moons" (natural satellites that orbit the natural satellite of another body) are known. In most cases, the tidal effects of the primary would make such a system unstable.
However, calculations performed after the recent detection<ref>Template:Cite web</ref> of a possible ring system around Saturn's natural satellite Rhea indicate that satellites orbiting Rhea would 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 any rings associated with Rhea.<ref>http://www.agu.org/pubs/crossref/2010/2010GL043663.shtml</ref>
It has also been proposed that Saturn's satellite Iapetus possessed a subsatellite 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>
Two natural satellites are known to have small companions at 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 Tethys; and Helene and Polydeuces, the leading and following companions of Dione.
Template:Main 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.
Proteus is the largest irregularly shaped natural satellite. All other known natural satellites the size of Miranda and above have lapsed into a rounded ellipsoid under hydrostatic equilibrium. The planets are not truly spherical but oblate spheroids, squatter at the pole than at the equator, but with a constant equatorial diameter. The larger natural satellites, however, since they are all tidally locked, are scalene, squat at the poles but with the equatorial axis directed at their planet longer than the axis along their direction of motion. The most distorted natural satellite is Mimas, where the major axis is 9% greater than its polar axis and 5% greater than its other equatorial axis, giving it a notable egg shape. The effect is smaller with the largest natural satellites, where self gravity is greater relative to tidal distortion, especially when they orbit a less massive planet or at a greater distance, as the Moon does.
|Name||Satellite of||Difference in axes|
|(km)||(% of mean diameter)|
|Mimas||Saturn||33.4 (20.4, 13.0)||8.4% (5.1%, 3.3%)|
Of the nineteen known natural satellites in the Solar System that are massive 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 gas giant 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, and presumably methane rain. 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
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 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, asteroids and other small Solar System bodies. Some studies estimate that up to 15% of all trans-Neptunian objects could have satellites.
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 predominately Greek, except for the Uranian natural satellites, which are named after Shakespearean characters. The nineteen bodies massive enough to have achieved hydrostatic equilibrium are in bold in the table below. Minor planets and satellites suspected but not proven to have achieved a hydrostatic equilibrium are italicized in the table below.
The first known natural satellite was the Moon, but it was considered a "planet" until Copernicus' introduction of heliocentrism in 1543. Until the discovery of the Galilean satellites in 1610, however, 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.
Christiaan Huygens, the discoverer of Titan, was the first to use the term moon for such objects, calling Titan Luna Saturni or Luna Saturnia – "Saturn's moon" or "The Saturnian moon", because it stood in the same relation to Saturn as the Moon did to the Earth.Template:Cn
The first to use of 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.
As additional natural satellites of Saturn were discovered the term "moon" was abandoned. Giovanni Domenico Cassini sometimes referred to his discoveries as planètes in French, but more often as satellites.Template:Cn
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 or 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 the 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 the Earth's natural satellite, but not when referring to other natural satellites.
The definition of a moon
There is not an established lower limit on what is considered a "moon", as natural satellites will be referred to in this section. Every natural celestial body with an identified orbit around a planet of the Solar System, some as small as a kilometer across, has been identified as 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.Template:Cn
The upper limit is also vague. Two orbiting bodies are sometimes described as a double body 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 commonTemplate:Cn 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, as it relies on distance as well as relative mass.
- Co-orbital moon
- Extrasolar moon
- Inner moon
- Irregular moon
- List of natural satellites
- Naming of moons
- Timeline of discovery of Solar System planets and their moons
- Trojan moon
- Tug of war (astronomy)
Moons of planets
Moons of dwarf planets and small Solar System bodies
- Natural Satellite Physical Parameters (JPL-NASA, with refs—last updated July 2006)
- Moons of the Solar System (The Planetary Society, as of March 2009)
- JPL's Solar System Dynamics page
- USGS list of named moons
- Upper size limit for moons explained
- Asteroids with Satellites
- Images of planets and major moons (not to scale)
- Scott Sheppard's page
- Jupiter's new moons (discovered in 2002)
- Jupiter's new moons (discovered in 2003)
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