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The Solar System's planets and officially recognized dwarf planets are known to be orbited by 182 natural satellites, or moons. 19 moons in the Solar System are large enough to be gravitationally rounded, and thus would be considered planets or dwarf planets if they were in direct orbit around the Sun.
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Moons are classed in two separate categories according to their orbits: regular moons, which have prograde orbits (they orbit in the direction of their planets' rotation) and lie close to the plane of their equators, and irregular moons, whose orbits can be pro- or retrograde (against the direction of their planets' rotation) and often lie at extreme angles to their planets' equators. Irregular moons are probably minor planets that have been captured from surrounding space. Most irregular moons are less than 10 kilometres (6.2 mi) in diameter.
The earliest published discovery of a moon other than the Earth's was by Galileo Galilei, who discovered the four Galilean moons orbiting Jupiter in 1610. Over the following three centuries only a few more moons were discovered. Missions to other planets in the 1970s, most notably the Voyager 1 and 2 missions, saw a surge in the number of moons detected, and observations since the year 2000, using mostly large, ground-based optical telescopes, have discovered many more, all of which are irregular.
Moons by primary
Mercury, the smallest and innermost planet, has no moons, or at least none that can be detected to a diameter of 1.6 km (1.0 mi). For a very short time in 1974, Mercury was thought to have a moon.
Venus also has no moons, though reports of a moon around Venus have circulated since the 17th century.
Earth has one Moon, the largest moon of any rocky planet in the Solar System. Earth also has at least two co-orbitals: the asteroids 3753 Cruithne and 2002 AA29; however, since they do not orbit Earth, they are not considered moons. (See Other moons of Earth and Quasi-satellite.)
Mars has two known moons, Phobos and Deimos ("fear" and "dread", after attendants of Ares, the Greek god of war, equivalent to the Roman Mars). Searches for more satellites have been unsuccessful, putting the maximum radius of any other satellites at 90 m (100 yd).
Jupiter has 67 known moons with confirmed orbits. Its eight regular moons are grouped into the planet-sized Galilean moons and the far smaller Amalthea group. They are named after lovers of Zeus, the Greek equivalent of Jupiter. Its 59 known irregular moons are organized into two categories: prograde and retrograde. The prograde satellites consist of the Himalia group and two others in groups of one. The retrograde moons are grouped into the Carme, Ananke and Pasiphae groups, as well as some isolated moons.
Saturn has 62 moons with confirmed orbits, 53 of which have names, most of which are quite small. Seven moons are large enough to be in hydrostatic equilibrium, including Titan, the second largest moon in the Solar System. Twenty-four of Saturn's moons are regular, and traditionally named after Titans or other figures associated with the mythological Saturn. The remaining thirty-eight, all small, are irregular, and classified by their orbital characteristics into Inuit, Norse, and Gallic groups, and their names are chosen from the corresponding mythologies. The rings of Saturn are made up of icy objects ranging in size from one centimetre to hundreds of metres, each of which is on its own orbit about the planet. Thus a precise number of Saturnian moons cannot be given, as there is no objective boundary between the countless small anonymous objects that form Saturn's ring system and the larger objects that have been named as moons. At least 150 "moonlets" embedded in the rings have been detected by the disturbance they create in the surrounding ring material, though this is thought to be only a small sample of the total population of such objects.
Uranus has 27 named moons, five of which are massive enough to have achieved hydrostatic equilibrium. There are another 13 inner moons that orbit within Uranus's ring system, and another nine outer irregular moons. Unlike most planetary moons, which are named from antiquity, all the moons of Uranus are named after characters from the works of Shakespeare and Alexander Pope's work The Rape of the Lock.
Neptune has 14 moons (one of which is unnamed); the largest, Triton, accounts for more than 99.5 percent of all the mass orbiting the planet. Triton is large enough to have achieved hydrostatic equilibrium, but, uniquely for a large moon, has a retrograde orbit, suggesting it was captured. Neptune also has six known inner regular satellites, and six outer irregular satellites.
The dwarf planet Pluto has five moons. Its largest moon Charon, named after the ferryman who took souls across the River Styx, is more than half as large as Pluto itself, and large enough to orbit a point outside Pluto's surface. In effect, each orbits the other, forming a binary system informally referred to as a double-dwarf-planet. Pluto's four other moons, Nix, Hydra, Kerberos and Styx are far smaller and orbit the Pluto–Charon system.
Among the other dwarf planets, Ceres has no known moons. It is 90 percent certain that Ceres has no moons larger than 1 km in size, assuming that they would have the same albedo as Ceres itself. Haumea has two moons, Hi'iaka and Namaka, of radii ~195 and ~100 km, respectively. Makemake has one moon, discovered in April 2016. A satellite having 1% Makemake's brightness would have been detected if it had been located at an angular distance from Makemake farther than 0.4 arcseconds (0.0001 degrees). Eris has one known moon, Dysnomia. Accurately determining its size is difficult: one indicative estimate of its radius is 7005257000000000000♠257±110 km, but on some assumptions could be as high as 7005342000000000000♠342±25 km. The Kuiper belt object 90482 Orcus, believed to be a dwarf planet, was found in 2005 to have a natural satellite, later named Vanth. As of October 2009, 190 asteroid moons and 63 trans-Neptunian moons had been discovered.
List
This is a list of the recognized moons of the planets and of the largest dwarf planets of the Solar System. The 19 moons that are large enough to have been rounded by their own gravity are listed in bold. The seven largest moons, which are larger than any of the known dwarf planets, are listed in bold and italic. Sidereal period differs from semi-major axis because it depends on the mass of the primary as well as the satellite's distance from it.