Of the Solar System's eight planets and its nine most likely dwarf planets, six planets and seven dwarf planets are known to be orbited by at least 300 natural satellites, or moons. At least 19 of them are large enough to be gravitationally rounded; of these, all are covered by a crust of ice except for Earth's Moon and Jupiter's Io.^[1] Several of the largest ones are in hydrostatic equilibrium and would therefore be considered dwarf planets or planets if they were in direct orbit around the Sun and not in their current states (orbiting planets or dwarf planets).

Moons are classed into 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 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).^[2] For a very short time in 1974, Mercury was thought to have a moon.

Venus also has no moons,^[3] 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 more than 20 known co-orbitals, including the asteroids 3753 Cruithne and 469219 Kamoʻoalewa, and the occasional temporary satellite, like 2020 CD₃; however, since they do not permanently 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).^[4]

Jupiter has 95 moons with known orbits; 72 of them have received permanent designations, and 57 have been named. Its eight regular moons are grouped into the planet-sized Galilean moons and the far smaller Amalthea group. They were named after lovers of Zeus, the Greek equivalent of Jupiter. Among them is Ganymede, the largest and most massive moon in the Solar System. Its 87 known irregular moons are organized into two categories: prograde and retrograde. The prograde satellites consist of the Himalia group and three others in groups of one. The retrograde moons are grouped into the Carme, Ananke and Pasiphae groups.

Saturn has 146 moons with known orbits; 66 of them have received permanent designations, and 63 have been named. Most of them are quite small. Seven moons are large enough to be in hydrostatic equilibrium, including Titan, the second largest moon in the Solar System. Including these large moons, 24 of Saturn's moons are regular, and traditionally named after Titans or other figures associated with the mythological Saturn. The remaining 122 are irregular, and classified by their orbital characteristics into Inuit, Norse, and Gallic groups, and their names are chosen from the corresponding mythologies the groups are named after. 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 28 known moons, five of which are massive enough to have achieved hydrostatic equilibrium. There are 13 moons that orbit within Uranus's ring system, and another ten 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 16 known moons; 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 a dwarf planet that was captured. Neptune also has seven known inner regular satellites, and eight outer irregular satellites.

Pluto, a dwarf planet, 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.^[5]

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.^[6] Eris has one large known moon, Dysnomia. Accurately determining its size is difficult: one indicative estimate of its radius is 350±57.5 km.^[7]

Two objects were named as dwarf planets, under the expectation that they would prove to be so (though this remains uncertain). Haumea has two moons, Hiʻiaka and Namaka, of radii ~195 and ~100 km, respectively.^[8] Makemake has one moon, discovered in April 2016.

A number of other objects in the Kuiper belt and scattered disk may turn out to be dwarf planets. Orcus, Quaoar, Gonggong, and Sedna are generally agreed to be dwarf planets among astronomers, and all but Sedna are known to have moons.^[9] A number of other smaller objects, such as Huya, Salacia, 2002 UX25, Varda, and 2013 FY27, also have moons, although their dwarf planethood is more doubtful. This list includes all objects with a best estimated diameter above 400 km that are considered likely or official dwarf planets by other sources and astronomers,^[10][11][12] including 2003 AZ84 whose satellite has not been seen since its initial discovery.

As of October 2022^[update], 365 asteroid moons and 128 trans-Neptunian moons (including those of Pluto and the other dwarf planets) had been discovered.^[13]

Summary – number of moons

Planets	Mercury	Venus	Earth	Mars	Jupiter	Saturn	Uranus	Neptune
Number of moons	0	0	1	2	95	146	28	16

Consensus dwarf planets	Ceres	Orcus	Pluto	Haumea	Quaoar	Makemake	Gonggong	Eris	Sedna
Number of moons	0	1	5	2	1	1	1	1	0

Minor planets
See list

Due to Earth's varying distance from these planets (as well as their distance to the Sun), the limits at which we are able to detect new moons are very inconsistent. As the below graph demonstrates, the maximum absolute magnitude (total inherent brightness, abbreviated H) of moons we have detected around planets occurs at H = 18 for Jupiter, H = 17 for Saturn, H = 14 for Uranus, and H = 12 for Neptune. Smaller moons may (and most likely do) exist around each of these planets, but are currently undetectable from Earth. Although spacecraft have visited all of these planets, Earth-based telescopes continue to outperform them in moon-detection ability due to their greater availability for wide-field surveys.^{[citation needed]}

Planetary moons by absolute magnitude

Graphs are unavailable due to technical issues. There is more info on Phabricator and on MediaWiki.org.

List

This is a list of the recognized moons of the planets and of the largest potential dwarf planets of the Solar System, ordered by their official Roman numeral designations. Moons that do not yet have official Roman numeral designations (because their orbits are not yet known well enough) are listed after those that do.

The 19 moons that are known to be 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 a moon's speed depends both on the mass of its primary and its distance from it.

Name	Image	Parent	Numeral	Mean radius (km)	Orbital semi-major axis (km)	Sidereal period (d) (r = retrograde)	Discovery year	Year Announced	Discovered by	Notes	Ref(s)
Moon		Earth	I (1)	1,738	384,399	27.321582	Prehistoric	Prehistory	—	Synchronous rotation (Binary)	[14]
Phobos		Mars	I (1)	11.267	9,380	0.319	1877	1877	Hall	Synchronous rotation	[15][16][17]
Deimos		Mars	II (2)	6.2±0.18	23,460	1.262	1877	1877	Hall	Synchronous rotation	[15][16][17]
Io		Jupiter	I (1)	1,821.6±0.5	421,800	1.769	1610	1610	Galileo	Main-group moon (Galilean)	[17][18]
Europa		Jupiter	II (2)	1,560.8±0.5	671,100	3.551	1610	1610	Galileo	Main-group moon (Galilean)	[17][18]
Ganymede		Jupiter	III (3)	2,634.1±0.3	1,070,400	7.155	1610	1610	Galileo	Main-group moon (Galilean)	[17][18]
Callisto		Jupiter	IV (4)	2,410.3±1.5	1,882,700	16.69	1610	1610	Galileo	Main-group moon (Galilean)	[17][18]
Amalthea		Jupiter	V (5)	83.5±2	181,400	0.498	1892	1892	Barnard	Inner moon (Amalthea)	[16][17][19]
Himalia		Jupiter	VI (6)	69.8	11,461,000	250.56	1904	1905	Perrine	Prograde irregular (Himalia)	[16][17][20][21]
Elara		Jupiter	VII (7)	43	11,741,000	259.64	1905	1905	Perrine	Prograde irregular (Himalia)	[16][17][22]
Pasiphae		Jupiter	VIII (8)	30	23,624,000	743.63 (r)	1908	1908	Melotte	Retrograde irregular (Pasiphae)	[16][17][23]
Sinope		Jupiter	IX (9)	19	23,939,000	758.90 (r)	1914	1914	Nicholson	Retrograde irregular (Pasiphae)	[16][17][24]
Lysithea		Jupiter	X (10)	18	11,717,000	259.20	1938	1938	Nicholson	Prograde irregular (Himalia)	[16][17][25]
Carme		Jupiter	XI (11)	23	23,404,000	734.17 (r)	1938	1938	Nicholson	Retrograde irregular (Carme)	[16][17][25]
Ananke		Jupiter	XII (12)	14	21,276,000	629.77 (r)	1951	1951	Nicholson	Retrograde irregular (Ananke)	[16][17][26]
Leda		Jupiter	XIII (13)	10	11,165,000	240.92	1974	1974	Kowal	Prograde irregular (Himalia)	[16][17][27]
Thebe		Jupiter	XIV (14)	49.3±2.0	221,900	0.675	1979	1980	Synnott (Voyager 1)	Inner moon (Amalthea)	[16][17][28]
Adrastea		Jupiter	XV (15)	8.2±2.0	129,000	0.298	1979	1979	Jewitt, Danielson (Voyager 1)	Inner moon (Amalthea)	[16][17][29]
Metis		Jupiter	XVI (16)	21.5±2.0	128,000	0.295	1979	1980	Synnott (Voyager 1)	Inner moon (Amalthea)	[16][17][30]
Callirrhoe		Jupiter	XVII (17)	4.5	24,103,000	758.77 (r)	1999	2000	Scotti, Spahr, McMillan, Larsen, Montani, Gleason, Gehrels	Retrograde irregular (Pasiphae)	[16][17][31]
Themisto		Jupiter	XVIII (18)	4	7,284,000	130.02	1975/2000	1975/2000	Kowal and Roemer (original); Sheppard, Jewitt, Fernández, Magnier (rediscovery)	Prograde irregular (Themisto)	[16][17][32][33]
Megaclite		Jupiter	XIX (19)	2.7	23,493,000	752.86 (r)	2000	2001	Sheppard, Jewitt, Fernández, Magnier, Dahm, Evans	Retrograde irregular (Pasiphae)	[16][17][34]
Taygete		Jupiter	XX (20)	2.5	23,280,000	732.41 (r)	2000	2001	Sheppard, Jewitt, Fernández, Magnier, Dahm, Evans	Retrograde irregular (Carme)	[16][17][34]
Chaldene		Jupiter	XXI (21)	1.9	23,100,000	723.72 (r)	2000	2001	Sheppard, Jewitt, Fernández, Magnier, Dahm, Evans	Retrograde irregular (Carme)	[16][17][34]
Harpalyke		Jupiter	XXII (22)	2.2	20,858,000	623.32 (r)	2000	2001	Sheppard, Jewitt, Fernández, Magnier, Dahm, Evans	Retrograde irregular (Ananke)	[16][17][34]
Kalyke		Jupiter	XXIII (23)	2.6	23,483,000	742.06 (r)	2000	2001	Sheppard, Jewitt, Fernández, Magnier, Dahm, Evans	Retrograde irregular (Carme)	[16][17][34]
Iocaste		Jupiter	XXIV (24)	2.6	21,060,000	631.60 (r)	2000	2001	Sheppard, Jewitt, Fernández, Magnier, Dahm, Evans	Retrograde irregular (Ananke)	[16][17][34]
Erinome		Jupiter	XXV (25)	1.6	23,196,000	728.46 (r)	2000	2001	Sheppard, Jewitt, Fernández, Magnier, Dahm, Evans	Retrograde irregular (Carme)	[16][17][34]
Isonoe		Jupiter	XXVI (26)	1.9	23,155,000	726.23 (r)	2000	2001	Sheppard, Jewitt, Fernández, Magnier, Dahm, Evans	Retrograde irregular (Carme)	[16][17][34]
Praxidike		Jupiter	XXVII (27)	3.5	20,908,000	625.39 (r)	2000	2001	Sheppard, Jewitt, Fernández, Magnier, Dahm, Evans	Retrograde irregular (Ananke)	[16][17][34]
Autonoe		Jupiter	XXVIII (28)	2	24,046,000	760.95 (r)	2001	2002	Sheppard, Jewitt, Kleyna	Retrograde irregular (Pasiphae)
Thyone		Jupiter	XXIX (29)	2	20,939,000	627.21 (r)	2001	2002	Sheppard, Jewitt, Kleyna	Retrograde irregular (Ananke)	[16][17][35]
Hermippe		Jupiter	XXX (30)	2	21,131,000	633.9 (r)	2001	2002	Sheppard, Jewitt, Kleyna	Retrograde irregular (Ananke)	[16][17][35]
Aitne		Jupiter	XXXI (31)	1.5	23,229,000	730.18 (r)	2001	2002	Sheppard, Jewitt, Kleyna	Retrograde irregular (Carme)	[16][17][35]
Eurydome		Jupiter	XXXII (32)	1.5	22,865,000	717.33 (r)	2001	2002	Sheppard, Jewitt, Kleyna	Retrograde irregular (Pasiphae)	[16][17][35]
Euanthe		Jupiter	XXXIII (33)	1.5	20,797,000	620.49 (r)	2001	2002	Sheppard, Jewitt, Kleyna	Retrograde irregular (Ananke)	[16][17][35]
Euporie		Jupiter	XXXIV (34)	1	19,304,000	550.74 (r)	2001	2002	Sheppard, Jewitt, Kleyna	Retrograde irregular (Ananke)	[16][17][35]
Orthosie		Jupiter	XXXV (35)	1	20,720,000	622.56 (r)	2001	2002	Sheppard, Jewitt, Kleyna	Retrograde irregular (Ananke)	[16][17][35]
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