List of natural satellites by diameter.html |
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The Solar System is known to contain 169 natural satellites, or moons, excluding those in orbit around small Solar System bodies.1 Nineteen moons in the Solar System are large enough to have achieved hydrostatic equilibrium, and thus would be considered planets or dwarf planets if they were in direct orbit around the Sun.
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 lie at extreme angles to their planets' equators. Irregular moons are likely minor planets that have been captured from surrounding space. Most irregular moons are less than 10 kilometres (6.2 mi) in diameter.
The earliest discovery of a moon other than the Earth's was by Galileo Galilei, who discovered the four Galilean moons 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.
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Moons by planet
Mercury, the innermost planet, has no moons, or at least any that can be detected to a diameter of 1.6 km (0.99 mi).2 However, for a brief time in 1974, Mercury was thought to have a moon. Venus has no moons,3 though reports of a moon around Venus have circulated since the 17th century. Earth has one Moon, the largest of any rocky planet in the Solar System. It also has at least two co-orbital satellites: the asteroids 3753 Cruithne and 2002 AA29;4 however, since they do not orbit Earth, they are not considered moons. Mars has two known satellites, Phobos and Deimos, ("fear" and "panic", 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 0.09 km (0.056 mi).5
Jupiter has 63 known moons, 62 with secured 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 54 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, Anake and Pasiphaë groups, as well as some isolated moons.
Saturn has 60 moons with confirmed orbits, 52 of which have names, and most of which are quite small. Seven moons are large enough to be in hydrostatic equilibrium. Twenty-two 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 1 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 twenty-seven named moons, five of which are massive enough to have achieved hydrostatic equilibrium. There are also thirteen inner moons that orbit within Uranus' 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 a massively lopsided moon system; one moon, Triton, accounts for more than 95 percent of all the mass orbiting the planet. Triton is large enough to have achieved hydrostatic equilibrium, but, uniquely for a large moon, is irregular, suggesting it was captured. Neptune also has six known inner regular satellites, and six outer irregular satellites.
Among the 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 Pluto has three 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, both moons orbit each other, and can thus be considered two parts of a double planet system. Pluto's two other moons, Nix and Hydra, are far smaller and orbit both Pluto and Charon. Makemake has no known moons. 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).7 Eris has one known moon, Dysnomia. Its radius is currently not known, but is expected to be of order 100 km.8
As of September 2008, 104 asteroid moons9 and 58 Trans-Neptunian moons had been discovered.9
Moons of planets and dwarf planets
This is a list of all the moons of planets and dwarf planets in the Solar System. Those 19 moons that are large enough to have achieved hydrostatic equilibrium are listed in bold.
| Satellite of Earth | Satellites of Mars |
| Satellites of Jupiter | Satellites of Saturn |
| Satellites of Uranus | Satellites of Neptune |
| Satellites of Pluto | Satellites of Haumea |
| Satellite of Eris |
| Image | Numeral | Name | Mean radius (km) | Semi-major axis (km) | Sidereal period (d) (r= retrograde) |
Discovery date | Discovered by | Notes | Ref(s) | Planet |
|---|---|---|---|---|---|---|---|---|---|---|
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— | Moon | 1,737.10 | 384,399 | 27.321582 | Prehistoric | — | Synchronous rotation | 10 | Earth |
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— | Phobos | 11.1 ± 0.15 | 9,380 | 0.319 | 1877 (18 August) | Hall | 111213 | Mars | |
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— | Deimos | 6.2 ± 0.18 | 23,460 | 1.262 | 1877 (12 August) | Hall | 111213 | Mars | |
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I | Io | 1,818.1 ± 0.1 | 421,800 | 1.769 | 1610 | Galileo | Main group moon (Galilean) | 1413 | Jupiter |
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II | Europa | 1,560.7 ± 0.7 | 671,100 | 3.551 | 1610 | Galileo | Main group moon (Galilean) | 1413 | Jupiter |
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III | Ganymede | 2,634.1 ± 0.3 | 1,070,400 | 7.155 | 1610 | Galileo | Main group moon (Galilean) | 1413 | Jupiter |
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IV | Callisto | 2,408.4 ± 0.3 | 1,882,700 | 16.69 | 1610 | Galileo | Main group moon (Galilean) | 1413 | Jupiter |
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V | Amalthea | 83.45 ± 2.4 | 181,400 | 0.498 | 1892 | Barnard | Inner moon (Amalthea) | 121315 | Jupiter |
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VI | Himalia | 85 | 11,461,000 | 250.56 | 1904 | Perrine | Prograde irregular (Himalia) | 121316 | Jupiter |
| VII | Elara | 43 | 11,741,000 | 259.64 | 1905 | Perrine | Prograde irregular (Himalia) | 121317 | Jupiter | |
| VIII | Pasiphaë | 30 | 23,624,000 | 743.63 (r) | 1908 | Melotte | Retrograde irregular (Pasiphae) | 121318 | Jupiter | |
| IX | Sinope | 19 | 23,939,000 | 758.90 (r) | 1914 | Nicholson | Retrograde irregular (Pasiphae) | 121319 | Jupiter | |
| X | Lysithea | 18 | 11,717,000 | 259.20 | 1938 | Nicholson | Prograde irregular (Himalia) | 121320 | Jupiter | |
| XI | Carme | 23 | 23,404,000 | 734.17 (r) | 1938 | Nicholson | Retrograde irregular (Carme) | 121320 | Jupiter | |
| XII | Ananke | 14 | 21,276,000 | 629.77 (r) | 1951 | Nicholson | Retrograde irregular (Ananke) | 121321 | Jupiter | |
| XIII | Leda | 10 | 11,165,000 | 240.92 | 1974 | Kowal | Prograde irregular (Himalia) | 121322 | Jupiter | |
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XIV | Thebe | 49.3 ± 2.0 | 221,900 | 0.675 | 1980 | Synnott (Voyager 1) | Inner moon (Amalthea) | 121323 | Jupiter |
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XV | Adrastea | 8.2 ± 2.0 | 129,000 | 0.298 | 1979 | Jewitt, Danielson (Voyager 1) | Inner moon (Amalthea) | 121324 | Jupiter |
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XVI | Metis | 21.5 ± 2.0 | 128,000 | 0.295 | 1979 | Synnott (Voyager 1) | Inner moon (Amalthea) | 121325 | Jupiter |
| XVII | Callirrhoe | 4.3 | 24,103,000 | 758.77 (r) | 2000 | Scotti, Spahr, McMillan, Larsen, Montani, Gleason, Gehrels | Retrograde irregular (Pasiphae) | 121326 | Jupiter | |
| XVIII | Themisto | 4.0 | 7,284,000 | 130.02 | 1975/2000 | Kowal and Roemer (original); Sheppard, Jewitt, Fernández, Magnier (rediscovery) | Prograde irregular | 12132728 | Jupiter | |
| XIX | Megaclite | 2.7 | 23,493,000 | 752.86 (r) | 2001 | Sheppard, Jewitt, Fernández, Magnier, Dahm, Evans | Retrograde irregular (Pasiphae) | 121329 | Jupiter | |
| XX | Taygete | 2.5 | 23,280,000 | 732.41 (r) | 2001 | Sheppard, Jewitt, Fernández, Magnier, Dahm, Evans | Retrograde irregular (Carme) | 121329 | Jupiter | |
| XXI | Chaldene | 1.9 | 23,100,000 | 723.72 (r) | 2001 | Sheppard, Jewitt, Fernández, Magnier, Dahm, Evans | Retrograde irregular (Carme) | 121329 | Jupiter | |
| XXII | Harpalyke | 2.2 | 20,858,000 | 623.32 (r) | 2001 | Sheppard, Jewitt, Fernández, Magnier, Dahm, Evans | Retrograde irregular (Ananke) | 121329 | Jupiter | |
| XXIII | Kalyke | 2.6 | 23,483,000 | 742.06 (r) | 2001 | Sheppard, Jewitt, Fernández, Magnier, Dahm, Evans | Retrograde irregular (Carme) | 121329 | Jupiter | |
| XXIV | Iocaste | 2.6 | 21,060,000 | 631.60 (r) | 2001 | Sheppard, Jewitt, Fernández, Magnier, Dahm, Evans | Retrograde irregular (Ananke) | 121329 | Jupiter | |
| XXV | Erinome | 1.6 | 23,196,000 | 728.46 (r) | 2001 | Sheppard, Jewitt, Fernández, Magnier, Dahm, Evans | Retrograde irregular (Carme) | 121329 | Jupiter | |
| XXVI | Isonoe | 1.9 | 23,155,000 | 726.23 (r) | 2001 | Sheppard, Jewitt, Fernández, Magnier, Dahm, Evans | Retrograde irregular (Carme) | 121329 | Jupiter | |
| XXVII | Praxidike | 3.4 | 20,908,000 | 625.39 (r) | 2001 | Sheppard, Jewitt, Fernández, Magnier, Dahm, Evans | Retrograde irregular (Ananke) | 121329 | Jupiter | |
| XXVIII | Autonoe | 2.0 | 24,046,000 | 760.95 (r) | 2002 | Sheppard, Jewitt, Kleyna | Retrograde irregular (Pasiphae) | 121330 | Jupiter | |
| XXIX | Thyone | 2.0 | 20,939,000 | 627.21 (r) | 2002 | Sheppard, Jewitt, Kleyna | Retrograde irregular (Ananke) | 121330 | Jupiter | |
| XXX | Hermippe | 2.0 | 21,131,000 | 633.9 (r) | 2002 | Sheppard, Jewitt, Kleyna | Retrograde irregular (Ananke?) | 121330 | Jupiter | |
| XXXI | Aitne | 1.5 | 23,229,000 | 730.18 (r) | 2002 | Sheppard, Jewitt, Kleyna | Retrograde irregular (Carme) | 121330 | Jupiter | |
| XXXII | Eurydome | 1.5 | 22,865,000 | 717.33 (r) | 2002 | Sheppard, Jewitt, Kleyna | Retrograde irregular (Pasiphae?) | 121330 | Jupiter | |
| XXXIII | Euanthe | 1.5 | 20,797,000 | 620.49 (r) | 2002 | Sheppard, Jewitt, Kleyna | Retrograde irregular (Ananke) | 121330 | Jupiter | |
| XXXIV | Euporie | 1.0 | 19,304,000 | 550.74 (r) | 2002 | Sheppard, Jewitt, Kleyna | Retrograde irregular (Ananke) | 121330 | Jupiter | |
| XXXV | Orthosie | 1.0 | 20,720,000 | 622.56 (r) | 2002 | Sheppard, Jewitt, Kleyna | Retrograde irregular (Ananke) | 121330 | Jupiter | |
| XXXVI | Sponde | 1.0 | 23,487,000 | 748.34 (r) | 2002 | Sheppard, Jewitt, Kleyna | Retrograde irregular (Pasiphae) | 121330 | Jupiter | |
| XXXVII | Kale | 1.0 | 23,217,000 | 729.47 (r) | 2002 | Sheppard, Jewitt, Kleyna | Retrograde irregular (Carme) | 121330 | Jupiter | |
| XXXVIII | Pasithee | 1.0 | 23,004,000 | 719.44 (r) | 2002 | Sheppard, Jewitt, Kleyna | Retrograde irregular (Carme) | 121330 | Jupiter | |
| XXXIX | Hegemone | 1.5 | 23,577,000 | 739.88 (r) | 2003 | Sheppard, Jewitt, Kleyna, Fernández | Retrograde irregular (Pasiphae) | 1213 | Jupiter | |
| XL | Mneme | 1.0 | 21,035,000 | 620.04 (r) | 2003 | Gladman, Allen | Retrograde irregular (Ananke) | 1213 | Jupiter | |
| XLI | Aoede | 2.0 | 23,980,000 | 761.50 (r) | 2003 | Sheppard, Jewitt, Kleyna, Fernández, Hsieh | Retrograde irregular (Pasiphae) | 1213 | Jupiter | |
| XLII | Thelxinoe | 1.0 | 21,164,000 | 628.09 (r) | 2003 | Sheppard, Jewitt, Kleyna, Gladman, Kavelaars, Petit, Allen | Retrograde irregular (Ananke) | 1213 | Jupiter | |
| XLIII | Arche | 1.5 | 23,355,000 | 731.95 (r) | 2002 | Sheppard, Meech, Hsieh, Tholen, Tonry | Retrograde irregular (Carme) | 121330 | Jupiter | |
| XLIV | Kallichore | 1.0 | 23,288,000 | 728.73 (r) | 2003 | Sheppard, Jewitt, Kleyna, Fernández | Retrograde irregular (Carme?) | 1213 | Jupiter | |
| XLV | Helike | 2.0 | 21,069,000 | 626.32 (r) | 2003 | Sheppard, Jewitt, Kleyna, Fernández, Hsieh | Retrograde irregular (Ananke) | 1213 | Jupiter | |
| XLVI | Carpo | 1.5 | 17,058,000 | 456.30 | 2003 | Sheppard, Gladman, Kavelaars, Petit, Allen, Jewitt, Kleyna | Prograde irregular | 1213 | Jupiter | |
| XLVII | Eukelade | 2.0 | 23,328,000 | 730.47 (r) | 2003 | Sheppard, Jewitt, Kleyna, Fernández, Hsieh | Retrograde irregular (Carme) | 1213 | Jupiter | |
| XLVIII | Cyllene | 1.0 | 23,809,000 | 752 (r) | 2003 | Sheppard, Jewitt, Kleyna | Retrograde irregular (Pasiphae) | 1213 | Jupiter | |
| XLIX | Kore | 1.0 | 24,543,000 | 779.17 (r) | 2003 | Sheppard, Jewitt, Kleyna | Retrograde irregular (Pasiphae) | 1213 | Jupiter | |
| — | S/2000 J 11 | 2.0 | 12,555,000 | 287 | 2000 | Sheppard, Jewitt, Fernández, Magnier, Dahm, Evans | Prograde irregular | 3132 | Jupiter | |
| — | S/2003 J 2 | 1.0 | 28,455,000 | 981.55 (r) | 2003 | Sheppard, Jewitt, Kleyna, Fernández, Hsieh | Retrograde irregular | 1213 | Jupiter | |
| — | S/2003 J 3 | 1.0 | 20,224,000 | 583.88 (r) | 2003 | Sheppard, Jewitt, Kleyna, Fernández, Hsieh | Retrograde irregular (Ananke) | 1213 | Jupiter | |
| — | S/2003 J 4 | 1.0 | 23,933,000 | 755.26 (r) | 2003 | Sheppard, Jewitt, Kleyna, Fernández, Hsieh | Retrograde irregular (Pasiphae) | 1213 | Jupiter | |
| — | S/2003 J 5 | 2.0 | 23,498,000 | 738.74 (r) | 2003 | Sheppard, Jewitt, Kleyna, Fernández, Hsieh | Retrograde irregular (Carme) | 1213 | Jupiter | |
| — | S/2003 J 9 | 0.5 | 23,388,000 | 733.30 (r) | 2003 | Sheppard, Jewitt, Kleyna, Fernández | Retrograde irregular (Carme) | 1213 | Jupiter | |
| — | S/2003 J 10 | 1.0 | 23,044,000 | 716.25 (r) | 2003 | Sheppard, Jewitt, Kleyna, Fernández | Retrograde irregular (Pasiphae?) | 1213 | Jupiter | |
| — | S/2003 J 12 | 0.5 | 17,833,000 | 489.72 (r) | 2003 | Sheppard, Jewitt, Kleyna, Fernández | Retrograde irregular (Ananke) | 1213 | Jupiter | |
| — | S/2003 J 15 | 1.0 | 22,630,000 | 689.77 (r) | 2003 | Sheppard, Jewitt, Kleyna, Fernández | Retrograde irregular (Ananke?) | 1213 | Jupiter | |
| — | S/2003 J 16 | 1.0 | 20,956,000 | 616.33 (r) | 2003 | Gladman, Sheppard, Jewitt, Kleyna, Kavelaars, Petit, Allen | Retrograde irregular (Ananke) | 1213 | Jupiter | |
| — | S/2003 J 17 | 1.0 | 22,983,000 | 714.51 (r) | 2003 | Gladman, Sheppard, Jewitt, Kleyna, Kavelaars, Petit, Allen | Retrograde irregular (Carme) | 1213 | Jupiter | |
| — | S/2003 J 18 | 1.0 | 20,426,000 | 596.58 (r) | 2003 | Gladman, Sheppard, Jewitt, Kleyna, Kavelaars, Petit, Allen | Retrograde irregular (Ananke) | 1213 | Jupiter | |
| — | S/2003 J 19 | 1.0 | 23,535,000 | 740.43 (r) | 2003 | Gladman, Sheppard, Jewitt, Kleyna, Kavelaars, Petit, Allen | Retrograde irregular (Carme) | 1213 | Jupiter | |
| — | S/2003 J 23 | 1.0 | 23,566,000 | 732.45 (r) | 2004 | Sheppard, Jewitt, Kleyna, Fernández | Retrograde irregular (Pasiphae) | 1213 | Jupiter | |
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I | Mimas | 198.8 ± 1.5 | 185,540 | 0.942 | 1789 | Herschel | Main group moon | 1213 | Saturn |
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II | Enceladus | 252.3 ± 0.6 | 238,040 | 1.370 | 1789 | Herschel | Main group moon | 1213 | Saturn |
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III | Tethys | 536.3 ± 1.5 | 294,670 | 1.888 | 1684 | Cassini | Main group moon | 1213 | Saturn |
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IV | Dione | 562.5 ± 1.5 | 377,420 | 2.737 | 1684 | Cassini | Main group moon | 1213 | Saturn |
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V | Rhea | 764.5 ± 2.0 | 527,070 | 4.518 | 1672 | Cassini | Main group moon | 1213 | Saturn |
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VI | Titan | 2,575.5 ± 2.0 | 1,221,870 | 15.95 | 1655 | Huygens | Main group moon | 1213 | Saturn |
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VII | Hyperion | 133.0 ± 8.0 | 1,500,880 | 21.28 | 1848 | W.Bond, G. Bond, and Lassell | Main group moon | 1213 | Saturn |
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VIII | Iapetus | 734.5 ± 4.0 | 3,560,840 | 79.33 | 1671 | Cassini | Main group moon | 1213 | Saturn |
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IX | Phoebe | 106.6 ± 1.1 | 12,947,780 | 550.31 (r) | 1899 | Pickering | Retrograde irregular (Norse) | 1213 | Saturn |
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X | Janus | 90.4 ± 3.0 | 151,460 | 0.695 | 1966 | Dollfus; Voyager 1 | Inner moon (co-orbital) | 1213 | Saturn |
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XI | Epimetheus | 58.3 ± 3.1 | 151,410 | 0.694 | 1980 | Walker; Voyager 1 | Inner moon (co-orbital) | 1213 | Saturn |
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XII | Helene | 16 ± 4 | 377,420 | 2.737 | 1980 | Laques, Lecacheux | Main group trojan | 1213 | Saturn |
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XIII | Telesto | 12 ± 3 | 294,710 | 1.888 | 1980 | Smith, Reitsema, Larson, Fountain (Voyager 1) | Main group trojan | 1213 | Saturn |
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XIV | Calypso | 9.5 ± 1.5 | 294,710 | 1.888 | 1980 | Pascu, Seidelmann, Baum, Currie | Main group trojan | 1213 | Saturn |
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XV | Atlas | 10 | 137,670 | 0.602 | 1980 | Terrile (Voyager 1) | Inner moon (shepherd) | 1213 | Saturn |
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XVI | Prometheus | 46.8 ± 5.6 | 139,380 | 0.613 | 1980 | Collins (Voyager 1) | Inner moon (shepherd) | 1213 | Saturn |
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XVII | Pandora | 40.6 ± 4.5 | 141,720 | 0.629 | 1980 | Collins (Voyager 1) | Inner moon (shepherd) | 1213 | Saturn |
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XVIII | Pan | 12.8 | 133,580 | 0.575 | 1990 | Showalter (Voyager 2) | Inner moon (shepherd) | 1213 | Saturn |
| XIX | Ymir | 9 | 23,040,000 | 1,315.14 (r) | 2000 | Gladman | Retrograde irregular (Norse) | 1213 | Saturn | |
| XX | Paaliaq | 11 | 15,200,000 | 686.95 | 2000 | Gladman | Prograde irregular (Inuit) | 1213 | Saturn | |
| XXI | Tarvos | 7.5 | 17,983,000 | 926.23 | 2000 | Gladman, Kavelaars | Prograde irregular (Gallic) | 1213 | Saturn | |
| XXII | Ijiraq | 6 | 11,124,000 | 451.42 | 2000 | Gladman, Kavelaars | Prograde irregular (Inuit) | 1213 | Saturn | |
| XXIII | Suttungr | 3.5 | 19,459,000 | 1,016.67 (r) | 2000 | Gladman, Kavelaars | Retrograde irregular (Norse) | 1213 | Saturn | |
| XXIV | Kiviuq | 8 | 11,110,000 | 449.22 | 2000 | Gladman | Prograde irregular (Inuit) | 1213 | Saturn | |
| XXV | Mundilfari | 3.5 | 18,628,000 | 952.77 (r) | 2000 | Gladman, Kavelaars | Retrograde irregular (Norse) | 1213 | Saturn | |
| XXVI | Albiorix | 16 | 16,182,000 | 783.45 | 2000 | Holman, Spahr | Prograde |