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A natural satellite is an object that orbits a planet or other body larger than itself and which is not man-made. Such objects are often called moons. Technically, the term could also refer to a planet orbiting a star, or even to a star orbiting a galactic center, but these uses are rare. Instead, the term is normally used to identify non-artificial satellites of planets or minor planets.

There are 240 known moons within the Solar system, including 162 orbiting the planets, 4 orbiting dwarf planets, and dozens more orbiting small solar system bodies. Other stars and their planets also have natural satellites.

The large gas giants have extensive systems of moons, including half a dozen comparable in size to Earth's moon. Of the inner planets, Mercury and Venus have no moons at all; Earth has one large moon ("the Moon" or "Luna"); and Mars has two tiny moons, Phobos and Deimos. Among the dwarf planets, Ceres has no moons (though many objects in the asteroid belt do), Eris has one, Dysnomia, and Pluto has three known satellites, Nix, Hydra, and a large companion called Charon. Charon is also unusual in that it does not orbit Pluto, but rather both bodies orbit a barycenter in empty space, making Pluto-Charon a binary system.

Origin

Moons orbiting relatively close to the planet on prograde orbits (regular satellites) are believed to have been formed out of the same collapsing region of protoplanetary disk that gave rise to its primary. Most irregular satellites (orbiting on distant, inclined, eccentric often retrograde orbits) are thought to be captured asteroids possibly further fragmented by collisions. Many special scenarios exist, including origin in a break-up of the planet. For example, in the case of Earth's Moon, a portion of the planet itself might have been blasted into orbit by a large impact. A very different scenario required to explain the origin of the large Neptunian satellites: circular but retrograde orbit of Triton and extremely eccentric but prograde orbit of Nereid.

Orbital characteristics

Tidal locking

Most regular moons in the solar system are tidally locked to their primaries, meaning that one side of the moon is always turned toward the planet. Exceptions include Saturn's moon Hyperion, which rotates chaotically because of a variety of external influences.

In contrast, the outer moons of the gas giants (irregular satellites) are too far away to become 'locked'. For example Jupiter's moon Himalia, Saturn's moon Phoebe and Neptune's Nereid have rotation period in the range of 10 hours to compare with their orbital periods of hundreds of days.

Satellites of satellites

No "moons of moons" (natural satellites that orbit the natural satelite of another body) are known. It is uncertain whether such objects can be stable in the long term. In most cases, the tidal effects of their primaries make such a system unstable; the gravity from other nearby objects (most notably the primary) would perturb the orbit of the moon's moon until it broke away or impacted its primary. In theory, a secondary satellite could exist in a primary satellite's Hill sphere, outside of which it would be lost due to the greater gravitational pull of the planet (or other object) that the primary satellite orbits. For example, the Moon orbits the Earth because the Moon is 370,000 km from Earth, well within Earth's Hill sphere, which has a radius of 1.5 million km (0.01 AU or 235 Earth radii). If a Moon-sized object were to orbit the Earth outside its Hill sphere, it would soon be captured by the Sun and become a dwarf planet in a near-Earth orbit.

Trojan satellites

However, two moons are known have small companions in their L₄ and L₅ Lagrangian points of their orbits, about sixty degrees ahead of and behind their positions. These companions are called Trojan moons, because their positions are comparable to the positions of the Trojan asteroids relative to Jupiter. They are Telesto and Calypso, the leading and following companions respectively of Tethys; and Helene and Polydeuces, the leading and following companions of Dione).

Asteroid satellites

The recent discovery of 243 Ida's moon Dactyl confirms that some asteroids also have moons. Some, like 90 Antiope, are double asteroids with two equal-sized components. The asteroid 87 Sylvia has two moons. See asteroid moon for further information.

Moons of the Solar system

The largest moons in the solar system (those bigger than about 3000 km across) are Earth's Moon, Jupiter's Galilean moons (Io, Europa, Ganymede, and Callisto), Saturn's moon Titan, Neptune's captured moon Triton and Button Moon. For smaller moons see the articles on the appropriate planet. In addition to the moons of the various planets there are also over 80 known moons 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 moons of the solar system by diameter. The column on the right includes some notable planets, dwarf planets, asteroids, and Trans-Neptunian Objects for comparison.

The definition of a moon

There has been some debate about the precise definition of a moon. This debate has been caused by the presence of orbital systems where the difference in mass between the larger body and its satellite, are not as pronounced as in normal systems (a commonly accepted defining point is where the satellite is larger than 1/40th the mass of the larger body) . Two examples include the Pluto-Charon system, and the Earth-Luna System. The presence of these systems has caused a debate about where to precisely draw the line between a double body system, and a main body-satellite system, the most commonly agreed upon definition is whether the barycentre is below the surface of the larger body, though this is not official. Another definition is whether the Sun or the larger body in the system has the most gravitational influence over the satellite in question. At the other end of the spectrum there are many ice/rock clumps that form the ring systems on the Solar System's gas giants and there is no set point to define when one of these clumps is large enough to be classified as a moon, though the term moonlet may be sometimes used to refer to extremely small objects in orbit around a larger body, there is again no official definition.

See also

• Asteroid moon
• Extrasolar moon
• Inner satellite
• Irregular satellite
• Naming of natural satellites
• Quasi-satellite
• Timeline of discovery of Solar System planets and their natural satellites
• Trojan moon

Natural satellites of planets and dwarf planets

• Earth's natural satellite, see Moon
• Mars' natural satellites
• Jupiter's natural satellites
• Saturn's natural satellites
• Uranus' natural satellites
• Neptune's natural satellites
• Pluto's natural satellites
• Eris' natural satellite, see Dysnomia

Notes and references

1. List of natural satellites orbiting the planets.
2. This column lists objects that are moons of small solar system bodies, not small solar system bodies themselves.
3. Also known as "Luna".
4. ^ Diameters of the new Plutonian satellites are still very poorly known, but they are estimated to lie between 44 and 130 km.
5. (617) Patroclus I Menoetius
6. (22) Kalliope I Linus
7. (87) Sylvia I Romulus
8. (45) Eugenia I Petit-Prince

External links

Jupiter's moons

• Data on Jupiter's satellites
• Jupiter's new moons (discovered in 2000)
• Jupiter's new moons (discovered in 2002)
• Jupiter's new moons (discovered in 2003)

Saturn's moons

• Saturn's new moons (discovered in 2000)
• Saturn's new moon (discovered in 2003)

Neptune's moons

• Neptune's new moons (discovered in 2003)

All moons

• Natural Satellite Physical Parameters (JPL-NASA, with refs)
• Moons of the Solar System (The Planetary Society)
• Scott Sheppard's page
• Major moons in order from the Sun
• JPL's Solar System Dynamics page
• Moon of an Object? First Photo of Satellite Beyond the Solar System
• USGS list of named moons
• Upper size limit for moons explained
• Asteroids with Satellites

v t e Natural satellites of the Solar System
v t e Moons of terrestrial planets and asteroids Earth Mars Asteroid moons The Moon Phobos Deimos Binaries List
v t e Moons of Jupiter Listed in increasing approximate distance from Jupiter Inner moons Metis Adrastea Amalthea Thebe Galilean moons Io Europa Ganymede Callisto Themisto Themisto Himalia group (9) Leda Ersa S/2018 J 2 Himalia Pandia Lysithea Elara S/2011 J 3 Dia Carpo group (2) S/2018 J 4 Carpo Valetudo Valetudo Ananke group (26) Euporie Jupiter LV Eupheme S/2021 J 3 Jupiter LII Jupiter LIV Mneme Euanthe S/2003 J 16 Harpalyke Orthosie S/2022 J 3 Helike S/2021 J 2 Praxidike Jupiter LXIV S/2021 J 1 S/2003 J 12 Jupiter LXVIII Thelxinoe Thyone S/2003 J 2 Ananke Iocaste Hermippe Jupiter LXX Carme group (30) S/2022 J 1 S/2022 J 2 S/2016 J 3 Pasithee Jupiter LXIX S/2021 J 6 S/2003 J 24 Chaldene Jupiter LXIII Isonoe S/2021 J 4 Kallichore Erinome Kale Eirene Aitne Eukelade Arche Taygete Jupiter LXXII Carme Herse Jupiter LXI Jupiter LI S/2003 J 9 Jupiter LXVI Kalyke S/2018 J 3 S/2021 J 5 S/2003 J 10 Pasiphae group (18) Philophrosyne Eurydome Jupiter LVI S/2003 J 4 S/2016 J 4 Jupiter LXVII Hegemone Pasiphae Sponde Megaclite Cyllene Sinope Jupiter LIX Aoede Autonoe Callirrhoe S/2003 J 23 Kore See also Rings of Jupiter Jupiter's moons in fiction Category
v t e Moons of Saturn Listed in approximate increasing distance from Saturn Ring moonlets S/2009 S 1 Aegaeon Ring shepherds Pan Daphnis Prometheus Epimetheus Janus Other inner moons Atlas Pandora Alkyonides Methone Anthe Pallene Large moons (with trojans) Mimas Enceladus Tethys Telesto Calypso Dione Helene Polydeuces Rhea Titan Hyperion Iapetus Inuit group (13) Kiviuq subgroup S/2019 S 1 Kiviuq S/2005 S 4 S/2020 S 1 Ijiraq Paaliaq subgroup Paaliaq Siarnaq subgroup S/2004 S 31 Tarqeq S/2019 S 14 Siarnaq S/2020 S 3 S/2019 S 6 S/2020 S 5 Gallic group (7) Albiorix Bebhionn S/2007 S 8 Saturn LX Erriapus Tarvos S/2020 S 4 Norse group (100) Phoebe subgroup Phoebe S/2006 S 20 S/2006 S 9 Skathi S/2007 S 5 S/2007 S 7 S/2007 S 2 S/2004 S 37 S/2004 S 47 S/2004 S 40 S/2019 S 2 S/2019 S 3 S/2020 S 7 Skoll S/2020 S 2 S/2019 S 4 S/2004 S 41 S/2004 S 42 Hyrrokkin Greip S/2004 S 13 S/2007 S 6 Mundilfari S/2006 S 1 S/2004 S 43 S/2006 S 10 S/2019 S 5 Gridr Bergelmir Jarnsaxa Narvi Suttungr S/2004 S 44 S/2007 S 3 S/2004 S 45 Hati S/2004 S 17 S/2006 S 11 S/2004 S 12 Eggther S/2006 S 13 S/2007 S 9 S/2019 S 7 S/2019 S 8 Farbauti Thrymr Bestla S/2019 S 9 S/2004 S 46 Angrboda S/2019 S 11 Aegir S/2019 S 10 Beli S/2019 S 12 Gerd S/2019 S 13 S/2006 S 14 Gunnlod S/2019 S 15 S/2020 S 6 S/2004 S 7 S/2006 S 3 S/2005 S 5 Skrymir S/2006 S 16 S/2006 S 15 S/2004 S 28 S/2020 S 8 Alvaldi Kari S/2004 S 48 Geirrod Fenrir S/2004 S 50 S/2006 S 17 S/2004 S 49 S/2019 S 17 Surtur S/2006 S 18 Loge Ymir S/2019 S 19 S/2019 S 18 S/2004 S 21 S/2004 S 39 S/2019 S 16 S/2004 S 53 S/2004 S 36 Thiazzi S/2019 S 20 S/2006 S 19 Saturn LXIV Fornjot S/2004 S 51 S/2020 S 10 S/2020 S 9 Saturn LVIII S/2019 S 21 S/2004 S 52 Outlier prograde irregular moons S/2006 S 12 S/2004 S 24 Rings of Saturn Cassini–Huygens Chiron Chrysalis Themis S/2004 S 3 S/2004 S 4 S/2004 S 6 Peggy Bleriot Moonlet In fiction
v t e Moons of Uranus Listed in approximately increasing distance from Uranus Inner Rings of Uranus Cordelia Ophelia Bianca Cressida Desdemona Juliet Portia Rosalind Cupid Belinda Perdita Puck Mab Major (spheroid) Miranda Ariel Umbriel Titania Oberon Outer (irregular) Prograde Margaret Retrograde Francisco Caliban Stephano S/2023 U 1 Trinculo Sycorax Prospero Setebos Ferdinand Geological features Arielian Kachina Chasmata Yangoor Mirandian Verona Rupes Oberonian Hamlet Mommur Chasma Puckian Titanian Gertrude Messina Chasmata Ursula Rousillon Rupes Umbrielian Wunda Vuver Skynd
v t e Moons of Neptune Listed in approximately increasing distance from Neptune Regular (inner) Naiad Thalassa Despina Galatea Larissa Hippocamp Proteus Irregular Triton Triton atmosphere climate geology Prograde Nereid Sao S/2002 N 5 Laomedeia Retrograde Halimede Psamathe Neso S/2021 N 1 See also Neptune trojans Rings of Neptune Category Portal
v t e Moons of likely dwarf planets Pluto Haumea Eris Makemake Gonggong Charon Styx Nix Kerberos Hydra ring Namaka Hiʻiaka Dysnomia S/2015 (136472) 1 Xiangliu Quaoar Orcus Salacia Varda 2013 FY27 rings Weywot Vanth Actaea Ilmarë 2013 FY27 I
v t e Natural satellites of the Solar System Planetary satellites of Earth Mars Jupiter Saturn Uranus Neptune Dwarf planet satellites of Orcus Pluto Haumea Quaoar Makemake Gonggong Eris Minor-planet moons Near-Earth Florence Didymos Dimorphos Moshup Squannit 1994 CC 2001 SN263 Main belt Kalliope Linus Euphrosyne Daphne Peneius Eugenia Petit-Prince Sylvia Romulus Remus Minerva Aegis Gorgoneion Camilla Elektra Kleopatra Alexhelios Cleoselene Ida Dactyl Roxane Olympias Pulcova Balam Dinkinesh (Selam) Jupiter trojans Patroclus Menoetius Hektor Skamandrios Eurybates Queta TNOs Lempo Hiisi Paha 2002 UX25 Sila–Nunam Salacia Actaea Varda Ilmarë Gǃkúnǁʼhòmdímà Gǃòʼé ǃHú 2013 FY27 Ranked by size Ganymede largest: 5268 km / 0.413 Earths Titan Callisto Io Moon Europa Triton Titania Rhea Oberon Iapetus Charon Umbriel Ariel Dione Tethys Dysnomia Enceladus Miranda Vanth Proteus Mimas Ilmarë Nereid Hiʻiaka Actaea Hyperion Phoebe ... Discovery timeline Inner moons Irregular moons List Planetary-mass moons Naming Subsatellite Regular moons Trojan moons
Discovery timeline Inner moons Irregular moons List Naming Subsatellite Regular moons Trojan moons

v t e Solar System
Sun Mercury Venus Earth Mars Ceres Jupiter Saturn Uranus Neptune Orcus Pluto Haumea Quaoar Makemake Gonggong Eris Sedna
Planets, dwarfs, minors Terrestrials Mercury Venus Earth Mars Giants Gas Jupiter Saturn Ice Uranus Neptune Dwarfs Ceres Orcus Pluto Haumea Quaoar Makemake Gonggong Eris Sedna Large Minor Planets Salacia Varuna Ixion List Moons Earth Moon Claimed Mars Phobos Deimos Jupiter Ganymede Callisto Io Europa all 95 Saturn Titan Rhea Iapetus Dione Tethys Enceladus Mimas Hyperion Phoebe all 146 Uranus Titania Oberon Umbriel Ariel Miranda all 28 Neptune Triton Proteus Nereid all 16 Pluto Charon Nix Hydra Kerberos Styx Orcus Vanth Haumea Hiʻiaka Namaka Quaoar Weywot Makemake S/2015 (136472) 1 Gonggong Xiangliu Eris Dysnomia Formation, evolution, contents, and History Star formation Accretion Accretion disk Excretion Capture theory Capture of Triton Circumplanetary disk Circumstellar disc Circumstellar envelope Coatlicue Cosmic dust Debris disk Detached object Disk instability EXCEDE Exozodiacal dust Extraterrestrial materials Curation Sample-return mission Frost/Ice/Snow line Giant-impact hypothesis Grand tack hypothesis Gravitational collapse Hills cloud Hill sphere Interplanetary dust cloud Interplanetary medium/space Interstellar cloud Interstellar medium Interstellar space Kuiper belt Kuiper cliff Late Heavy Bombardment Molecular cloud Nebular hypothesis Nice model Nice 2 model Five-planet Nice model Oort cloud Outer space Planet Disrupted Migration System Planetesimal Formation Merging stars Protoplanetary disk Ring system Roche limit vs. Hill sphere Rubble pile Scattered disc Co-orbital configuration Saturn Moons Kordylewski cloud Rings Earth? Jovian Saturnian (Rhean?) Charikloan Chironean Uranian Neptunian Haumean Quaoarian Hypothetical objects Bagby's moon Chiron Coatlicue Counter-Earth Chrysalis Fifth Giant Hyperion Lilith Mercury's moon Neith Nemesis Nibiru Petit's moon Phaeton Planet Nine Planet Ten Planet V Planet X Subsatellites Synestia Theia Themis Tyche Vulcan Vulcanoids Waltemath's moons Exploration (outline) Colonization Discovery astronomy historical models timeline Space probes timeline list Human spaceflight space stations list programs Mercury Venus Moon mining Mars Ceres Asteroids mining Comets Jupiter Saturn Uranus Neptune Pluto Deep space Small Solar System bodies Comets Damocloids Meteoroids Minor planets names and meanings moons Planetesimal Planetary orbit-crossers Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Trojans Venus Earth Mars Jupiter Trojan camp Greek camp Saturn Uranus Neptune Near-Earth objects NEAs Asteroid belt Asteroids Ceres Vesta Pallas Hygiea active List families PHA exceptional Kirkwood gap Centaurs Trans-Neptunian objects Kuiper belt Cubewanos Plutinos Detached objects Sednoids Scattered disc Hills cloud Oort cloud Lists Comets Possible dwarf planets Gravitationally rounded objects Minor planets Natural satellites Solar System models Solar System objects by size by discovery date Interstellar and circumstellar molecules Related Double planet Lagrange point Moonlet Syzygy Tidal locking
Outline of the Solar System Solar System portal Astronomy portal Earth sciences portal Solar System → Local Interstellar Cloud → Local Bubble → Gould Belt → Orion Arm → Milky Way → Milky Way subgroup → Local Group → Local Sheet → Virgo Supercluster → Laniakea Supercluster → Local Hole → Observable universe → Universe Each arrow (→) may be read as "within" or "part of".

• Moons