--- title: Natural satellite summary: A moon, a natural body orbiting a planet, dwarf planet or asteroid. The Solar System's planets have more than 430 known moons, from kilometre-sized captured fragments to Ganymede, which is larger than Mercury. science_status: [observed, model, sim] categories: [Moons, Moon types, Moons of the Solar System, Planetary systems] aliases: [Moon types, Moons, Satellite, Regular moon, Irregular moon, Exomoon] infobox: type: moon_class name: Natural satellite (moon) definition: "A natural body in a bound orbit around a planet, dwarf planet or smaller body, inside its host's Hill sphere and outside its Roche limit." size_range_km: {min: 1, max: 5262, note: "kilometre-scale irregular moons to Ganymede (diameter)"} largest: "Ganymede, 2,631 km radius, 0.025 Earth masses" solar_system_count: as_of: 2026-09 Mercury: 0 Venus: 0 Earth: 1 Mars: 2 Jupiter: 115 Saturn: 293 Uranus: 29 Neptune: 16 total_planetary: 456 note: "Counts rise as surveys find more small irregular moons; always date them" regular_vs_irregular: "Regular moons orbit close in, prograde, on near-circular orbits near the planet's equator, and formed with it. Irregular moons orbit far out on eccentric, inclined and often retrograde orbits, and were captured. More than 90 percent of Jupiter's and Saturn's known moons are irregulars a few kilometres across." formation_channels: [circumplanetary disk accretion, giant impact, capture] satellite_mass_fraction: "about 1e-4 of the planet for the regular moons of gas and ice giants (Jupiter 2.1e-4, Saturn 2.5e-4, Uranus 1.0e-4); Earth's Moon 0.012; Charon 0.12 of Pluto" exomoons: "None confirmed as of September 2026; Kepler-1625 b-i and Kepler-1708 b-i are disputed candidates" sim_types: - {code: M0-B1, name: "Barren rocky", analogue: "the Moon"} - {code: M0-B2, name: "Iron-rich", analogue: "none known"} - {code: M1-A, name: "Ancient ice", analogue: Callisto} - {code: M1-C, name: "Cryovolcanic ice", analogue: Enceladus} - {code: M1-E, name: "Exotic nitrogen and CO ice", analogue: Triton} - {code: M1-M, name: "Mixed ice and rock", analogue: Ganymede} - {code: M2-V, name: "Volcanic", analogue: Io} - {code: M2-M, name: "Magma ocean", analogue: "none known"} - {code: M3-A, name: "Captured asteroid", analogue: Phobos} - {code: M3-K, name: "Captured Kuiper belt object", analogue: Phoebe} - {code: M3-R, name: "Roche-disrupting", analogue: "Phobos in its final tens of millions of years"} - {code: M4-A, name: "Thick atmosphere", analogue: Titan} - {code: M4-O, name: "Subsurface ocean", analogue: Europa} - {code: M4-T, name: "Thin atmosphere", analogue: "none known"} - {code: "M5-G, M5-H, M5-W", name: "Super-moons of very massive giants", analogue: "none known (speculative)"} examples: [the Moon, Io, Europa, Ganymede, Callisto, Titan, Enceladus, Triton, Charon, Phobos] sim: entity: moon_class refs: - id: iau2026 type: webpage author: [{literal: International Astronomical Union}] title: "IAU Minor Planet Center Confirms New Moons of Saturn and Jupiter" container-title: IAU announcements issued: 2026-03-26 URL: https://www.iau.org/IAU/IAU/News/Ann2026/MPC-New-Moons-Saturn-Jupiter.aspx accessed: 2026-09-27 - id: nasa_jupiter_moons type: webpage author: [{literal: "NASA"}] title: "Jupiter Moons" container-title: NASA Science URL: https://science.nasa.gov/jupiter/jupiter-moons/ accessed: 2026-09-27 note: "115 moons officially recognised by the IAU; page updated 21 September 2026" - id: nasa_saturn_moons type: webpage author: [{literal: "NASA"}] title: "Saturn Moons" container-title: NASA Science URL: https://science.nasa.gov/saturn/moons/ accessed: 2026-09-27 note: "293 confirmed moons as of August 2026; page updated 22 September 2026" - id: carnegie2024 type: webpage author: [{literal: Carnegie Science}] title: "New moons of Uranus and Neptune announced" issued: 2024-02-23 URL: https://carnegiescience.edu/new-moons-uranus-and-neptune-announced accessed: 2026-09-27 - id: webb2025 type: webpage author: [{literal: ESA/Webb}] title: "New moon of Uranus" issued: 2025-08-19 URL: https://esawebb.org/images/uranus-moon-S2025U1/ accessed: 2026-09-27 - id: jplsats type: webpage author: [{literal: JPL Solar System Dynamics}] title: "Planetary Satellite Physical Parameters" URL: https://ssd.jpl.nasa.gov/sats/phys_par/ accessed: 2026-09-27 - id: canup2006 type: article-journal author: [{family: Canup, given: Robin M.}, {family: Ward, given: William R.}] title: "A common mass scaling for satellite systems of gaseous planets" container-title: Nature volume: 441 page: 834-839 issued: 2006 DOI: 10.1038/nature04860 - id: canup2001 type: article-journal author: [{family: Canup, given: Robin M.}, {family: Asphaug, given: Erik}] title: "Origin of the Moon in a giant impact near the end of the Earth's formation" container-title: Nature volume: 412 page: 708-712 issued: 2001 DOI: 10.1038/35089010 - id: canup2005 type: article-journal author: [{family: Canup, given: Robin M.}] title: "A Giant Impact Origin of Pluto-Charon" container-title: Science volume: 307 page: 546-550 issued: 2005 DOI: 10.1126/science.1106818 - id: jewitt2007 type: article-journal author: [{family: Jewitt, given: David}, {family: Haghighipour, given: Nader}] title: "Irregular Satellites of the Planets: Products of Capture in the Early Solar System" container-title: Annual Review of Astronomy and Astrophysics volume: 45 page: 261-295 issued: 2007 DOI: 10.1146/annurev.astro.44.051905.092459 - id: agnor2006 type: article-journal author: [{family: Agnor, given: Craig B.}, {family: Hamilton, given: Douglas P.}] title: "Neptune's capture of its moon Triton in a binary-planet gravitational encounter" container-title: Nature volume: 441 page: 192-194 issued: 2006 DOI: 10.1038/nature04792 - id: rosenblatt2016 type: article-journal author: [{family: Rosenblatt, given: Pascal}, {family: Charnoz, given: Sebastien}, {family: Dunseath, given: Kevin M.}, {family: others}] title: "Accretion of Phobos and Deimos in an extended debris disc stirred by transient moons" container-title: Nature Geoscience volume: 9 page: 581-583 issued: 2016 DOI: 10.1038/ngeo2742 - id: black2015 type: article-journal author: [{family: Black, given: Benjamin A.}, {family: Mittal, given: Tushar}] title: "The demise of Phobos and development of a Martian ring system" container-title: Nature Geoscience volume: 8 page: 913-917 issued: 2015 DOI: 10.1038/ngeo2583 - id: domingos2006 type: article-journal author: [{family: Domingos, given: R. C.}, {family: Winter, given: O. C.}, {family: Yokoyama, given: T.}] title: "Stable satellites around extrasolar giant planets" container-title: Monthly Notices of the Royal Astronomical Society volume: 373 page: 1227-1234 issued: 2006 DOI: 10.1111/j.1365-2966.2006.11104.x - id: barnes2002 type: article-journal author: [{family: Barnes, given: Jason W.}, {family: O'Brien, given: D. P.}] title: "Stability of Satellites around Close-in Extrasolar Giant Planets" container-title: The Astrophysical Journal volume: 575 page: 1087-1093 issued: 2002 DOI: 10.1086/341477 - id: kane2017 type: article-journal author: [{family: Kane, given: Stephen R.}] title: "Worlds without Moons: Exomoon Constraints for Compact Planetary Systems" container-title: The Astrophysical Journal Letters volume: 839 page: L19 issued: 2017 DOI: 10.3847/2041-8213/aa6bf2 - id: murray1999 type: book author: [{family: Murray, given: Carl D.}, {family: Dermott, given: Stanley F.}] title: "Solar System Dynamics" publisher: Cambridge University Press issued: 1999 DOI: 10.1017/CBO9781139174817 - id: gladman1996 type: article-journal author: [{family: Gladman, given: Brett}, {family: Quinn, given: D. Dane}, {family: Nicholson, given: Philip}, {family: Rand, given: Richard}] title: "Synchronous Locking of Tidally Evolving Satellites" container-title: Icarus volume: 122 page: 166-192 issued: 1996 DOI: 10.1006/icar.1996.0117 - id: williams2016 type: article-journal author: [{family: Williams, given: James G.}, {family: Boggs, given: Dale H.}] title: "Secular tidal changes in lunar orbit and Earth rotation" container-title: Celestial Mechanics and Dynamical Astronomy volume: 126 page: 89-129 issued: 2016 DOI: 10.1007/s10569-016-9702-3 - id: peale1979 type: article-journal author: [{family: Peale, given: S. J.}, {family: Cassen, given: P.}, {family: Reynolds, given: R. T.}] title: "Melting of Io by Tidal Dissipation" container-title: Science volume: 203 page: 892-894 issued: 1979 DOI: 10.1126/science.203.4383.892 - id: lainey2009 type: article-journal author: [{family: Lainey, given: Valery}, {family: Arlot, given: Jean-Eudes}, {family: Karatekin, given: Ozgur}, {family: Van Hoolst, given: Tim}] title: "Strong tidal dissipation in Io and Jupiter from astrometric observations" container-title: Nature volume: 459 page: 957-959 issued: 2009 DOI: 10.1038/nature08108 - id: iess2012 type: article-journal author: [{family: Iess, given: Luciano}, {family: Jacobson, given: Robert A.}, {family: Ducci, given: Marco}, {family: others}] title: "The Tides of Titan" container-title: Science volume: 337 page: 457-459 issued: 2012 DOI: 10.1126/science.1219631 - id: postberg2018 type: article-journal author: [{family: Postberg, given: Frank}, {family: Khawaja, given: Nozair}, {family: Abel, given: Bernd}, {family: others}] title: "Macromolecular organic compounds from the depths of Enceladus" container-title: Nature volume: 558 page: 564-568 issued: 2018 DOI: 10.1038/s41586-018-0246-4 - id: postberg2023 type: article-journal author: [{family: Postberg, given: Frank}, {family: Sekine, given: Yasuhito}, {family: Klenner, given: Fabian}, {family: others}] title: "Detection of phosphates originating from Enceladus's ocean" container-title: Nature volume: 618 page: 489-493 issued: 2023 DOI: 10.1038/s41586-023-05987-9 - id: teachey2018 type: article-journal author: [{family: Teachey, given: Alex}, {family: Kipping, given: David M.}] title: "Evidence for a large exomoon orbiting Kepler-1625b" container-title: Science Advances volume: 4 page: eaav1784 issued: 2018 DOI: 10.1126/sciadv.aav1784 - id: kipping2022 type: article-journal author: [{family: Kipping, given: David}, {family: Bryson, given: Steve}, {family: Burke, given: Chris}, {family: others}] title: "An exomoon survey of 70 cool giant exoplanets and the new candidate Kepler-1708 b-i" container-title: Nature Astronomy volume: 6 page: 367-380 issued: 2022 DOI: 10.1038/s41550-021-01539-1 - id: kipping2020 type: article-journal author: [{family: Kipping, given: David}] title: "An Independent Analysis of the Six Recently Claimed Exomoon Candidates" container-title: The Astrophysical Journal Letters volume: 900 page: L44 issued: 2020 DOI: 10.3847/2041-8213/abafa9 - id: heller2024 type: article-journal author: [{family: Heller, given: René}, {family: Hippke, given: Michael}] title: "Large exomoons unlikely around Kepler-1625 b and Kepler-1708 b" container-title: Nature Astronomy volume: 8 page: 193-206 issued: 2024 DOI: 10.1038/s41550-023-02148-w images_wanted: - file: File:Galilean_satellites_NASA_PIA01400.jpg subject: "Jupiter's four large moons, Io, Europa, Ganymede and Callisto, shown to scale in order of distance from the planet" source: nasa source_page: https://science.nasa.gov/photojournal/the-galilean-satellites-2 credit: "NASA/JPL/DLR" licence: public domain (NASA) - file: File:Halden_Mare_from_orbit_sim.png subject: "Halden Mare, an airless, heavily cratered moon of a gas giant in Pax Abyssi, seen from 1,500 km with dark maria filling old basins" source: sim source_ref: QA:halden_mare_round_four_final_in_game/alt1500km_disc.png note: "prefix legend in content/wiki/_notes/writer-c.md; crop off the DIRECTOR debug line along the bottom edge before use" --- A **natural satellite**, or **moon**, is a body that orbits a planet, a dwarf planet or even an asteroid. As of September 2026 the Solar System's eight planets have 456 known moons, 408 of them around Jupiter and Saturn alone :cite[nasa_jupiter_moons] :cite[nasa_saturn_moons]. They range from captured fragments a kilometre or two across to Ganymede, whose 5,262 km diameter exceeds Mercury's :cite[jplsats]. Some of the most promising places to look for life beyond Earth are moons, and so are the most volcanically active world known and the only moon with a thick atmosphere. ## How moons form Moons come from three main routes, and a moon's orbit usually says which one it took. **Growing in a disk around the planet.** A young giant planet is surrounded by a disk of gas and dust, a small version of the disk around the young Sun, and solid material in it gathers into moons. These are the **regular moons**: they orbit prograde (in the direction the planet spins), on nearly circular orbits close to the planet's equator. The four large moons of Jupiter, Saturn's major moons and the five large moons of Uranus are regular. Canup and Ward showed that the total mass of such a system settles at about one ten-thousandth of the planet's mass, because moons that grow too large spiral into the planet through the gas and are replaced by new ones :cite[canup2006]. The real numbers bear this out: Jupiter's regular moons hold $2.1 \times 10^{-4}$ of its mass, Saturn's $2.5 \times 10^{-4}$ and Uranus's $1.0 \times 10^{-4}$ :cite[jplsats]. **A giant impact.** Earth's Moon is thought to have formed from debris thrown out when a Mars-sized body struck the young Earth :cite[canup2001], and Pluto's large moon Charon from a similar collision :cite[canup2005]. Impact-made moons can be large relative to their hosts: the Moon has 1.2 percent of Earth's mass and Charon 12 percent of Pluto's, far above the giant planets' one part in ten thousand. **Capture.** The **irregular moons** orbit far from their planets on eccentric, steeply inclined and often retrograde orbits. They are captured bodies, and more than 90 percent of Jupiter's and Saturn's known moons are irregulars only a few kilometres across, many of them fragments of a few captured parents broken up by later collisions :cite[jewitt2007]. The largest captured moon is Neptune's Triton, 2,705 km across, which circles Neptune backwards on an orbit inclined 157 degrees. The favoured explanation is that Triton was one half of a binary pair that passed too close to Neptune: the planet kept Triton and flung its partner away :cite[agnor2006]. The origin of Mars's two small moons, Phobos and Deimos, is still argued. Their dark, asteroid-like surfaces suggest capture, but their near-circular, equatorial orbits suggest they accreted from debris after an impact on Mars :cite[rosenblatt2016]. ## Where a moon can orbit A moon must stay inside its planet's **Hill sphere**, the region where the planet's gravity outweighs the star's tidal pull, of radius $r_H \approx a\,(m/3M)^{1/3}$ (see [[Orbit]]). Only the inner part is stable over long times. Numerical experiments put the limit at about half the Hill radius for prograde moons and about 0.93 of it for retrograde ones, which is why the most distant irregular moons tend to orbit backwards :cite[domingos2006]. At the inner edge sits the **Roche limit**, inside which tides pull a moon apart; material there stays as a ring. Planets close to their stars have small Hill spheres, and tides between planet and moon then drive the moon either into the planet or out of the Hill sphere. Barnes and O'Brien found that a hot Jupiter cannot keep a sizeable moon for the age of its system :cite[barnes2002], and Kane showed that most planets in compact systems such as TRAPPIST-1 have too little room between the Roche limit and the Hill sphere for moons at all :cite[kane2017]. ## Tides: locked faces, heat and slow drift Tides raised by a planet slow a moon's spin until one face always points at the planet; close moons lock quickly, because the despinning time grows as the sixth power of distance :cite[gladman1996]. The Moon, the Galilean moons and most regular moons are locked. The tides a moon raises on its planet change the orbit too. Earth spins faster than the Moon orbits, so the tidal bulge runs ahead of the Moon and tows it outward: lunar laser ranging to reflectors left by the Apollo astronauts shows the Moon receding by about 3.8 cm a year :cite[williams2016]. Phobos orbits Mars faster than Mars spins, so the drag works the other way. Phobos is spiralling inward and is expected to break apart within 20 to 40 million years, leaving Mars a ring :cite[black2015]. A moon on an eccentric orbit is squeezed and relaxed once per orbit, and the flexing heats its interior. The heating rate is $$ \dot E = \frac{21}{2}\,\frac{k_2}{Q}\,\frac{G M_p^2 R^5\, n\, e^2}{a^6} $$ where $M_p$ is the planet's mass, $R$ the moon's radius, $n$ its mean motion, $e$ its eccentricity and $a$ its orbital distance; $k_2$ and $Q$ describe how readily the moon deforms and how much of that energy it turns into heat :cite[murray1999]. Because $n$ itself falls as $a^{-3/2}$, heating drops as $a^{-15/2}$: halve the distance and the heat rises about 180-fold. Tides would circularise the orbit and switch the heating off, unless something keeps pumping the eccentricity. At Jupiter, the orbital periods of Io, Europa and Ganymede lock in the ratio 1:2:4, and the repeated tugs keep Io's orbit eccentric. Peale, Cassen and Reynolds used this to predict volcanoes on Io just before Voyager 1 found them in 1979 :cite[peale1979]. Io now radiates about 100 terawatts of internal heat, some 2 to 2.5 watts per square metre, which is 20 to 30 times Earth's average heat flow, and astrometry of the Galilean moons over more than a century confirms that tides supply it :cite[lainey2009]. ## The kinds of moon | Kind | Solar System example | Radius (km) | Density (g/cm³) | What defines it | |---|---|---|---|---| | Airless rocky | The Moon | 1,737 | 3.34 | Cratered highlands, dark lava plains, no atmosphere | | Volcanic | Io | 1,821 | 3.53 | Tidal heating, hundreds of active volcanoes, sulfur surface | | Subsurface ocean | Europa | 1,561 | 3.01 | Ice shell over a global salt-water ocean | | Mixed ice and rock | Ganymede | 2,631 | 1.94 | Largest moon; its own magnetic field; buried ocean | | Ancient ice | Callisto | 2,410 | 1.83 | Saturated with craters; little internal activity | | Thick atmosphere | Titan | 2,575 | 1.88 | 1.5 bar nitrogen air, methane rain, hydrocarbon lakes | | Cryovolcanic | Enceladus | 252 | 1.61 | Jets of ocean water from its south pole | | Captured, nitrogen ice | Triton | 1,353 | 2.06 | Retrograde; nitrogen geysers seen by Voyager 2 | | Captured small body | Phobos | 11 | 1.87 | Irregular shape, dark surface, low density | Radii and densities from JPL :cite[jplsats]. Titan's tidal flexing, measured by Cassini, is too large for a solid interior and points to a global ocean of liquid water beneath its ice :cite[iess2012]. Enceladus's jets carry salty ice grains that Cassini sampled directly: they contain large organic molecules :cite[postberg2018] and phosphates, the form of phosphorus that life on Earth uses, at concentrations well above those in Earth's oceans :cite[postberg2023]. ::figure{src="File:Galilean_satellites_NASA_PIA01400.jpg" size=wide alt="Four moons side by side at the same scale: orange-yellow Io, pale cracked Europa, large grey-brown Ganymede and dark cratered Callisto" caption="Observation: Jupiter's four large moons to scale, from Galileo spacecraft images. Io and Europa are rocky; Ganymede and Callisto are about half ice. Credit: NASA/JPL/DLR."} ## How many moons, and why the number keeps changing | Planet | Known moons (as of September 2026) | |---|---| | Mercury, Venus | 0 | | Earth | 1 | | Mars | 2 | | Jupiter | 115 | | Saturn | 293 | | Uranus | 29 | | Neptune | 16 | NASA lists 115 moons of Jupiter recognised by the International Astronomical Union and 293 confirmed moons of Saturn :cite[nasa_jupiter_moons] :cite[nasa_saturn_moons]; as recently as March 2026, after the IAU's Minor Planet Center confirmed four and eleven new moons, the counts stood at 101 and 285 :cite[iau2026]. Neptune reached 16 in 2024 :cite[carnegie2024], and Uranus 29 after the James Webb Space Telescope found a moon about 10 km across in 2025 :cite[webb2025]. Almost all new discoveries are small irregular moons found by deep, repeated imaging, so the totals measure survey depth as much as anything about the planets. The number of large moons changes very little: Jupiter has eight regular moons, four small inner ones and the four Galilean moons, and the rest are captures. ## Moons around other planets No moon outside the Solar System has been confirmed as of September 2026. The two leading candidates, Kepler-1625 b-i and Kepler-1708 b-i, are each suggested by transits of a Jupiter-sized planet and would be moons about the size of Neptune :cite[teachey2018] :cite[kipping2022]. Both are disputed: a reanalysis in 2024 found that the Kepler-1625 signal can be explained by how the star's brightness varies across its disk, and that models without a moon fit Kepler-1708 as well as models with one :cite[heller2024]. Earlier claims based on transit timing alone did not survive independent tests :cite[kipping2020]. A moon as small as Io or the Moon is beyond current transit sensitivity. ::figure{src="File:Halden_Mare_from_orbit_sim.png" size=wide alt="A grey, heavily cratered moon seen from orbit, half lit, with dark smooth plains filling several round basins" caption="Sim render: Halden Mare, an airless moon of a gas giant in Pax Abyssi, drawn to the Moon's measured albedos and crater statistics."} :::callout{type=sim title="In Pax Abyssi"} Moons in generated systems are chosen by the type of their parent planet. A cold gas giant gets three to eight major moons, about five on average, and an ice giant two to six. Rocky planets and small super-Earths get none in just over half of cases and up to three otherwise; super-Earths above six Earth masses have only a one-in-ten chance of a single small moon. Gas giants within 0.1 AU of their star or with years shorter than 30 days get none, and neither do lava worlds or mini-Neptunes. Close-in planets of small red dwarfs lose some or all of their moons. Each system's moons share a mass budget of one to three ten-thousandths of the planet's mass, after Canup and Ward. Orbits step outward in period ratios between 1.5 and 2.1, beginning just outside the Roche limit, and anything beyond a third of the Hill radius is dropped. A snow line in the planet's formation disk at about 12 planet radii separates rocky inner moons from icy outer ones, and captured moons get inclined orbits, a third to a half of them retrograde. The simulation has about 15 moon types, from airless rock and volcanic moons to ocean moons and hazy Titans, plus speculative super-moons for giants above about five Jupiter masses. The generated systems hold 7,495 moons. The game models major moons only: the hundreds of kilometre-sized irregulars around real giants are not generated. Sol carries its 28 best-known moons, each on elements fitted to JPL Horizons ephemerides and referenced to its planet's equator; tidally locked moons keep one face to their planet. ::: ## See also - [[Orbit]] - [[Sol]] - [[Gas giant]] - [[Ice giant]] - [[Subsurface ocean world]] - [[Volcanic world]] - [[Ice world]] - [[Planetary system archetypes]] - [[Asteroid belt]]