Natural satellite

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 ^{[1] [2]}. They range from captured fragments a kilometre or two across to Ganymede, whose 5,262 km diameter exceeds Mercury's ^[3]. 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 ^[4]. 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 $1.0 \times 10^{-4}$ ^[3].

A giant impact. Earth's Moon is thought to have formed from debris thrown out when a Mars-sized body struck the young Earth ^[5], and Pluto's large moon Charon from a similar collision ^[6]. 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 ^[7]. 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 ^[8]. 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 ^[9].

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 ^[10]. 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 ^[11], 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 ^[12].

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 ^[13]. 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 ^[14]. 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 ^[15].

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 ^[16]. 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 ^[17]. 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 ^[18].

The kinds of moon

Radii and densities from JPL ^[3].

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 ^[19]. Enceladus's jets carry salty ice grains that Cassini sampled directly: they contain large organic molecules ^[20] and phosphates, the form of phosphorus that life on Earth uses, at concentrations well above those in Earth's oceans ^[21].

How many moons, and why the number keeps changing

NASA lists 115 moons of Jupiter recognised by the International Astronomical Union and 293 confirmed moons of Saturn ^{[1] [2]}; 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 ^[22]. Neptune reached 16 in 2024 ^[23], and Uranus 29 after the James Webb Space Telescope found a moon about 10 km across in 2025 ^[24]. 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 ^{[25] [26]}. 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 ^[27]. Earlier claims based on transit timing alone did not survive independent tests ^[28]. A moon as small as Io or the Moon is beyond current transit sensitivity.

Image pendingFile:Halden_Mare_from_orbit_sim.pngA grey, heavily cratered moon seen from orbit, half lit, with dark smooth plains filling several round basins

See also

• Orbit
• Sol
• Gas giant
• Ice giant
• Subsurface ocean world
• Volcanic world
• Ice world
• Planetary system archetypes
• Asteroid belt

References

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