Moons
Natural satellites orbiting planets and smaller bodies.
Overview
A moon, or natural satellite, orbits a planet, dwarf planet, or smaller body. The Solar System's moons range from irregular captured fragments a few kilometres across to worlds larger than Mercury, some with atmospheres and subsurface oceans.
- Ganymede and Titan are both larger than the planet Mercury.
- Tidal locking — one hemisphere permanently facing the primary — is the normal outcome for close-in moons.
- Several icy moons are believed to hold liquid water oceans beneath their surfaces.
- The count of known moons rises with every survey; the catalogue is the authority, not any fixed number.
How a planet acquires a moon
- Co-accretion: forming alongside the planet from the same circumplanetary disc. The regular satellite systems of Jupiter and Saturn — orbiting near the equatorial plane, in the same direction, on near-circular orbits — indicate this route.
- Capture: a passing body caught by the planet's gravity. Captured moons typically have inclined, eccentric, and often retrograde orbits. Neptune's Triton, orbiting backwards, is almost certainly a captured Kuiper Belt object.
- Giant impact: a collision ejecting material that reaccretes in orbit. This is the leading explanation for Earth's Moon, supported by its low iron content and by the isotopic similarity between lunar and terrestrial material.
- Mars's two small irregular moons, Phobos and Deimos, have contested origins — capture and impact-debris scenarios both have supporters and neither is settled.
Tidal locking and tidal heating
Tidal forces raise bulges on a moon, and friction as those bulges are dragged around by rotation dissipates energy until rotation synchronises with orbit. The result is tidal locking: the moon keeps one face permanently toward its planet, as Earth's Moon does. Given enough time and proximity it is the expected outcome, not a coincidence.
Where a moon's orbit is kept eccentric by resonance with other moons, the tidal flexing continues and generates internal heat. This is the energy source for Io's continuous volcanism — the most volcanically active body in the Solar System — and it is what plausibly keeps subsurface oceans liquid on Europa and Enceladus far outside the region where sunlight could.
The worlds among them
- Ganymede, the largest moon in the Solar System, is bigger than Mercury and is the only moon known to generate its own magnetic field.
- Titan has a dense nitrogen atmosphere thicker than Earth's and stable liquid methane and ethane lakes on its surface — the only other body known to have surface liquids.
- Europa's fractured ice shell overlies a probable global saltwater ocean, making it a primary astrobiology target.
- Enceladus vents plumes of water vapour and ice from its south polar region; Cassini flew through them and detected salts and organic molecules.
- Io is caught in an orbital resonance with Europa and Ganymede that sustains extreme tidal heating and hundreds of active volcanoes.
- Triton orbits Neptune retrograde, is geologically active with nitrogen plumes, and is almost certainly a captured Kuiper Belt object.
Why the count keeps changing
Announced moon totals for Jupiter and Saturn have risen repeatedly as surveys detect ever smaller and more distant irregular satellites, many only a few kilometres across. These discoveries reflect improving detection limits rather than anything new happening at the planets.
For that reason this page does not state a total. The catalogue pages carry the current recorded set with their sources, and that is the honest place for a number that changes with each survey release.
Continue in the data
Catalogues, hubs, and reference pages that hold the underlying records for this topic.
Frequently asked
- Why do we always see the same side of the Moon?
- Because the Moon is tidally locked: it rotates once per orbit, so the same hemisphere faces Earth continuously. Tidal friction over long timescales drove the rotation into synchronisation with the orbit. There is a far side, not a dark side — it receives just as much sunlight, and it was first photographed by Luna 3 in 1959.
- Which moons might have oceans?
- Europa and Enceladus have the strongest cases, both supported by multiple independent lines of evidence including magnetic induction signatures, surface geology, and in Enceladus's case direct sampling of erupted plume material. Ganymede, Callisto, Titan and possibly several other icy bodies are also thought to have subsurface liquid layers.
- How did the Moon form?
- The leading model is a giant impact: a Mars-sized body struck the early Earth, and debris thrown into orbit reaccreted into the Moon. It accounts for the Moon's low iron content, the Earth–Moon angular momentum, and the close isotopic match between lunar and terrestrial material — although that last similarity is also the detail that continues to drive refinement of the model.
- Do all planets have moons?
- No. Mercury and Venus have none. Earth has one, Mars two small ones, and the four giant planets have extensive systems. Several asteroids and trans-Neptunian objects also have satellites, so having a moon is not restricted to planets.