What a moon is

A moon is a natural object that orbits a planet. The planet orbits the Sun, and the moon goes along with it, circling as they travel.

Earth has one, and we call it the Moon. Mars has two small ones. Mercury and Venus have none at all.

The giant planets have most of them. In November 2026 the count stood at 115 for Jupiter, 292 for Saturn, 29 for Uranus and 16 for Neptune. Even little Pluto has five. The numbers keep going up, because the ones still being found are small, dark and very far away.

Moons are not all alike. Ganymede and Titan are bigger than the planet Mercury. Most of the others are lumps of rock and ice only a few kilometres across, too small for their own gravity to pull them round.

Two families. Look at where a giant planet’s moons are and they fall into two groups that could hardly be more different.

Regular moons sit close to the planet, on nearly circular orbits, level with its equator, all going round the same way the planet spins. They formed where they are, from a disc of gas and dust around the young planet, much as the planets formed around the Sun. Jupiter’s four big moons and Saturn’s Titan are regular moons.

Irregular moons are far out, on stretched and steeply tilted orbits. Many go round backwards. They are small, and there are a great many of them: they make up more than four in every five of the moons known. They are thought to be passing bodies that the planet caught early on.

Planet	Moons known	Of which far-out and irregular
Earth	1	0
Mars	2	0
Jupiter	115	107
Saturn	292	267
Uranus	29	10
Neptune	16	8
Pluto	5	0

The counts are from NASA/JPL’s list in November 2026. Triton is counted with the regular moons here because it is close in, but see below.

The odd ones out. A few moons fit neither pattern, and each one records an accident.

Our own Moon is far too big for the Earth to have grown it from a disc. The best explanation is that a body about the size of Mars struck the young Earth some 4.5 billion years ago, and the Moon gathered from the wreckage.

Triton, Neptune’s one large moon, goes round backwards. No moon can form that way, so it must have been caught. It looks very like Pluto and probably came from the same region.

Mars’s two moons are tiny. Phobos is so close in that it creeps 1.8 metres nearer to Mars every century. In about 50 million years it will either hit the planet or be pulled apart into a ring.

How far a planet can reach. A planet can only hold a moon inside the region where its own pull beats the Sun’s. Astronomers call it the Hill sphere. Saturn’s reaches about 65 million km, and its outermost moons range as far as 40 million km from it. Mercury and Venus are so close to the Sun that their regions are small, and nothing has stayed.

One face inward. Nearly every regular moon keeps the same face towards its planet, as ours does. The lesson on tides explains why.