How moons form
Why do planets have moons, and how are they made?
Three ways, and you can tell them apart. Some moons form from the same disc as their planet. Some form from the debris of a giant impact. Some are captured — they were going past and got stuck. Triton’s backward orbit on Neptune is the capture giveaway. Earth’s Moon on the Earth and Moon view is the impact story.
Disc, smash, or grab
A disc moon sits on a tidy, forward, nearly circular orbit near the equator — Io, Europa, Titan. An impact moon can be huge compared with its planet: our Moon is a quarter of Earth’s width; Charon is half of Pluto’s. A captured moon often comes in on a tilted or backward path, like Triton.
The same falling-and-missing that keeps a planet beside the Sun keeps a moon beside a planet. More mass, a larger volume of space where “beside the planet” wins.
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Earth’s Moon is the odd one in the inner system
A Mars-sized body hit the young Earth. Debris went into orbit and became the Moon. That much is broadly agreed. The puzzle is that the Moon’s rocks look almost identical to Earth’s, while most impact models make a moon out of the impactor. Getting a Moon that looks like Earth’s mantle out of a collision with something else is still an unsolved modelling problem.