Question: Why do some planets have more moons than others?

Sam Geen answered on 24 Jun 2013:

Moons are formed out of debris that used to orbit a planet – or still does, in the case of planets with rings like Jupiter or Saturn. A recent study found that moons are made by clumps of matter moving outwards from the rings and merging together. This is apparently still happening for Saturn, although very slowly.

So why do rings of debris form? Either they’re captured by the planet from debris in the solar system, or formed when something big collides with the planet (which is how the Moon was formed around Earth). Venus doesn’t have a moon, which means that it probably didn’t collide with anything big enough to make one.