3 What makes a moon?
With moons so widely distributed in the Solar System, it’s natural to ask what they are. There are a large number, with the majority in the outer Solar System, orbiting the giant planets Jupiter, Saturn, Uranus and Neptune. It is not believed that a moon can have a smaller moon in a stable orbit around it, and this has never been observed. What makes a moon? There are many sizes and compositions and there doesn’t seem to be a single explanation that holds for all.
Even our own Moon’s origin remains a topic of research.