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Moons of our Solar System
Moons of our Solar System

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3 What makes a moon?

This is a diagram of planets, moons and asteroids to scale.
Figure 35 Planets, moons and asteroids to scale

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.