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The Question

(Submitted October 26, 1998)

Why do some planets have more moons than others?

The Answer

1) The giant gas planets have many more moons than the terrestrial planets (like Mercury, Venus, Earth, and Mars) - this is because of their larger mass, which means they have a correspondingly greater gravitational field. Whether moons are captured by the gravity of a large planet or formed in place when our solar system was created, larger planets have the *potential* to have more simply because their gravitational "reach" allows them to "control" more space and hold more mass around them.

Your question, a very good one, by the way, is one that has no single correct answer - it is one that is being actively researched now!

Hope this helps!

Gail Rohrbach, Maggie Masetti & Koji Mukai
for Ask an Astrophysicist

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