Journey Into Space: Gravity, Orbits, and Collisions
Space Astronomy and Space
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Why Are Planets Round?

Q: Why are all planets round?

Lilliana M., Arizona

A: Gravity (an attractive force) makes planets round. Just as gravity pulls you toward Earth's center, keeping you on the ground, it also pulls all parts of a planet -- its inside and outside layers -- toward the planet's center to round and smoothen it.
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