Why Are Planets Round?

Recently after a planetarium program about the Solar System, a young girl, maybe 4 years old asked from her mother's arms, "Why are all planets round?".  Answering a question like this in the planetarium environment can be challenging due to time constraints, but especially so for very young visitors with little physical experience.

I started answering on too high a level, but fortunately the child's mother who had been listening along quietly saved the day by asking the child, "Do you remember what happens when you try to squeeze a piece of Play Doh into the smallest size you can?" "Yes," said the child, "It makes a ball."  Ah, perfect - she now had a concrete experience to attach to the situation, one that was easy to remember and illustrated something about the nature of gravity. So I wrapped an answer around that, and given her age, it seemed a good way to leave the question, assuming that the next time she wondered about it she could take in more of the subtleties.

Planets are spherical (or roughly so) because they form from cooling, molten rock, with gravity pulling equally on all parts of the forming planet toward the center.  As the material moves toward the center it produces rotation, just as a spinning ice skater increases their rotational speed by pulling in legs and arms. Rotation helps the sloshing, molten material make a ball by sort of smoothing out any bumps that might have remained otherwise. (Paradoxically, rotation also makes the larger planets appear LESS spherical, bulging visibly at their equators due a centrifugal effect.)

Though the planets of the Solar System are nearly spherical, bodies having the smallest amounts of material are the most likely to be irregularly shaped, for example moons and asteroids. Check out the Moons of Pluto Nix and Hydra at right, as seen close up last summer by NASA's New Horizons spacecraft.