ASK TOM WHY: Why are planets spherical?

Dear Tom,

 

Why are planets spherical like balls, instead of being shaped like broken bits after a big bang?

 

Blanchard Gephart, Wheeling

 

Dear Blanchard,

 

It’s due to gravity, a force that pulls everything evenly toward the center of a planet. Over time, this results in a spherical planetary shape. Even “solid” materials like rock flow like a liquid, albeit incredibly slowly, when they are pulled by a strong, steady force for a very, very long time. “Softer” solids such as ice respond more readily, and that is one of the mechanisms of glacial motion.

 

Despite gravity, planets are not perfectly spherical. Every feature of the terrain — mountains, valleys, buildings — constitute a deviation from a perfectly spherical planet. Another force is also at work. Gravity maintains a planet’s near-spherical shape, but rotation introduces centrifugal force that causes a planet to bulge at its Equator.

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