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.
TheDudeAbides
November 12, 2012
In 2004, astronaut Donald Pettit did an interesting experiment while orbiting Earth. He put salt and sugar in a plastic baggie and then observed what the particles did in zero gravity. They clumped together. The theory on planet formation is that dust particles clump together like in the experiment, and gravity causes them to for spherical shapes.
Here’s the experiment:
http://www.skyandtelescope.com/news/3308986.html?page=1&c=y