SCIENCE FILE

Q: Why are all the planets and moons in the solar system nice and round?

A: Simply put, it is because of gravity, according to astronomer Harold Zirin of Caltech. Because gravity pulls equally in every direction, it tends to pull any high points down to the same level as other points on the surface of a sphere. “The extra weight would bring it down,” he said. Mountains exist because geological forces are acting in opposition to gravity to raise them up, but even so, they represent only very small blips on the surface of a sphere and are worn down relatively quickly, on a geological time scale.