Have you ever wondered why the planets are round? The eight planets in our solar system differ in lots of ways. They are different sizes. They are different distances from the sun. Some are small and rocky, and others are big and gassy. But they’re all nice and round.
Why are planets round?
A planet is round because of gravity. A planet’s gravity pulls equally from all sides. Gravity pulls from the center to the edges like the spokes of a bicycle wheel. This makes the overall shape of a planet a sphere, which is a three-dimensional circle.
Planets form when material in space starts to bump and clump together. After a while it has enough stuff to have a good amount of gravity. That’s the force that holds stuff together in space. When a forming planet is big enough, it starts to clear its path around the star it orbits. It uses its gravity to snag bits of space stuff.
Are they perfect spheres?
While all the planets in our solar system are nice and round, some are rounder than others. Mercury and Venus are the roundest of all. But some planets aren’t quite so perfectly round.
Along the equator of a planet, a circle half way between the north and south poles, gravity is holding the edges in but, as it spins, stuff wants to spin out like mud flying off a tire. Saturn and Jupiter are really big and spinning really fast but gravity still manages to hold them together. That’s why they bulge in the middle. We call the extra width the equatorial bulge.
Earth and Mars are small and don’t spin around as fast as the gas giants. They aren’t perfect spheres, but they are rounder than Saturn and Jupiter.
Source: NASA