Why planets spherical

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. 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. They are nearly perfect spheres, like marbles.

Saturn and Jupiter are bit thicker in the middle. As they spin around, they bulge out along the equator. Why does that happen? When something spins, like a planet as it rotates, things on the outer edge have to move faster than things on the inside to keep up.

This is true for anything that spins, like a wheel, a DVD, or a fan. Things along the edge have to travel the farthest and fastest. 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. Saturn bulges the most of all the planets in our solar system. If you compare the diameter from pole to pole to the diameter along the equator, it's not the same.

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