Why Are Planets Round

Have you ever wondered why all the planets are mostly round? The spherical shape of all the planets is caused by gravity. When the planets in our solar system began to form, space was filled with billions and billions of pieces of dust and gas.

As these pieces bumped into each other, they began to form clumps that slowly grew larger and larger. Eventually these clumps of material grew large enough to develop their own gravitational fields. As the forming planets continued to grow, the force of the collisions with additional matter caused them to become hot and molten. Each planet's gravitational force is centered at its core. It pulls equally in every direction from the core, thus pulling all the molten material into a spherical shape. Scientists call this process "isostatic adjustment."

Think of a bicycle wheel. The center of the wheel is like the core of a planet, where the planet's center of gravity is. The gravity pulls equally in every direction, like the spokes of the wheel. The natural shape of the wheel formed as a result is a circle. In the case of a three-dimensional object, it becomes a sphere.

Planets are not actually perfectly round, because they also spin. When planets spin, the forces created by spinning work against gravity, causing planets to bulge out around their equators. Scientists call this extra width the "equatorial bulge." None of this has anything to do with getting older and developing our own human 'equatorial bulge' around the middle.