Why are all the planets round?

Planets are basically round because of the way that gravity works – if you imagine all of a planet’s mass concentrated at its core, then it’s pulling with equal force in every direction. Provided the gravity is strong enough and the material the planet’s made from is relatively weak (or even molten), then all the planet’s rock and other material will arrange itself in the most efficient way so that every part of the surface is at more or less the same distance from the centre. Often, planetary interiors even separate into layer of different density, like Earth’s own core, mantle and crust.

Earth’s internal layers have separated by density under the uniform inward pull of gravity, producing round shells