This is because of Gravity
 

. All the atoms in an OBJECT pull towards a common center of gravity, and they’re resisted outwards by whatever force is holding them apart. The FINAL RESULT could be a sphere… but not always, as we’re about to learn.

Consider a glass of water. If you could see the individual molecules jostling around, you’d see them trying to fit in as snugly as they can, tension making the top of the water smooth and even.

Imagine a planet made entirely of water. If there were no winds, it would be perfectly smooth. The water molecules on the north pole are pulling towards the molecules on the south pole. The ones on the left are pulling towards the right. With all points pulling towards the center of the mass you would get a PERFECT sphere.

Gravity and surface tension pull it in, and molecular forces are pushing it outward. If you could hold this massive water droplet in an environment where it would remain undisturbed, EVENTUALLY the water would reach a perfect balance. This is known as “hydrostatic equilibrium”