text: Because the closer to the center of mass you are, the mass in this case being the planet, the stronger the gravity.  Knowing that, you can imagine how there are gravity gradients surrounding the earth, and it is natural for energy to return to its lowest state, in the case of mass, its lowest state of energy without changing state woud be to locate its self in a position with the lowest potential.\n\nThe location with the lowest potential is the location closest to the center of mass, and given enough time, and the planets have had plenty, the potential will be equally distributed amoung all matter, which results in a circle...\n\nSame as a water droplet in zero gravity.