Earth and the Moon formed together as part of the same planetary system, with the Moon being a natural satellite of Earth. The leading scientific theory is that the Moon was formed approximately 4.51 billion years ago, shortly after the formation of the Earth, as a result of a giant impact between Earth and a Mars-sized proto-planet called Theia. The debris from this collision is thought to have coalesced into the Moon, which was then gravitationally bound to Earth.