The fact that the oldest rocks on the Moon are about the same age as the oldest rocks found on Earth tells indicates that the two were formed around the same time. Rather than forming by gravitational clumping of matter in orbit around the Earth, however, the Moon is probably a captured asteroid. Studies of the differences in the compositions of the Earth and Moon indicate that, during the early stages of the Earth's formation, a large asteroid struck the Earth a glancing blow, and pieces of it, as well as pieces of Earth, flew back up to orbit the Earth in a ring-like system around the planet. Eventually, the force of gravity caused the fragments to coalesce and form our Moon. This is called the "ring ejection theory" of lunar origin.