Earth-Moon system formation. Illustration showing a late stage of the process that formed the Moon. The Moon is thought to have been formed by the collision of a Mars-sized body with the young Earth, early in its history around 4.5 billion years ago. The proto-Moon was captured by the Earth and remained in orbit around it. The collision melted the surface of both bodies and led to the formation of this debris ring around the Earth. Over time, the debris fell back onto the surfaces of the Moon and Earth.