So how did our planet end up with such a special moon? The answer is that, surprisingly enough, the moon is a piece of our ...

The powerful impact instantly melted the crust and outer part of the mantle of proto-Earth, mixing them with bits from the rogue planet. The moon quickly formed out of the resulting cloud of debris.

According to a new theory, there were two giant impacts in a row, separated by about one million years, involving a Mars-sized 'Theia' and proto-Earth.