Moon rock: Scientists find traces of Moon's birth
5 June 2014 Last updated at 19:01 BST
Four and a half billion years ago it is thought that another large world - called Theia - crashed into the Earth, and that debris from both objects clumped together to form our Moon.
Now, a German-led team has found very tiny amounts of this alien world in Moon rock.
Here, Dr Caroline Smith of the Natural History Museum in London explains these first indications of this object.