How do we know that another planet collided with the Earth?

Chris - Well, we have to go by what the models are telling us and what samples we've got. We've got a very large moon around the earth. In fact, it's unfeasibly large for a planet of our size. Why have we got such a big moon where it is? What the prediction is, and based on what we know about the composition of the moon from samples that the Apollo astronauts have brought back is that the moon is made of exactly the same stuff, give or take, as the surface of the Earth - the Earth's crust. The big question is how did something made pretty much of the same material as the Earth end up orbiting the Earth unless something bashed it an put it up there? The best suggestion that scientists can come up with, based on all the evidence we have, is that during the early phases of the formation of the solar system (something like 4.5 billion years ago) there were two planets. One a future Earth, one another planet which they've notionally called Thea. These were very similar in terms of their orbital pattern. One ran into another - it was like cosmic billiards that went on. As a consequence of their massive great collision the cores of both planets effectively fused. In the course of this collision a lot of the surface material from the Earth got ejected into space and it formed a sort of shroud around the Earth which slowly coalesced in the same way that rings around Saturn have coalesced to what would have originally been an envelope. They then coalesced and aggregated to form the moon. It's on the basis of there's no other better explanation than that one to explain why we have this phenomenon of this big moon and what the moon's made of.