Dr Caroline Smith of the Natural History Museum in London explains the indications of the planet believed to have formed the Moon.
Researchers have found evidence of the world that crashed into the Earth billions of years ago to form the Moon.
Analysis of lunar rock brought back by Apollo astronauts shows traces of the "planet" called Theia.
The researchers claim that their discovery confirms the theory that the Moon was created by just such a cataclysmic collision.
The study has been published in the journal Science.
It was getting to the stage where some people were suggesting that the collision had not taken place” Dr Daniel Herwartz University of Cologne
The accepted theory since the 1980s is that the Moon arose as a result of a collision between the Earth and Theia 4.5 billion years ago.
Theia was named after a goddess in Greek mythology who was said to be the mother of Selene, goddess of the Moon. It is thought to have disintegrated on impact with the resulting debris mingling with that from the Earth and coalescing into the Moon.
It is the simplest explanation, and fits in well with computer simulations. The main drawback with the theory is that no-one had found any evidence of Theia in lunar rock samples.
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