(CNN) It's the only world besides Earth that man has set foot on . But we still don't know exactly how it got there.

Now, rocks brought back by the Apollo astronauts in the late '60s and early '70s are providing new clues about how Earth got its moon.

Many scientists have long theorized that the moon formed after a planet called Theia crashed into Earth about 4.5 billion years ago.

This photograph of the full moon was taken from the Apollo 11 spacecraft as the crew headed home in July 1969. On board was Neil Armstrong, the first man to walk on the moon, and astronauts Buzz Aldrin and Michael Collins.

"The moon was formed by a violent, head-on collision between the early Earth and a 'planetary embryo' called Theia approximately 100 million years after the Earth formed," the team said in on UCLA's website.

The team reported its new research in the journal Science

The researchers analyzed seven moon rocks brought back to the Earth by the Apollo 12 15 and 17 astronauts. They compared them with six volcanic rocks from Earth, five from Hawaii and one from Arizona.

They found the oxygen in all the rocks had a similar chemical signature.

"We don't see any difference between the Earth's and the moon's oxygen isotopes; they're indistinguishable," said Edward Young , the study's lead author.

Young said that if Earth had only received a glancing blow, most of the moon would have been made up of Theia, and the Earth and moon rocks would not be so similar.

So what happened to Theia? Scientists say it didn't survive the collision, except for some pieces that got mixed in with the Earth and moon. If it hadn't crashed with Earth, it might have become a full-fledged planet, Young said.