A geologist from UCLA says in a new study he has found evidence to support the theory the moon was created by a head-on collision between the Earth and another planet billions of years ago.

In a story on Forbes, Edward Young, lead author of the study, says a comparison of lunar rocks and rocks found here on Earth showed both contained identical oxygen isotopes, leading the researchers to conclude the two bodies were made from the same well-mixed material.

“We don’t see any difference between the Earth’s and the moon’s oxygen isotopes; they’re indistinguishable,” said Young in a statement.

The new study supports the theory that nearly 4.5 billion years ago, the Earth collided with another planet, Theia.  Previously, most scientists thought it was likely a glancing blow at a high angle, or by just swiping the side of the Earth.  But, the authors say that type of impact would lead you to believe the Moon and Earth would not have identical oxygen isotopes, and the findings suggest the impact was a direct hit.

Although there is no clear indication of the size of the planet Theia, most theories place the planet’s mass somewhere between that of the Earth and Mars.  At the  time of the impact, the newly-formed Earth would have had a thin outer crust and a large amount of volcanic activity, and the researchers believe the planet was incorporated into the Earth and Moon after the collision.

Young added, “Theia was thoroughly mixed into both the Earth and the moon, and evenly dispersed between them.  This explains why we don’t see a different signature of Theia in the moon versus the Earth.”

The lunar rocks used in the new study were collected on the moon during Apollo missions 12, 15 and 17, and from a lunar meteorite found on Earth.

Young’s findings were published in the journal Science.