A smaller planet called Theia hit the younger Earth 4.5 billion years ago with a force strong enough that caused both to fuse, creating the Earth as we know of today. The researchers added that the small piece broke off during the impact and formed the moon.

Lead author Edward Young and other researchers from the University of California did not find any difference between the Earth’s and the moon’s oxygen isotopes after studying six volcanic rocks from Hawaii and Arizona and seven Moon rocks brought back after the Apollo 12, 15 and 17 missions. If Theia only collided with Earth at a 45-degree angle, the moon would have been made mainly of Theia and the isotopes would have been different, but this new study proves that this was not the case.



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

Theia was still developing into a planet during that time. If the crash did not happen, some experts believe it would have been the same size as Earth while others theorise that it would have been smaller, most likely the same size as Mars.

Young also came up with the idea that Theia might have removed any water from the early Earth during the impact. It was not until small water-rich asteroids hit the Earth tens of millions of years after the crash and filled the planet with water once again.

The idea has been around for some time when Matija Ćuk, a research scientist with the Search for Extraterrestrial Intelligence (SETI) Institute, and Sarah Stewart, a professor at UC Davis proposed the collision in 2012. Robin Canup of the Southwest Research Institute also raised the theory in the same year.