It was believed that the forming planet Theia had sideswiped the Earth and blasted the moon into orbit before heading off into space about 100 million years after the formation of the Earth. However, a new research suggests that instead of merely brushing off, the two planets actually had a head-on collision, fusing them together.

A team of researchers from the University of California, Los Angeles gathered seven lunar rocks brought by the Apollo missions and six volcanic rocks from the Earth's mantle to compare their oxygen isotopes ratio — the number of protons and neutrons present in oxygen atoms. The importance of testing this lies to the fact that each planetary body in the Solar System has a unique "fingerprint" ratio of oxygen isotopes, which help scientists test out where certain materials came from.

Earth's oxygen is O-16, which means each oxygen atom contains eight protons and eight neutrons.

In their test, they found out that the rocks on Earth and the Moon have identical oxygen isotopes, which is contrary to a 2014 report from a team of German scientists that the two have their own unique ratios.

The researchers argued that if Theia simply sideswiped the Earth and formed the Moon, then the lunar body would be made up mainly of the said planetary embryo. With that, the rocks they tested would have different oxygen isotopes ratio.

"We don't see any difference between the Earth's and the moon's oxygen isotopes; they're indistinguishable," said Edward Young, lead author of the new study. "Theia was thoroughly mixed into both the Earth and the moon, and evenly dispersed between them."

While there are not much details about Theia, Young explained that there is evidence this planet is growing, and if it survived the crash, it would have been another planet in the Solar System.

The study has been published in the journal Science.