Earth isn't one planet.It's two, according to a study from UCLA published in " Science ." As the study contends, a "planetary embryo" named Theia collided with the early Earth and split evenly into two parts: one subsumed by the Earth, and another that became the Earth's moon.Scientists already knew about Earth and Theia's collision, but they thought it was a "glancing side blow." This new research suggests that the crash was much more violent.If the collision had been a side blow, the moon would be made up largely of Theia. Instead, it's made up largely of Earth.“We don’t see any difference between the Earth’s and the moon’s oxygen isotopes; they’re indistinguishable,” Edward Young, UCLA professor of geochemistry and cosmochemistry and lead author of the study, explained in a news release.“Theia was thoroughly mixed into both the Earth and the moon, and evenly dispersed between them,” Young added. “This explains why we don’t see a different signature of Theia in the moon versus the Earth.”Hence, Earth is really a composite of two planets: Earth and Theia.Some scientists, including Young, believe Theia was approximately the same size as the Earth and would have continued growing had the collision not occurred.