Planet Earth is Actually Two Planets, Study Suggests

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.