For years, scientists have believed that the Moon was caused by a terrible collision between the Earth and a rogue alien planet. The hypothesis was that the alien planet, called Theia, smashed into the Earth 4.5 million years ago, and the resulting dust and fragments and debris ultimately coalesced to form the Moon.
The theory looked good on the computer simulations and sounded right. But there was one problem: there was no physical evidence of the cataclysmic crash. If Theia had, in fact, collided with Earth and been pulverized, why wouldn’t we find pieces of Theia and its alien geology scattered about like Indian arrowheads? So scientists refined the theory and concluded that most of Theia ended up forming the Moon. And they had a way to test the theory — checking out the rocks that the Apollo astronauts gathered from the lunar surface and testing them for signs of their Theianic origin.
Initial tests, however, indicated that the Moon’s geology looked just like the Earth. It was so perplexing for our scientific friends! But they kept testing, and now — more than 40 years after the first Moon rocks were retrieved — scientists think they have found traces of Theia. Some of the lunar rocks show slight differences in their oxygen composition that scientists believe reflect an alien origin, and therefore would be Theia’s fingerprint.
The findings aren’t without controversy, and some scientists argue that the differences are so slight the rocks are still of Earth origin. Others theorize that the rocks are alien after all, and that scientists were wrong to expect huge differences in planetary composition — a theory that has intriguing implications for the history of our solar system and the possibility that old Earth really isn’t as unique as we once thought.