The prevailing theory for how our moon formed is that an enormous collision between the early Earth and a Mars-sized object blasted it into space. If true, much of the substance that makes up the moon should come from the Earth, but a sizable chunk should have originated from the other planetary object, dubbed Theia. Surprisingly, Junjun Zhang and her colleagues from the University of Chicago have found that this is not the case. Essentially all of the moon’s material appears to have come solely from the Earth.
How did the cosmologists reach this conclusion? They compared the ratios of isotopes (atomic variations) of titanium found both on the Earth and in samples brought back from the moon. Different objects throughout the solar system have different ratios of these isotopes. For example, asteroids that have been collected on Earth have highly varied titanium isotope signatures. It’s extremely unlikely that Theia would have had the exact same ratio of titanium isotopes as Earth. Therefore, if the moon truly were a child of their union, it should contain a titanium ratio somewhere between the two.
That’s not what the researchers found at all. Instead, the titanium ratio of the moon is virtually identical to that of the Earth. This strongly suggests that all of the material in the moon came from the Earth and none of it came from the hypothetical Theia.
Unfortunately, without Theia colliding into the Earth, there isn’t a good explanation for how the moon got here. Other hypotheses, such as that the moon spun out of a rapidly rotating Earth or that the moon was a free-floating entity that got captured by Earth’s gravity, have serious flaws. Perhaps Theia was involved but didn’t contribute any material to the building of the moon. This doesn't seem particularly likely either.
I guess it’s back to the drawing board for moon formation theorists.