The giant impact hypothesis goes like this: 4.5 billion years ago, a Mars-sized body named Theia slammed into the Earth. The collision launched magma—some from Theia and some from Earth—into orbit around our planet. The magma condensed and cooled into the rocky sphere that we see in the sky, our Moon.

This scenario—explored through collision models—handily explains the way that our Moon spins, its small core, and its lack of water. It is the most widely accepted scientific response to the question of how the Moon came to be hung in our sky.

But the giant impact hypothesis has suffered from one major problem: multiple analyses of lunar rocks suggest that the moon is made up of the same material as Earth. Collision models peg the moon at 70-90 percent material from Theia, and most bodies in our Solar System have very different compositions.

But based on examinations of lunar rocks, it's as though the big collision never happened. It is “maybe the last major problem” with the giant impact hypothesis, says geochemist Daniel Herwartz, who also thinks he has solved it. In a paper published in Science on Thursday, he reported that the Moon does, in fact, contain a tell-tale sign of alien material.

To Herwartz—or anyone studying lunar rocks with the intention of figuring out their origin—all rocks have a sort of cosmic address label. Since ratios of isotopes (versions of an element with different numbers of neutrons) on the Earth, Mars, and asteroids are unique, we know that our young Solar System was “isotopically heterogeneous.” The isotopes in a rock reveal exactly where in the Solar System it formed.

Differences in oxygen isotope levels are particularly dramatic. According to previous readings of moon-rock oxygen, the difference between a key oxygen isotope measurement—the ratio between three variations of oxygen, specifically—appeared to be just 3 parts per million higher on the Moon than on Earth, a difference so small as to be negligible. Against the evidence from collision models, the Moon’s address label suggested that it was made mostly or entirely of Earth.

Herwartz thought that maybe the difference was more than statistical variance. He took samples from the Apollo 11, 12, and 16 missions (lunar samples that fell to Earth were too contaminated). Using a high-precision method published earlier this year, he released the oxygen by heating it in a container with fluorine gas, purified it, and then measured the isotope ratios in a gas mass spectrometer.

On this re-evaluation, he found that the ratio between to oxygen isotopes on the Moon was, in fact, different: 12 parts per million higher on the Moon than on Earth.

This difference confirms that the Moon is not made of material that formed in the same region as Earth, and, most importantly, that it’s not merely a chunk of Earth. The isotope difference is still not terribly large—Mars and the Earth differ by a factor of 300 ppm, for example. But that suggests Theia probably formed in a region of the solar system near Earth.

As for how much of the Moon is Theia and how much is Earth—that’s still a mystery. After colliding with Earth, Theia ceased to exist as an independent body. Collision models peg the ratio at 70 percent to 90 percent. Herwartz suspects that it's closer to 50/50, but that’s just an informed guess at this point.

The details may be fuzzy, but as Herwartz said in a statement: “we can now be reasonably sure that the giant collision took place.”

Science, 2014. DOI: 10.1126/science.1251117 (About DOIs).