Scientists have wondered for decades where xenon (the heaviest stable noble gas on Earth) came from. Now they have a partial answer: at least one-fifth of it in Earth's atmosphere may have originated in comets, a new study finds.

Eight of the nine different isotopes of xenon on Earth have been traced to different parts of the cosmos that came together as the planet and our solar system formed. But the source of the remaining xenon isotope was a mystery.

How they solved the mystery: Data collected by the Rosetta spacecraft, whose mission is to study a comet known as 67P, showed that xenon leaking from it had been trapped in the comet's icy surface from a time before our solar system was formed. It matched one of the isotopes of xenon on Earth that scientists had been unable to source – strongly suggesting it came from comets just like 67P.

What it means: If such a significant portion of xenon found in Earth's atmosphere came from comets, then it's also possible that comets raining down on our planet for millions of years also delivered other important materials, like water and the building blocks of life.