We found big chunks of the impactor Theia.” With those five words to The New York Times, Caltech geophysicist Qian Yuan summed up the significance of a find that’s transforming our knowledge of ...

In the aftermath of this collision, it’s usually thought that material from Theia mixed with Earth’s mantle, so that the planet and the newborn Moon share the same general composition.

But one glaring mystery at the center of this hypothesis has endured: What ever happened to Theia? Direct evidence of its existence has remained elusive. No leftover fragments from the planet have ...