Set a grapefruit beside a basketball. That is the chapter's rough picture of Theia beside young Earth. Now imagine them crashing together. Rock melts, and glowing scraps fly into space.
Theia (thee-uh) is the name for the lost world in the giant-impact idea. A crash could have made the Moon from those scraps. Scientists test the idea with computer models and rocks brought back from the Moon.
The rocks pose a puzzle. Oxygen atoms come in forms with different weights, called isotopes. Think of their mix as a fingerprint. Earth's rocks and Moon rocks have a very close match, though a Moon made mostly from Theia ought to carry that world's mark.
Scientists still argue about how to make the pieces fit. Theia may have grown from stuff much like Earth's. Or the crash may have stirred both worlds into a glowing cloud of rock vapor shaped like a doughnut. That cloud has a name: a synestia (sih-ness-tee-uh).
There is another place to look: deep below your feet. Earthquake waves reveal huge blobs in Earth's rocky mantle. One model says they could hold bits of Theia. The blobs are there; their link to the lost planet is still a guess. Even the crash's date is disputed.
