What it was
A grapefruit sideswipes a basketball. Make both into glowing worlds and you have the chapter's rough picture of Theia hitting young Earth. Melted rock flies into space. Some of those scraps could gather into a Moon.
Theia (thee-uh) is the name for the lost planet in this giant-impact idea. Nobody has photographed it. Scientists build crashes inside computers, changing the blow to see where the rock goes. Then they check that picture against real rocks from Earth and the Moon.
When and where
The proposed crash belongs near Earth's beginning, roughly 4.5 billion years ago. Earth was still growing as space rubble fell in. The exact crash date is disputed: clocks made from changing atoms in ancient rocks do not yet agree on a birthday.
How big
The familiar model uses a planet roughly as wide as Mars, with about a tenth of Earth's mass. Those sizes describe a model, so the grapefruit and basketball give you a rough feel for it. They are no scale drawing of a world we have measured.
How we know
Moon rocks pose the hardest clue. Their mix of oxygen isotopes, atoms with different weights, closely matches Earth's. Think of a chemical fingerprint. If most of the Moon came from another planet, why does its fingerprint look so much like ours?
Still a mystery
Scientists still argue about that match. Theia may have grown from similar stuff, or the crash may have stirred both worlds together. Huge blobs deep in Earth's mantle offer another possible clue. Earthquake waves reveal them; one model says they could hold Theia's remains. The link is still unproved.