A lost planet called Theia may be buried inside Earth right now
About 4.5 billion years ago, a world perhaps the size of Mars is thought to have slammed into the young Earth, and the debris gathered into the Moon. Nobody has ever seen Theia. Yet two enormous blobs deep in Earth's mantle might be its wreckage, and 2023 simulations made that idea more plausible.
Theia is a proposal, not an observation. It belongs to the giant-impact hypothesis, one of several explanations offered for the Moon's origin alongside capture by Earth's gravity and the two bodies forming together from the same disc of material. Rock samples brought back by Apollo astronauts proved strikingly similar to Earth's crust, suggesting lunar material had been torn from Earth in some violent event.
Where did Theia come from? A popular idea places it at one of two relatively stable Lagrange points shared by Earth and the Sun, known as L4 and L5, where it could have grown undisturbed until the pull of Venus, Jupiter or both nudged it onto a collision course. A 2017 study favoured an origin in a fairly stable orbit beyond Earth's, and some researchers, more controversially, have suggested it formed in the outer Solar System and might have delivered much of Earth's water.
Its size is uncertain. The usual estimate is about 10% of Earth's present mass, but some authors argue for 30% or even 40 to 45%, almost matching the proto-Earth, and a newer study proposes 13 to 15%. Composition is disputed too. Moon rocks share many isotope ratios with Earth rocks, which implies either that the impact blended the two bodies thoroughly or that Theia formed from very similar material. Under a classic scenario with a smaller impactor, simulations credit Theia with roughly 70 to 90% of the Moon's mass.
Early versions imagined a glancing blow, since a head-on hit seemed likely to destroy both worlds. Evidence published in January 2016 pointed instead to a direct collision, leaving Theia's remains in both Earth and Moon. The name fits: in Greek myth Theia was a Titan and the mother of Selene, goddess of the Moon.
Source: Theia (hypothetical planet)
