Theia

Theia is a planet nobody has ever seen, and nobody ever will. It is the impactor in the leading explanation for the Moon. About 4.5 billion years ago, a body about the size of Mars struck the young Earth a glancing blow. Most of Theia merged into Earth, and part of the debris thrown into orbit gathered into the Moon. The idea was proposed in 1975 and 1976 by two pairs of scientists, Hartmann and Davis, and Cameron and Ward.

Why a collision

The Moon is strange for a moon. Its iron core makes up only about 1.6 to 1.8% of its mass, where Earth’s makes up about 30%. That fits a Moon made mostly of rocky mantle material blasted off in an impact, after Earth’s iron had already sunk to its centre. In the standard version of the idea, Theia hits at about 45 degrees and about 9 kilometres per second.

The trouble is the rocks. If the Moon were made largely of Theia, its chemistry should differ from Earth’s. In 2014 a team reported a tiny difference in oxygen isotopes, 12 parts per million, and called it the signature of Theia. Later studies found less, and a 2024 measurement found none at all at below one part per million. Earth and Moon look almost identical, which suggests the two bodies were thoroughly mixed, or that Theia was built from much the same material as Earth.

Where it came from

In 2025 alone three studies drew three different Theias. Hopp and colleagues used iron isotopes to argue that Theia formed in the inner Solar System, possibly closer to the Sun than Earth. A dynamical study by Branco, Raymond and Machado gave roughly even odds that it was carbon-rich material from further out. A team at the University of Bern argued that it formed further out and brought Earth much of its water.

Even the timing is disputed. Zircon crystals from Apollo 14 suggest the Moon formed within about 60 million years of the Solar System’s birth. A study of iron-loving metals in Earth’s mantle put the impact at 95 million years, give or take 32 million.

It may still be down there

Deep under Africa and the Pacific Ocean, at the base of Earth’s mantle, lie two vast regions where earthquake waves slow down. In 2023 Yuan and colleagues proposed that they are pieces of Theia’s mantle, 2 to 3.5% denser than Earth’s own, that sank and never mixed in. Each is about twice the size of the Moon. Other geologists see them as old ocean floor that sank from above.

Hours, centuries or millions of years

Not everyone agrees there was one Theia. In 2017 Rufu and colleagues built the Moon from about 20 smaller impacts over millions of years. Other models have the impact turn Earth into a spinning cloud of vapour and molten rock, a synestia, in which the Moon condenses over perhaps a few hundred years. A 2026 study led by Denton found that a hot, partly molten Theia could leave an intact Moon behind within about five hours. Sarah Stewart, who was not involved, calls the Moon’s origin “basically unsolved”.