An artist's impression of the Earth–Theia impact, with the Sun in the background, illustrating the idea that Theia originated close to the Sun. Image credit: MPS / Mark A. Garlick.
A study led by Professor Nicolas DAUPHAS of the Department of Earth and Planetary Sciences at The University of Hong Kong (HKU) and Dr Timo HOPP of the Max Planck Institute for Solar System Research in Göttingen, Germany, together with their collaborators reveals that Theia—the planet-sized body that collided with the early Earth to form the Moon—was not an icy wanderer from the outer Solar System, but a rocky neighbour born close to the Sun.
For decades, scientists have accepted that a giant impact between a Mars-sized planet, Theia, and the young Earth created the Moon. Yet Theia's origin has remained uncertain: was it a distant visitor from the Solar System's cold outskirts, or a local world formed alongside Earth?
Planetary scientists have now traced Theia's birthplace using the iron isotopic 'fingerprints' found in ancient meteorites and lunar rocks. "The samples returned from the Moon by space missions such as Apollo, Luna, and Chang'e are invaluable for understanding where we come from," said Dr Timo Hopp. The team analysed iron isotopic patterns that act like DNA markers, revealing where planetary building blocks originally formed.
The results show that Earth and the Moon share indistinguishable iron isotopic compositions — a match found only in material forged in the inner Solar System. This finding confirms that Theia and proto-Earth grew from the same reservoir of rocky matter close to the Sun.
The study also refines the widely accepted giant-impact theory: the nearly identical isotopic signatures indicate that Earth and Theia were either already similar in composition before the impact or became completely mixed during it.
By showing that both bodies originated near the Sun, the research suggests that the early Solar System was more orderly than previously believed, with most rocky planets forming locally from shared, sun-baked materials.
"Our results show that the Moon-forming impactor came from nearby. While theory allows for it to have come from afar, the measurements tell a different story. The ingredients that built our planet — and made it habitable — came from our neighbourhood," said Professor Dauphas.
