Researchers have identified the likely chemical composition and birthplace of Theia, the protoplanet that collided with early Earth about 4.5 billion years ago to form the Moon. By analyzing isotope ratios in Earth and lunar rocks, the team traced Theia to the inner Solar System. The findings, published on November 20, 2025, in Science, suggest Theia formed closer to the Sun than Earth.
Approximately 4.5 billion years ago, a massive collision between the young Earth and a protoplanet named Theia reshaped our planet and ejected material that coalesced into the Moon. This event altered Earth's size, structure, and orbit, leaving chemical traces in rocks from both bodies.
A new study, led by scientists from the Max Planck Institute for Solar System Research and the University of Chicago, reconstructs Theia's composition using isotopic analysis. The researchers examined iron isotope ratios in 15 Earth samples and six lunar samples from Apollo missions with unprecedented precision. They found no measurable differences in these ratios between Earth and the Moon, consistent with prior studies on chromium, calcium, titanium, and zirconium isotopes.
To pinpoint Theia's origins, the team modeled possible pre-collision compositions. "The composition of a body archives its entire history of formation, including its place of origin," said Thorsten Kleine, director at the Max Planck Institute and co-author. By incorporating iron, chromium, molybdenum, and zirconium isotopes, they determined that Theia's building materials came from the inner Solar System, interior to Earth's orbit.
Meteorite comparisons revealed that while Earth's makeup aligns with known types, Theia's does not fully match any group. Instead, it indicates material from regions even closer to the Sun. "The most convincing scenario is that most of the building blocks of Earth and Theia originated in the inner Solar System. Earth and Theia are likely to have been neighbors," stated lead author Timo Hopp.
The study, published in Science (volume 390, issue 6775, page 819; DOI: 10.1126/science.ado0623), highlights how early planetary differentiation—such as Earth's core formation—preserved clues about Theia's role in delivering mantle elements like iron.