Scientists have uncovered new clues about where Theia — the planet that collided with Earth around 4.5 billion years ago and led to the formation of the Moon — may have originated. A study published in Science indicates that Theia formed in the inner region of the Solar System, in an area relatively close to Earth.
The giant-impact hypothesis, first proposed in the 1970s, predicted that the Moon was composed largely of material from Theia. This implied that Earth and the Moon should show clear chemical differences. However, decades of research revealed that the two bodies are almost chemically identical — a puzzle that has remained unresolved.
Thorsten Kleine and his team at the Max Planck Institute for Solar System Research analyzed Moon samples brought back during the Apollo missions alongside terrestrial rocks, searching for clues in heavy elements. The researchers studied molybdenum and iron isotopes that remained within Earth's mantle after the ancient collision.
The study shows that Theia was a rocky planet with a metallic core, estimated to be 5–10% of Earth's mass. A comparison of iron, molybdenum, and zirconium isotope ratios revealed that Theia likely formed closer to the Sun than Earth did.
According to scientists, objects that form nearer to the Sun tend to be richer in heavy elements. The unexpectedly high concentrations of these elements in both Earth and the Moon can therefore be explained only by a collision with a body like Theia.
Sara Russell, a planetary scientist at London's Natural History Museum, described the research as "an analysis offering highly valuable insights through extraordinarily precise measurements."
Russell emphasized that the findings significantly advance our understanding of how the Earth–Moon system became hospitable to life.
The research team plans next to test the giant-impact scenario using computer simulations and to analyze additional isotope signatures from new samples. Russell noted that upcoming lunar sample-return missions could push this work even further, adding: "Thanks to the rocks collected by Apollo, we are still learning new things about the Moon and Earth even 50 years later."
