A new study in Science reports that Theia, the impactor that formed the Moon, may have originated in the inner Solar System—possibly even closer to the Sun than early Earth. Researchers measured precise isotopic ratios of iron, chromium, zirconium and other elements in Moon samples, Earth rocks, and meteorites to infer Theia's birthplace. Their models suggest the Moon‑forming collision was likely a local event between neighboring bodies. The results refine models of early Solar System dynamics and the formation of terrestrial planets.
Theia May Have Been Earth's Neighbor: Isotope Study Recasts Moon‑Forming Impact
A new study in Science reports that Theia, the impactor that formed the Moon, may have originated in the inner Solar System—possibly even closer to the Sun than early Earth. Researchers measured precise isotopic ratios of iron, chromium, zirconium and other elements in Moon samples, Earth rocks, and meteorites to infer Theia's birthplace. Their models suggest the Moon‑forming collision was likely a local event between neighboring bodies. The results refine models of early Solar System dynamics and the formation of terrestrial planets.
12:40 AM, 11/21/2025Science
About 4.5 billion years ago, a catastrophic collision reshaped the young Earth. A Mars‑sized object, commonly called Theia, slammed into proto‑Earth roughly 100 million years after the Solar System began to form. That violent encounter not only altered Earth's size, composition, and orbit but also produced the debris that coalesced into the Moon.
A new study published in Science presents the most detailed inventory yet of Theia's chemical building blocks. Researchers led by geoscientist Timo Hopp of the Max Planck Institute for Solar System Research measured extremely precise isotopic ratios of elements such as iron, chromium, and zirconium in lunar samples, terrestrial rocks, and various meteorites. Because isotopic compositions vary with where material formed in the protoplanetary disk, these fingerprints can be used to infer an object's birthplace.
“The most convincing scenario is that most of the building blocks of Earth and Theia originated in the inner Solar System,” said Timo Hopp, lead author of the study.
Combining newly generated measurements with existing datasets, the team modeled the likely formation region for Theia. Their results indicate Theia's components are consistent with material from the inner Solar System—possibly even somewhat closer to the Sun than Earth was when it formed. In other words, the Moon‑forming impact may have been a collision between true neighbors rather than a late arrival from the outer system.
These findings refine our view of early Solar System dynamics and the processes that assembled terrestrial planets. If Theia and Earth formed from nearby reservoirs of material, that proximity helps explain subtle isotopic similarities between Earth and Moon samples and constrains simulations of planet formation and migration in the inner Solar System.
Although the study strengthens the case for a local origin of Theia, open questions remain about the precise timing, impact angle, and subsequent mixing of material during the giant impact. Future high‑precision isotope work and new lunar samples could further clarify how that singular event shaped both Earth and its constant companion.
Study source: T. Hopp et al., Science (publication reporting isotopic measurements and modeling).