Theia, the colossal, Mars-sized impactor that collided with the Earth to birth our Moon, may have come from the direction of the Sun, answering a fundamental question about the formation of our solar system.
Conflicts between theory and real-world data measurements have left a cloud of mystery over the formation of Earth’s moon and the transition from proto-Earth to our present Earth. One of the fundamental questions has been whether Theia formed in the inner or outer Solar System, which would have provided access to very different raw materials for these enormous space rocks.
The Moon is Born
Understanding the makeup of this ancient body—which reshaped Earth forever—is key to decoding the early solar system. Rocky worlds form from a protoplanetary disk, a rotating band of gas and dust left over from a newborn star. Within this disk, particles collide, stick, and gradually assemble into planets. Because the disk’s components are distributed unevenly, they leave chemical clues that help identify where particular bodies originated.
By tracking iron isotopes in terrestrial and lunar rocks, scientists have been able to narrow down possibilities. In the new study, researchers propose that Theia approached from the Sunward direction before striking Earth roughly 100 million years after the solar system formed.
Theia or Proto-Earth?
Previous modeling has suggested that Theia contributed most of the material that became the Moon. One way to test this was to compare isotopic signatures between lunar and terrestrial samples. Because isotopic decay follows predictable timelines, it can reveal whether two samples arose from different sources.
But results have been puzzling: many isotopic signatures between Earth and the Moon are nearly identical. That lack of distinction has complicated efforts to determine Theia’s origin. It raises three possibilities: the Moon formed mostly from proto-Earth material; the impact homogenized the material of both bodies; or Theia formed very close to the proto-Earth in the first place.
Inner Solar System Formation
The team revisited the “canonical” model—where the Moon is composed primarily of Theia’s material—and sought to test it directly. They analyzed isotopic data from lunar rocks, terrestrial samples, and meteorites to reconstruct the reservoirs that fed both Theia and the proto-Earth.
Earth and the Moon both match most closely with non-carbonaceous meteorites from the inner solar system. Subtle negative deviations in the isotopes of iron, zirconium, and molybdenum allowed the researchers to link lunar samples to specific meteorite groups. From this, the team concluded that Theia originated in the inner solar system—but even closer to the Sun than the region that produced the proto-Earth.
With this long-running mystery potentially resolved, scientists gain a clearer view of how the solar system’s early architecture shaped the world we live in today.
The paper, “The Moon-Forming Impactor Theia Originated from the Inner Solar System,” appeared in Science on November 20, 2025.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.