About 4.5 billion years ago, a massive object collided with the early Earth, resulting in debris that eventually formed the moon. While scientists have long accepted this general scenario, many details about the impactor—known as Theia—have remained unclear, including its composition and origin.
A new study published on November 20 in Science provides fresh insights into these questions. The research was conducted by scientists from the University of Chicago, Max Planck Institute for Solar System Research, and the University of Hong Kong.
“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 and former postdoctoral researcher at the University of Chicago who now works at the Max Planck Institute for Solar System Research. “Earth and Theia are likely to have been neighbors,” he added.
To reach their conclusions, researchers analyzed lunar rocks collected during NASA’s Apollo missions along with terrestrial rocks and meteorite samples from various regions of the solar system. By measuring isotope ratios—small variations in elements such as iron, chromium, calcium, titanium, molybdenum, and zirconium—the team sought to determine where Theia may have come from.
Isotopes serve as unique fingerprints that can trace celestial objects’ origins because they reflect distinct proportions inherited from different regions of the solar system. As Prof. Nicolas Dauphas explained: “As a result, different regions inherited distinct isotopic proportions, which now serve as a fingerprint to trace the origins of meteorites and other celestial bodies.”
The process required highly precise measurements due to tiny differences between isotopes and limited available samples from lunar material. Dauphas’ laboratory developed specialized techniques for this purpose.
By combining isotope data with knowledge about how metals behave during planetary formation—for example, understanding that much of Earth’s iron core formed before it was struck—the researchers inferred that a significant portion of iron found today in Earth's crust likely came from Theia.
Simulations further supported these findings by indicating which compositions would yield observed isotope ratios. According to their calculations, Theia probably did not originate far out in space but rather formed closer to the sun than Earth did; its composition closely matches meteorites from inner solar system zones.
"During the early solar system's game of cosmic billiards, Earth was struck by a neighbor,” said Dauphas. “It was a lucky shot. Without the moon's steadying influence on our planet's tilt, the climate would have been far too chaotic for complex life to ever flourish."
The study was funded by NASA, National Science Foundation, U.S. Department of Energy, Deutsche Forschungsgemeinschaf (DFG), and European Research Council.