“Where did the Mars-sized celestial body Theia come from, which collided with the young Earth 4.5 billion years ago, thus giving rise to the Moon? Did it originate from outside the solar system, or did it form near our star like the other planets? A research team led by Timo Hopp from the Max Planck Institute for Solar System Research (MPS) in Göttingen is convinced that Theia orbited in the inner solar system together with the proto-Earth before the fateful collision occurred. The similarity in the chemical composition of the Moon and our home planet supports this theory, the researchers write in the journal "Science." "Earth and Theia were likely neighbors," says Hopp.
It is well known that neighbors don't always live harmoniously side by side. For Theia—in Greek mythology, the name of a daughter of the Earth goddess Gaia—the encounter ended in her Total destruction and a complete transformation of the outer layers of the proto-Earth. Yet this planetary disaster was simultaneously the birth of the Moon, as various findings show: Earth and Moon consist of similar proportions of the same chemical elements. If the Moon had been captured by our planet, greater differences would be evident. However, the two celestial bodies obviously did not originate together from the primordial cloud of the solar system: Had they formed together, the Moon's orbit around Earth would be closer and our planet's rotation slower. A massive collision, on the other hand, satisfactorily explains all the observational data.
But details about Theia are scarce and extremely difficult to ascertain. Its material and that of the proto-Earth are intermingled and almost indistinguishable. Whether a rock sample from the lunar surface or Earth was originally contained in Theia or in the proto-Earth is therefore almost impossible to determine with certainty. And yet, the original composition of Theia must have decisively influenced that of today's Earth as well as the Moon, explains Timo Hopp: "Because the Moon and Earth are a product of the proto-Earth and Theia." "We can try to reconstruct the composition of the bodies before the collision."
To this end, the researchers characterized the elemental composition of Earth and the Moon using specific isotopes of the element iron. Hopp and his team discovered that the ratio of different iron isotopes on Earth and the Moon resembles that of so-called non-carbonaceous chondrites. These are a specific group of meteorites from the inner solar system. They originated within the region roughly bounded by Jupiter's orbit. Theia must therefore have come from this region as well. If the celestial body had come from the outer solar system, it would have altered the isotopic ratio on Earth and the Moon relative to that in the chondrites. In fact, the measurements even suggest that Theia formed significantly closer to the Sun than the early Earth. Theia thus originated in the zone where the planets Mercury and Venus now orbit the Sun.
The study thus provides, for the first time, reliable evidence for the origin of Theia. "It was possible after the necessary precision of isotope measurements was achieved before "It has only become technically feasible in about five years," explains Timo Hopp. During a research stay at the University of Chicago, the scientist was able to apply the technique for the first time to lunar samples brought back to Earth by Apollo astronauts. The result could also confirm a finding by American researchers from 2023: At that time, a team led by Qian Yuan from Arizona State University found evidence that two large regions in Earth's mantle, characterized by particularly high densities, could be remnants of Theia. The two so-called "low-shear provinces" lie beneath the Pacific Ocean and the African continent, respectively, and were discovered using seismic waves.
"An origin in the inner solar system explains why Theia had a higher density than the early Earth and why the provinces were able to withstand the movements of Earth's mantle over billions of years," says Qian Yuan, who himself was not involved in the study by Timo Hopp's team. [1]
1. Die Katastrophe näherte sich aus Richtung Sonne: Der Protoplanet Theia und die junge Erde waren einst enge Nachbarn, bevor sie miteinander kollidierten. Frankfurter Allgemeine Zeitung; Frankfurt. 26 Nov 2025: N2.
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