The formation of the Moon remains one of the most decisive episodes in Earth's history, but also one of the most uncertain. If the “great impact” hypothesis – according to which a celestial body struck the young Earth – remains widely accepted, the origin and exact nature of this impactor, named Theia, have long eluded researchers. A new study, published on November 20, 2025 in the journal Science, finally clarifies the profile of this disappeared planet.
Based on isotopic analysis of lunar and terrestrial rocks, a team from the Max Planck Institute for Solar System Research and the University of Chicago demonstrates that Theia would have been a close neighbor of the Earth, born in the same sector of the inner solar system. This discovery calls into question the idea of an impactor coming from afar and sheds new light on the chemical origins of our planet.
A founding collision with lasting effects
The great impact hypothesis, formulated in the 1970s, suggests that the Moon came from debris resulting from a titanic collision between the young Earth and a celestial body named Theia. This collision, which occurred 4.5 billion years ago, would have melted a large part of the Earth's mantle. It would also have ejected a mass of materials into space. This debris would then have clustered in orbit to form the Moon.
This scenario explains the current position of the Moon and its rock composition. But a major problem persisted: the near chemical identity between lunar and terrestrial rocks. Indeed, the original model predicted an origin mainly from Theia for the Moon. It should therefore present notable isotopic differences compared to Earth. However, the samples brought back by the Apollo missions show almost identical isotopic signatures for key elements such as chromium, calcium or titanium.
Researchers proposed several hypotheses to resolve this enigma. One of them assumes a complete mixing of materials from Theia and Earth during the impact. Another considers that the Moon was formed mainly from the Earth's mantle. However, these models remained theoretical, in the absence of precise data on the origin of Theia.
In this context, the recent study led by Timo Hopp (Max Planck Institute for Solar System Research) and Nicolas Dauphas (University of Chicago) provides concrete elements to better understand the provenance and nature of Theia. Based on a detailed isotopic analysis of lunar and terrestrial rocks, it makes it possible, for the first time, to locate the place of formation of this disappeared planet.
Precise isotope signatures to turn back time
Isotopes are different forms of the same element, with a varying number of neutrons. They are particularly effective tracers for identifying the origin of materials, because their distribution in the early solar system was not uniform.
The research team measured the isotopic ratios of iron, molybdenum, zirconium and chromium. Fifteen terrestrial samples and six lunar rocks from the Apollo 12 and 17 missions were screened. The goal: to detect tiny differences in isotopic compositions, particularly of iron, that could reveal the origin of Theia.
Iron plays a central role here. On the early Earth, during the formation of the core, heavy elements such as iron and molybdenum were drawn towards the center. This means that the current Earth's mantle should contain little iron. However, we find some. The only plausible explanation? This iron was added after the formation of the core, probably by an external impact. The iron contained in the mantle therefore constitutes a residual signature of Theia. Isotopic analysis revealed that the iron ratios between Earth and Moon are not only identical, but also have a slight common anomaly, very subtle, but significant.
Theia: a little-known neighbor born near the Sun
Furthermore, contrary to the widespread idea of an impactor coming from the confines of the solar system, the study demonstrates that Theia probably saw the light of day in an internal region, closer to the Sun than the Earth. This conclusion results from a comparison between the isotopes measured in lunar and terrestrial rocks, and those present in twenty types of meteorites originating from different areas of the solar system.
These meteorites serve as references because they retain the chemical signature of their region of origin. However, none of the known compositions corresponds exactly to that deduced for Theia. This suggests that this celestial body came from a previously unsampled reservoir of material, probably located very close to the Sun. Thorsten Kleine, co-author of the study, explains this in a press release. “ The composition of a body records its entire formation history, including its place of birth“.
The team's modeling determined that Theia was a rocky body with a metallic core. It represents approximately 5 to 10% of the earth's mass. Its isotopic composition of molybdenum, an element particularly sensitive to the distance from the Sun during planetary formation, confirms an internal origin.
Timo Hopp specifies that the most consistent models remain those where Theia and Earth shared a common formation zone. This changes the perspective on the dynamics of the early solar system. Far from being a place of random collisions between wandering bodies, it would have been marked by interactions between “sister” planets, formed side by side. The violence of the impact would be no less real. But the local nature of the event redefines the planetary formation scenarios.
Rethinking the chemical history of the Earth and the Moon
The implications of this discovery go far beyond Theia's identity. By showing that the Earth and the Moon share a common ancestor formed nearby, the study allows us to reconsider how our planet acquired its current chemical characteristics. In particular, certain elements present in the Earth's mantle, such as molybdenum and zirconium, seem partly inherited from Theia.
This means that the impact didn't just create the Moon! It also enriched the Earth with heavy elements coming from this disappeared body. These elements play an important role in the dynamics of the mantle and in the chemical differentiation of the planet. Nicolas Dauphas explains that “molybdenum and zirconium provide a window into different phases of planetary formation. Because they react differently depending on whether they are found in the core or the mantle“.
The unexpected presence of iron in the Earth's mantle, already mentioned, confirms that Theia has modified the internal structure of the planet. This kind of process could have contributed to the creation of a chemical environment conducive to the development of plate tectonics or to the maintenance of a stable magnetic field, two essential elements for the emergence of life.
Finally, this study highlights the importance of sample return space missions. Sara Russell, from the Natural History Museum in London, cited byScientificAmericanrecalls that “rocks brought back by Apollo more than 50 years ago continue to reveal new information thanks to ever more precise analysis techniques“. She calls for future missions to Mercury or Venus to complete this understanding by bringing materials from other internal regions of the solar system.
Source: Timo Hopp et al., “The Moon-forming impactor Theia originated from the inner Solar System”. Science 390,819-823 (2025).
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