A NASA and Durham University simulation puts forth the theory that the moon may have formed in a matter of hours, when material from the Earth and a Mars sized-body were launched directly into orbit after the impact. [NASA/Durham University/Jacob Kegerreis]

By Sarah Cope | 2026-01-13

Earth's moon may have formed as the result of a collision in the inner Solar System, according to new research published in Science, the peer-reviewed academic journal of the American Association for the Advancement of Science.

The study, published in November, casts fresh light on the giant impact hypothesis, which is the prevailing theory of how the Earth and its moon formed.

This theory posits that the protoplanet Theia collided with Earth roughly 4.5 billion years ago, creating a debris field that coalesced to form Earth's moon.

The original supposition was that Theia was a visitor from deep space.

But the new study suggests Theia and Earth were in fact close neighbors, and formed side-by-side in the inner Solar System, sharing similar building blocks.

Analyzing isotopes

While the giant impact hypothesis explains many aspects of the Earth and moon system, such as the moon’s size and angular momentum, it fails to account for a mystery known as the isotopic crisis, which is the focus of the new study.

If the moon did form as the result of a collision between Earth and Theia, it would be mostly comprised of the remnants of Theia, which -- per the giant impact hypothesis -- would have formed outside the Solar System.

This would give the moon distinct chemical signatures.

But rock samples taken from the moon's surface during NASA's Apollo missions show nearly identical chemical compositions to the Earth’s surface, with both composed largely of silicate minerals and metals.

The mantles of the Earth and moon are similarly made up of iron and nickel.

After analyzing iron isotopes in lunar and terrestrial rocks and finding striking similarities, researchers from Germany's Max Planck Institute for Solar System Research (MPS) have turned the giant impact hypothesis on its head.

The chemical overlap between the lunar and Earth surface samples indicates the two may have formed much closer together than previously believed, they suggest, positing that Theia likely originated in the inner Solar System.

Led by geoscientist Tim Hopp, lab manager at MPS and the University of Chicago's Department of Geophysical Sciences, the research team examined iron isotopes in 15 rock samples from Earth and six lunar samples.

Isotope ratios provide evidence as to when these rocks formed. These ratios are different from those measured in meteorites believed to originate from farther out in the solar system.

The results suggest Theia formed in the same inner Solar System reservoir as Earth, rather than migrating inward toward Earth before the collision.

New understanding

This discovery alters the giant impact hypothesis without discarding it. While the moon did appear to form from a collision between the Earth and Theia, new evidence implies the impact was not a random encounter.

Instead, Earth and Theia likely formed from similar building blocks and co-evolved in nearby orbits before colliding.

This suggests the impact, and the moon’s formation, may have been a natural outcome of planetary growth in the inner Solar System and that local planetary collisions could have played a systematic role in shaping rocky planets.

The findings also help to explain some of the unusual components of the Earth-moon relationship when compared to other planet-moon pairs.

Earth’s moon is large relative to its planet and closely mirrors Earth’s chemical composition, whereas many moons in the solar system form through processes such as gravitational capture or through co-formation.

The co-formation theory suggests that dust and debris left over from planetary formation consolidate into a moon.

The study also underscores how advances in isotopic analysis are making it possible to understand the Solar System’s origins.

Subtle differences in isotope composition and ratios provide insights into where planetary bodies originated and how they interacted during the Solar System's earlier phases.

While key questions remain, particularly those regarding how Earth and Theia’s impact occurred, the study offers a resolution to the longstanding isotopic crisis.