It sounds like the plot of a science fiction blockbuster: a young planet colliding with another world, triggering a catastrophic explosion that scatters incandescent debris across space. Yet this dramatic scenario is not fiction — it is the leading scientific explanation for the origins of Earth and its moon.
Around 4.5 billion years ago, when Earth existed as a molten sphere of iron and other elements, it is widely believed to have been struck by a protoplanet roughly the size of Mars. This lost world, named Theia after a titaness in Greek mythology, is thought to have been obliterated in the collision, although fragments of it may still lie buried deep within our planet’s interior.
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While scientists broadly agree on the giant-impact hypothesis, several uncertainties remain — particularly regarding Theia’s origins.
A new study published in Science proposes that Theia was not a distant wanderer but rather a close neighbour of the early Earth, forming somewhere between our planet and the Sun.
The research, led by experts at the Max Planck Institute for Solar System Research (MPS), draws on detailed comparisons of the elemental composition of Earth, the moon, and meteorites that orbit near our planet. According to the findings, isotopes — the different atomic forms of elements — in lunar and terrestrial samples are virtually identical.
Isotopes share the same number of protons but differ in their neutron count, resulting in variations in mass and behaviour. For example, hydrogen exists as hydrogen-1, hydrogen-2 (deuterium), and hydrogen-3 (tritium), each with its own nuclear structure.
Understanding the link between Earth, the moon, and Theia has advanced significantly over the past decade. Until 2015, many scientists assumed the moon must contain material from beyond Earth, but more recent research revealed an identical match in oxygen isotopes between the two bodies.
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The latest work by geoscientist Timo Hopp and his colleagues goes even further, showing that iron and other metals also share near-identical isotopic signatures. Hopp told DW that although similarities in iron were anticipated, he was struck by how different these isotopes were in meteorites found near Earth, which appear to have formed from a separate chemical reservoir.
The evidence suggests that Earth and the moon were forged from the same suite of isotopes, while nearby meteorites originated elsewhere. If Theia had formed far from Earth, the team argues, its composition would likely have been noticeably different — and this difference should have been carried into lunar samples. Instead, the data points to Theia having formed from the same primordial material as Earth, almost like a planetary twin developing in our immediate cosmic vicinity. However, the study does not close the book on the early history of the solar system. Despite areas of broad agreement, new findings to challenge established theories.
Earlier in 2025, another German research team proposed that the moon was composed largely of material torn from Earth in the collision, with only a small contribution from Theia. Other analyses, drawing on different lunar samples or simulation models, have suggested that Theia may have come from farther afield or that the moon may have formed almost instantaneously after the impact. Several researchers involved in these earlier studies did not respond to requests for comment before publication.
Hopp noted that his team’s conclusions assume the colossal impact thoroughly mixed material from both worlds, creating a chemically blended Earth–Theia system. Exactly how such complete homogenisation might have occurred remains uncertain.
“The precise mechanisms and conditions required for this mixing process are still unknown and will need further study through simulations and modelling,” Hopp said.
