The planet that created the Moon was our neighbor

New research from the Max Planck Institute argues that Theia and Earth were actually cosmically neighbors: Theia was born in the interior of the Solar System.

Around 4.5 billion years ago, the most remarkable event in the history of our planet took place: a huge celestial body called Theaia with young Earth.

How the collision unfolded and what exactly happened afterwards has not yet been conclusively determined. What is certain, however, is that Earth’s size, composition, and orbit changed as a result—and that the impact marked the birth of our constant companion in space, the Lua.

What kind of body was it that so dramatically altered the course of our planet’s development? How big was Theia? What was it made of? And from what part of the Solar System did it rush towards Earth?

It is difficult to find answers to these questions. After all, Theia was completely destroyed in the collision. However, it can still be found today, for example, in the current compositions of the Earth and the Moon. In a recent study, published on November 20, 2025 in the journal Science, researchers from the Max Planck Institute for Solar System Research and the University of Chicago use this information to deduce the possible Theia’s “list of ingredients” and, consequently, its place of origin.

The proportions in which certain metal isotopes are present in a body are particularly revealing. Isotopes are variants of the same element that differ only in the number of neutrons in their atomic nucleus and, therefore, in their weight. Even in the molecular cloud from which our Sun and the protoplanetary disk formed, the isotopes of these elements were apparently not evenly distributed.

On the contrary: even at that time, depending on the distance from the center, there must have been differences in the proportions of the isotopes. Thus, the planetary bodies that were still growing were formed from building material with different isotopic compositions, depending on whether the material clustered near or far from the Sun.

Information about the origin of its original building blocks is thus stored in the isotopic composition of a planetary body.

Search for traces of Theia on Earth and the Moon

In the current study, the research team determined the proportion of different iron isotopes in rocks from the Earth and the Moon with unprecedented precision. To this end, they examined 15 Earth rocks and six lunar samples that astronauts from the Apollo missions brought to Earth.

The result is not surprising: as previous measurements of the isotopic ratios of chromium, calcium, titanium and zirconium had already demonstrated, the Earth and the Moon are indistinguishable in this aspect.

However, the great similarity does not allow for any direct conclusions about Theia. There are simply too many possible collision scenarios. Although most models assume that the Moon formed almost exclusively from material from Theia, it is also possible that it was made up mainly of material from the early Earth’s mantle or that rocks from Earth and Theia were inseparably mixed.

Reverse engineering a planet

To learn more about Theia, researchers applied a kind of reverse engineering to planets. To do this, they do not use complex computer models that simulate various impact scenarios involving Theia, but focus on isotope mixtures in Earth and lunar rocks.

Based on the corresponding isotope ratios in current Earth and lunar rocks, the team analyzed what compositions and sizes of Theia and what composition of early Earth could have led to this final state.

In their investigations, scientists looked not only at isotopes of iron, but also those of chromium, molybdenum and zirconium. The different elements give access to different phases of planetary formation.

Thanks to Theia

But how can we know which material was already there and which was brought to the Earth-Moon system by Theia? Long before the devastating encounter with Theia, a kind of selection process took place within the primitive Earth. With the formation of the iron core, some elements, such as iron and molybdenum, accumulated there; after that, they were practically absent from the rocky mantle. The iron that is currently found in the Earth’s mantle can therefore only have “arrived” after the formation of the nucleus, for example, on board Theia.

The ubiquitous metal, from which humans made tools, ships and bridges, can therefore be attributed primarily to Theia. Another element is the zirconiumwhich is very resistant and hardly undergoes changes. It has been in the mantle for as long as the Earth has existed and has not sunk into the core. Zirconium thus documents not just a window of time, but the entire history of the formation of our planet.

Researchers use these different sources of information to define what material and mixture of materials Theia must have been made of and, finally, from what part of the primitive gas and dust disk this material originated before forming Theia.

Meteorites as a reference

According to the research results, the isotopic ratios of material from Theia differ significantly from those on Earth. Therefore, they are not “of this world” and can still be identified as such in the mix of Earth’s material.

However, mathematical calculations reveal various scenarios and compositions of Earth and Theia before their collision. Some of these scenarios are implausible because they are incompatible with knowledge about early planetary formation that has also been obtained through meteorite analysis.

Meteorites that can be analyzed after impact with Earth are as old as the Solar System. They allow us to know the time in which planets and other bodies were formed. They therefore serve as reference material for the building material that was available during the formation of early Earth and Theia.

They are divided into different classes based on their isotopic ratios: some meteorites originate from the inner region of the planet-forming disk, while others originate from the outer region.

The isotope ratios of the Earth’s mantle most closely resemble those of meteorites from the inner Solar System.

The isotopes that the team attributes to Theia in this study have proportions that were previously unknown and do not correspond to the building blocks of Earth.

By comparing them with classes of meteorites, researchers conclude that Theia must have originated in the inner part of the early Solar System, closer to the Sun than Earth’s current orbit.