Theia And Earth Were Neighbors

New research suggests that the body that collided with Earth 4.5 billion years ago, creating the Moon, originated in the inner Solar System.

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To the point:

• "Ingredients" of impactor: In the current issue of the journal Science, researchers determine the possible composition of Theia.
• Search for birthplace: The impactor's composition allows conclusions about its place of origin. It is located in the inner Solar System, likely closer to the Sun than Earth.
• Moon samples in the laboratory: Lunar rocks from the Apollo missions were used in the analyses. For the first time, researchers used their ratio of iron isotopes to determine the origin of Theia.

About 4.5 billion years ago, the most momentous event in the history of our planet occurred: a huge celestial body called Theia collided with the young Earth. How the collision unfolded and what exactly happened afterwards has not been conclusively clarified. What is certain, however, is that the size, composition, and orbit of the Earth changed as a result - and that the impact marked the birth of our constant companion in space, the Moon.

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 which part of the Solar System did it hurtle toward Earth? Finding answers to these questions is difficult. After all, Theia was completely destroyed in the collision. Nevertheless, traces of it can still be found today, for example in the composition of present-day Earth and Moon. In the current study, published on November 20, 2025, in the journal Science, researchers led by the Max Planck Institute for Solar System Research (MPS) and the University of Chicago use this information to deduce the possible "list of ingredients" of Theia - and thus its place of origin.

The ratios 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 thus in their weight. Even in the molecular cloud from which our sun and the protoplanetary disk first formed, the isotopes of these elements were apparently not evenly distributed. Rather, depending on the distance from the center, there must have been differences in the isotope ratios even back then. Accordingly, the planetary bodies, which were still growing at that time, were formed from building material with different isotopic compositions, depending on whether the material clumped together close to 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.

Searching for traces of Theia in Earth and Moon

In the current study, the research team determined the ratio of different iron isotopes in Earth and Moon rocks with unprecedented precision. To this end, they examined 15 terrestrial rocks and six lunar samples that astronauts from the Apollo missions brought back to Earth. The result is hardly surprising: as earlier measurements of the isotope ratios of chromium, calcium, titanium, and zirconium had already shown, Earth and Moon are indistinguishable in this respect.

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

Reverse engineering of a planet

In order to learn more about Theia, the researchers applied a kind of reverse engineering for planets. To do this, they do not use complex computer models that simulate various impact scenarios involving Theia, but instead focus on the isotope mixtures in Earth and lunar rocks. Based on the matching isotope ratios in today's terrestrial and lunar rocks, the team played through which compositions and sizes of Theia and which composition of the early Earth could have led to this final state. In their investigations, the researchers looked not only at iron isotopes, but also at those of chromium, molybdenum, and zirconium. The different elements give access to different phases of planetary formation.

Steel 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 sorting process had taken place inside the early Earth. With the formation of the iron core, some elements such as iron and molybdenum accumulated there; they were afterwards largely absent from the rocky mantle. The iron found in the Earth's mantle today can therefore only have "arrived" after the formation of the core, for example on board Theia.

The ubiquitous metal, from which humans made tools, ships, and bridges, could therefore be attributed primarily to Theia. Another element is zirconium, which is very resistant and hardly undergoes any 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 only a window of time, but the entire history of our planet's formation. Researchers use these different information carriers to define what material and mixture of materials Theia must have consisted of and, finally, from which part of the early gas and dust disc this material originated before it formed Theia.

Meteorites as a reference

According to the research results, the isotope ratios of Theia's material differ significantly from those of Earth. They are therefore "not of this world" and can still be identified as such in the mixture of Earth's material today. However, mathematical calculations reveal several scenarios and compositions of Earth and Theia prior to their collision. Some of these are implausible, however, as they are incompatible with knowledge on early planetary formation that has also been gained through the analysis of meteorites.

Meteorites that can be analyzed after impacting Earth are as old as the solar system. They provide insight into a time when planets and other bodies were formed. They therefore serve as reference material for the building material that was available during the formation of the early Earth and Theia. They are divided into different classes based on their isotope 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 assigns to Theia in the study have ratios that were previously unknown and do not match the building blocks of Earth. By comparing them with meteorite classes, the researchers conclude that Theia must have originated from the inner part of the early solar system, closer to the Sun than Earth's current orbit.

MPG/BEU