It's the stuff of a science fiction movie: a planet in the stages of formation collides with another. The massive impact causes both worlds to break apart, sending hot space rock flying.

But it's not sci-fi, it's the story of Earth.

Space scientists are largely agreed that about 4.5 billion years ago, Earth, then a hot ball of molten iron and other elements, was hit by another Mars-sized protoplanet. This hypothetical world is called Theia, named for a titaness of Greek mythology.

Theia was completely destroyed by this impact, but likely lives on beneath our feet, as fragments from this doomed world fused with the early Earth.

But while researchers are in broad agreement that this is how the Earth's earliest history in the solar system played out, many other questions remain unresolved — such as where Theia came from.

New research published in the journal Science suggests this protoplanet was actually a close neighbor of the early Earth, and formed somewhere between our home world and the sun. 

Theia: Space forensics analyzing a billion-year-old murder

The findings from a study led by researchers at the Max Planck Institute for Solar System Research (MPS) come from a detailed analysis of the elemental makeup of the Earth, our moon and nearby meteorites. 

It found that the elemental isotopes are practically indistinguishable between the Earth and the moon. 

Isotopes are chemical element "species" that have the same number of protons in their nucleus, but vary in the number of neutrons, resulting in variations with differing weights and properties. 

Hydrogen, for example, has a single proton in its nucleus, and no neutrons. Its isotopes are hydrogen-2 (known as deuterium), which has a proton and a neutron, and hydrogen-3 (tritium), with two neutrons and a proton in its nucleus.

Understanding of the relationship between Earth, the moon and Theia has shifted significantly in the last decade. It was as recent as 2015 that isotopic analysis found that Earth and the moon have identical oxygen isotopes. 

A recent study led by Timo Hopp, a geoscientist at MPS, goes further, showing that iron and other metal isotopes are all-but identical. 

"The similar iron isotopic composition of the moon and Earth was not that surprising, because this was shown before for other elements," Hopp told DW.

On the other hand, he was surprised to find that samples from near-Earth meteorites had very different iron isotopes. These, said Hopp, were found to "have distinct iron isotopic composition relative to Earth [and] the moon."

It's as if Earth and the moon were built from one set of isotopes, and meteorites from a nearby region of space from another.

Hopp's team argues that, if Theia had been formed beyond Earth's cosmic backyard, it would probably have a distinct composition, drawn from a different reservoir, that would have carried over to lunar samples.

However, since Earth and the moon are practically identical at this chemical level, Hopp's team suggests, Theia must have formed from the same isotopic building blocks as Earth — much as if this ancient world was a chemical twin, forming next door to our own planet.

Formation of the moon still needs modeling

This new study is not the final word in the story of our early solar system. And though parts of the story are agreed upon, scientists continue to make new findings that reshape the understanding of our solar system's early stages.

Earlier in 2025, another German research team suggested the moon was mainly formed by chunks of Earth that had been severed by the collision with Theia, but very little of Theia itself. 

Other studies had performed analyses that suggested Theia originated beyond the Earth, drawing on different lunar samples, or performed simulations suggesting that the moon could have formed almost instantly after Theia's impact. DW contacted several of these authors for comment on the latest research published by Hopp's group, but didn't receive a response before deadline. 

Hopp also confirmed that his team assumed that the giant impact "fully homogenized" material from the early Earth with Theia — effectively intermixing the two worlds. It's not clear, however, how this process may have happened.

"The actual mechanism and conditions of how this can be achieved are unknown, and need further investigation based on simulations and modeling," Hopp said.

Edited by: Maren Sass