It’s like a science fiction movie: a planet collides with another planet in the formation stage. The massive impact causes both worlds to shatter, sending hot space rocks flying.
But this is not science-fiction, this is Earth’s story.
Space scientists largely agree that about 4.5 billion years ago, Earth, then a hot ball of molten iron and other elements, collided with another Mars-sized protoplanet. This fictional world is called Theia, named after the title figure in Greek mythology.
Theia was completely destroyed by this impact, but possibly survives beneath our feet, as fragments of this destroyed world fused with the early Earth.
But while researchers widely agree that Earth’s early history in the Solar System has been similar, many other questions remain unresolved – such as where Theia came from.
new research Published in Science Journal Turns out that this protoplanet was actually a close neighbor of the early Earth, and formed somewhere between our homeworld and the Sun.
Theia: Space forensics analyzing a billion-year-old murder
The findings of a study led by researchers at the Max Planck Institute for Solar System Research (MPS) come from a detailed analysis of the elemental composition of Earth, our Moon and nearby meteorites.
It found that the elemental isotopes between the Earth and the Moon are practically indistinguishable.
Isotopes are chemical element “species” that have the same number of protons in their nuclei, but differing numbers of neutrons, resulting in different weights and properties.
For example, the nucleus of hydrogen contains one proton, and no neutrons. Its isotopes are hydrogen-2 (known as deuterium), which has one proton and one neutron in its nucleus, and hydrogen-3 (tritium), which has two neutrons and one proton in its nucleus.
The understanding of the relationship between the Earth, the Moon, and Theia has changed significantly over the past decade. This was most recently isotopic analysis in 2015 found That the Earth and the Moon have similar oxygen isotopes.
A recent study by Hopp goes even further, showing that iron and other metal isotopes are exactly the same.
“The similar iron isotope composition of the Moon and Earth was not as surprising, as it had previously been shown for other elements,” MPS geologist and study leader Timo Hopp told DW.
On the other hand, Hope was surprised to find that samples of meteorites near Earth had very different iron isotopes. Hope said, these “were found to have a unique iron isotope composition relative to Earth.” [and] Moon.”
It is as if the Earth and Moon were formed from one set of isotopes, and meteorites from a nearby region of space were formed from another.
Hope’s team argues that, if Theia was formed beyond Earth’s cosmic backyard, it would likely have a different composition, derived from a different reservoir than the one that would have led to the lunar samples.
However, since Earth and the Moon are practically identical at this chemical level, Hope’s team suggests, Theia must have been formed from the same isotopic building blocks as Earth – as if this ancient world were a chemical twin, forming next to our own planet.
Moon’s formation still requires modeling
This new study is not the last word on the story of our early solar system. And although there is consensus on some parts of the story, scientists continue to make new findings that reshape our understanding of the early stages of our solar system.
Earlier in 2025, another German research team suggested The Moon was formed primarily from pieces of Earth that were broken apart by the collision with Theia, but very little of Theia itself.
Other studies have analyzed Suggested that Theia originated beyond EarthDrawing on various lunar samples, or performing simulations suggesting the Moon could be made almost immediately After Thea’s influence. DW contacted several of these authors for comment on the latest research published by the Hope Group, but did not receive a response before deadline.
Hopp also confirmed that his team assumed that the massive impact of “completely homogeneous” material from the early Earth with Theia was effectively merging the two worlds. However, it is unclear how this process might have occurred.
“The actual mechanism and conditions of how this can be achieved are unknown, and require further investigation based on simulations and modeling,” Hopp said.
Edited by: Maren Sass