New evidence shows Theia formed near Earth in the inner Solar System
A recent study published in Science shows that Theia, the planetary body that collided with early Earth around 4.5 billion years ago and formed the Moon, originated in the inner Solar System, likely closer to the Sun than Earth.
Artist’s impression of the collision between the early Earth and Theia. Since Theia originated in the inner Solar System, in this perspective the Sun can be seen in the background. Credit: MPS/Mark A. Garlick
A new study in Science published on November 20, reveals that Theia, the long-lost planetary body that struck the young Earth 4.5 billion years ago, formed in the inner Solar System, possibly even closer to the Sun than Earth. This finding challenges decades of models that assumed the Moon-forming impactor came from a distant, unrelated region of the early Solar System.
The collision between Theia and the proto-Earth was one of the most transformative events in planetary history. The impact created the Moon, altered Earth’s rotation, and changed the chemistry of our planet’s interior forever. Understanding where Theia came from helps scientists answer a deeper question: how did the Earth–Moon system, so unique in the Solar System, come to exist?
Researchers from the Max Planck Institute for Solar System Research (MPS) and the University of Chicago have used high-precision isotope analyses to trace Theia’s origin. Their results show that Theia’s material carries isotopic fingerprints typical of the inner Solar System—indicating that Earth and Theia once shared the same neighborhood before their catastrophic merger.
“The composition of a body archives its entire history of formation, including its place of origin,” said Thorsten Kleine, Director at MPS and co-author of the new study. His statement underscores the idea that planetary chemistry holds an enduring record of where and how a world was born.
The fingerprints of a lost world
To track down Theia, the team examined what it left behind—its atoms. Isotopes, variants of an element with different numbers of neutrons, preserve clues to the conditions and distance from the Sun where a body formed. In the young Solar System, isotopes were distributed unevenly in the disk of gas and dust surrounding the Sun. Planets that formed closer in inherited different isotopic compositions than those that formed farther out.
In the new analysis, the researchers measured the ratios of iron isotopes in both Earth and Moon rocks with record-breaking precision. They analyzed 15 terrestrial samples and six lunar samples collected during the Apollo missions. Previous measurements had already shown that Earth and Moon are nearly identical in other isotope systems, such as chromium, calcium, titanium, and zirconium.
Those similarities confirm the Moon’s origin in a shared event but also make the puzzle more difficult. If Earth and Moon are isotopically indistinguishable, where is Theia’s signature? Some models suggest the Moon is mostly made from Theia; others claim it formed mainly from Earth’s mantle after the impact. The new isotope data help constrain which scenarios make physical sense.
For the first time, researchers relied on the precise ratio of mass-independent iron isotopes, a metric unaffected by geological processes, to identify Theia’s contribution. This approach provides a window into the earliest chemical sorting in the Solar System, unaffected by billions of years of planetary evolution.
Reconstructing Theia without simulations
Instead of relying on computer models of the impact, the scientists used what they call a “reverse engineering” method. They began with the known compositions of Earth and the Moon and worked backwards, calculating which combinations of Theia and proto-Earth materials could have produced the observed isotopic results.
This method lets them test multiple possible mixtures of elements. They studied not only iron but also chromium, molybdenum, and zirconium, each of which traces different stages of formation. Iron and molybdenum reveal what materials entered Earth’s mantle after core formation, while zirconium remains chemically stable and records the planet’s entire formation history.
Before Theia’s impact, Earth had already formed an iron-rich core. When metals segregated into the core, they carried away most of the iron and molybdenum. That means the iron now found in Earth’s rocky mantle must have arrived later, brought by Theia during the collision.
The study concludes that the metal humans use for tools, ships, and bridges is partly an inheritance from this vanished world.
Zirconium, by contrast, stayed behind through every phase of planetary evolution. Its isotopic consistency provides an anchor that lets scientists reconstruct Earth’s earliest material. Together, these elements formed a detailed record of how Earth and Theia mixed, merged, and transformed.
What meteorites reveal about Theia’s birthplace
To locate Theia’s home in the Solar System, the researchers compared its reconstructed isotopic profile to those of ancient meteorites. Meteorites are remnants of the planet-forming era, and their isotope ratios record where they formed in the protoplanetary disk. Some meteorites, such as enstatite chondrites, come from the inner Solar System within about 1 astronomical unit of the Sun, while others, such as carbonaceous chondrites, originated farther out.
The team found that Theia’s isotopic pattern differs from Earth’s but still matches the general signature of inner Solar System meteorites. In other words, Theia’s material was “not of this world,” but it was still from the same region of the Solar System as Earth. The calculations ruled out outer Solar System origins, which would have produced distinct isotopic contrasts.
Mathematical mass-balance modeling allowed the team to test multiple pre-impact scenarios. Some mixtures produced unrealistic compositions inconsistent with known meteorite chemistry. Others fit perfectly, indicating both Earth and Theia accumulated from material formed close to the Sun.
“The most convincing scenario is that most of the building blocks of Earth and Theia originated in the inner Solar System. Earth and Theia are likely to have been neighbors,” said Timo Hopp, lead author of the study and planetary scientist at MPS.
Why it matters for planetary formation
If Theia and Earth formed side by side, it challenges older theories that planets assembled from material mixed across vast Solar System distances. Instead, the study supports the view that the early Solar System was chemically divided into zones. Each terrestrial planet—Mercury, Venus, Earth, and Mars—grew mainly from local material in its orbital region.
This discovery reshapes how scientists think about planetary growth. The giant impact that produced the Moon was not a random collision between distant bodies but a natural outcome of local accretion and orbital instability among planetary embryos. Earth’s close neighbor became its most defining influence.
The isotopic similarities between Earth, the Moon, and inner Solar System meteorites suggest a shared chemical reservoir from which all terrestrial planets formed. The differences that remain record subtle variations in temperature and composition near the Sun. These patterns provide a chemical map of our Solar System’s earliest structure.
The research explains where the Moon came from and refines how scientists understand the origin of habitable worlds. By learning how material was distributed and recycled in the Solar System’s infancy, researchers can better predict how similar processes might shape rocky planets around other stars.
The legacy of a shared origin
Theia may be gone, but its legacy lives on in every iron atom within Earth’s mantle and in every rock on the Moon’s surface. The collision that destroyed Theia also forged the conditions for a stable Earth–Moon system, one that moderates our planet’s tilt, lengthens our days, and creates the tides that have influenced life for billions of years.
What once was a violent catastrophe now appears as a neighborly encounter—two planets born close together, colliding, and becoming one. “The composition of the Earth–Moon system tells us that our planet’s history is deeply local,” said Kleine. “It was shaped by material from the same inner Solar System region where we still orbit today.”
This study bridges planetary chemistry, astrophysics, and history. It suggests that at the dawn of the Solar System, proximity determined destiny. Earth’s closest neighbor became the agent of its transformation, and from that encounter arose the world we now call home.
References:
1 Theia and Earth were neighbors – Max Planck Institute for Solar System Research – November 20, 2025
2 The Moon-forming impactor Theia originated from the inner Solar System – Timo Hopp et al. – Science – November 20, 2025 – DOI: 10.1126/science.ado0623
I’m a science journalist and researcher at The Watchers, contributing to the Epicenter edition, where I cover peer-reviewed scientific research and emerging discoveries across Earth and space sciences. With a background in astronomy and a passion for environmental science, I’ve worked in shark and coral conservation in Fiji, conducting reef and shark-behavior research, contributing to mangrove restoration, and earning PADI Open Water and Coral Reef Certifications. I bring a blend of scientific rigor and storytelling to illuminate the discoveries shaping our planet and beyond.
Numerous years ago NASA reported another planet with the same compounds as earth. Only that planet was still hot. How could that planet or world have the same compounds as earth and not have another object hit it to make it the same? Perhaps it did have, but unlikely. Sorry but I do not have the article stating the other world with the same compounds as earth. I do remember it was NASA that reported it.