A groundbreaking new study reveals that Theia—the Mars-sized protoplanet that collided with early Earth and formed the Moon—originated in the inner Solar System, not the outer regions as long believed. Using advanced iron isotope analysis, researchers show Earth and Moon share the same chemical “DNA,” reshaping our understanding of planetary formation and habitability.
TheInterviewTimes.com | 24 November 2025: In a landmark discovery reshaping theories of planetary formation, scientists have revealed compelling evidence that Theia, the ancient protoplanet that collided with early Earth and created the Moon, formed within the inner Solar System—possibly even closer to the Sun than proto-Earth itself. The breakthrough, published just four days ago in Science, closes a decades-long debate over where Earth’s most influential cosmic neighbor originated.
Led by Timo Hopp of the Max Planck Institute for Solar System Research and the University of Chicago, the new research provides the clearest chemical proof yet that Theia and Earth were “local siblings,” forged from the same rocky building blocks more than 4.5 billion years ago. As the world marks 56 years since Apollo 12 brought home the lunar samples that made such discoveries possible, scientists say this new evidence brings us closer than ever to solving the origin story of the Moon.
A Violent Birth: Revisiting the Giant-Impact Hypothesis
The giant-impact hypothesis, first proposed in the 1970s, states that Theia, a Mars-sized world weighing about 10% of Earth’s mass, slammed into our young planet roughly 95 million years after the Solar System formed. The glancing collision melted Earth’s mantle, vaporized huge amounts of rock, and launched molten debris into orbit—material that cooled and eventually became the Moon.
Despite its scientific acceptance, the model faced two major challenges:
- The Moon lacks volatile elements, indicating formation close to the Sun in an extremely hot environment.
- Earth and Moon share nearly identical isotopic signatures, which should not be possible if Theia came from the outer Solar System.
Carbonaceous chondrites—meteorites from beyond 2.5 AU—carry distinct chemical fingerprints. If Theia originated there, lunar rocks should reflect it. Instead, Moon samples look nearly indistinguishable from Earth’s mantle.
The new research finally resolves this mystery.
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Iron Isotopes Reveal Theia’s True Home
Hopp’s team analyzed iron isotopes in:
- 15 terrestrial samples
- 6 Apollo lunar rocks
- Multiple meteorite groups
Their focus was iron because its isotopes separate during core formation. Earth’s core drained heavy iron isotopes from the mantle, meaning any heavier iron found today must have arrived later—most likely from Theia.
The results were striking.
Theia’s iron signature closely matched enstatite chondrites, dry, rocky materials found between 0.7 and 1.2 AU—solid proof of an inner Solar System origin. Carbonaceous chondrites, by contrast, showed sharp chemical differences.
Researchers concluded that Theia likely formed sunward of Earth, possibly between 0.8 and 0.9 AU, before gravitational nudges—perhaps from Venus—triggered its fatal journey toward Earth.
This solves the isotopic puzzle: Earth and Theia were born from the same regional material, so their chemical “DNA” was nearly identical from the start.
Buried Remnants of Theia Still Inside Earth
One of the most intriguing implications is that parts of Theia may still exist deep underground.
A 2023 Caltech model proposed that dense remnants of Theia’s mantle survived the impact and sank to the base of Earth’s mantle, forming two enormous structures known as Large Low-Shear-Velocity Provinces (LLSVPs) beneath Africa and the Pacific.
The new isotopic analysis strengthens this theory:
LLSVP compositions resemble enstatite-like materials, not outer-Solar-System matter.
If true, Theia may have shaped not only the Moon—but also Earth’s geology, influencing volcanic hotspots, mantle plumes, and even plate tectonics.
A New Understanding of How Habitable Worlds Form
This study does more than rewrite the Moon’s origin story. It reframes planetary evolution itself.
If Earth’s stability, rotation, magnetic field, and the Moon’s protective presence arose from local building blocks in the inner Solar System, then the recipe for habitability may be far more common than previously believed.
Future missions such as NASA’s Artemis program may soon drill deeper into the Moon’s mantle to test these predictions, while seismic missions explore Mars and other rocky worlds for Theia-like signatures.
For now, one truth stands out:
The Moon is not only Earth’s companion—it is the fossilized remains of a lost world that once orbited our Sun.
Key Takeaways
- Theia formed in the inner Solar System, likely closer to the Sun than Earth.
- New iron isotope data explains why Earth and Moon share near-identical chemistry.
- Remnants of Theia may still be buried deep inside Earth as LLSVP structures.
- The discovery offers new insights into planetary formation and habitability.
- Future Moon missions may uncover more direct evidence of Theia’s ancient past.