Theia The Celestial Body That Crashed Into Earth Originated In Inner Solar System

Researchers led by the Max Planck Institute for Solar System Research (MPS) and the University of Chicago have determined that Theia, the celestial body that collided with Earth approximately 4.5 billion years ago, originated in the inner Solar System. Published on November 20, 2025, in the journal Science, the study utilized unprecedentedly precise measurements of iron isotope ratios in 15 terrestrial rocks and six lunar samples obtained from the Apollo missions. These isotopic compositions reveal clues about Theia’s formation history and place of origin, suggesting it formed closer to the Sun than Earth, thereby advancing understanding of the Moon’s birth and the early Solar System.

Theia’s Composition and Origin

Researchers determined Theia’s possible composition by analyzing iron isotope ratios in 15 terrestrial and six lunar rocks obtained from the Apollo missions. This analysis, published November 20, 2025, in Science, aimed to deduce Theia’s place of origin. The team also examined isotopes of chromium, molybdenum, and zirconium, considering how these elements partition into planetary mantles—explaining why some, like gold, are rare. This ‘reverse engineering’ approach seeks to understand the building blocks of both Earth and Theia.

The study suggests Theia and Earth likely originated as “neighbors” in the inner Solar System, closer to the Sun than Earth currently resides. Calculations indicate that while Earth’s building blocks resemble known meteorite classes from various outer Solar System locations, Theia’s composition may include previously unknown material. Researchers used meteorite classes as reference material for available building materials during the early stages of planetary formation.

Determining the origin of Theia is complex as models vary regarding the source of Moon material. While some models suggest the Moon formed almost exclusively from Theia, others propose significant contributions from Earth’s mantle or an inseparable mixing of materials. The isotopic composition of elements like iron, which accumulated in Earth’s core after its formation, provides key clues about material arriving “on board” Theia, helping refine potential scenarios.

Isotopic Analysis of Earth and Moon

Researchers utilized isotopic analysis of iron, chromium, molybdenum, and zirconium in terrestrial and lunar rocks—specifically 15 terrestrial and 6 Apollo mission lunar samples—to deduce the composition of Theia, the celestial body that collided with Earth. The unprecedented precision of these measurements revealed Earth and Moon share indistinguishable isotope ratios for these elements. This similarity, however, didn’t directly reveal Theia’s origin, necessitating a “reverse engineering” approach to planetary formation scenarios.

To understand Theia’s composition, researchers played through various collision scenarios, considering different compositions and sizes for both Theia and the early Earth. They leveraged the different affinities of elements like iron and molybdenum for metal, noting how these elements partitioned into planetary mantles. The team used known meteorite classes as reference material for the building blocks of early Earth and Theia, recognizing that Theia may have incorporated previously unknown material.

The most compelling scenario suggests both Earth and Theia originated in the inner Solar System, implying they were neighbors. Calculations indicate Theia’s building materials originated closer to the Sun than Earth’s, differing from the source of materials found in many meteorite classes. This conclusion stems from analyzing isotope ratios and considering element partitioning during planetary formation, offering new insights into the Moon’s origins.

These elements have different affinities for metal and therefore partition into planetary mantles in different proportions; this is why gold is so rare and precious!

Nicolas Dauphas of the University of Chicago and the University of Hong Kong

Tracing Theia Through Reverse Engineering

Researchers are using a “reverse engineering” approach to understand Theia, the celestial body that collided with Earth to form the Moon. By analyzing the ratios of iron isotopes in 15 terrestrial and six lunar rocks—samples brought back by Apollo missions—the team is reconstructing possible compositions of both Theia and the early Earth. This method aims to determine what combination of compositions and sizes could have resulted in the isotopic similarities observed in present-day Earth and Moon rocks.

The study focuses on iron, chromium, molybdenum, and zirconium isotopes, as these elements behave differently during planetary formation. Elements like iron and molybdenum accumulated in the early Earth’s core, while others like zirconium remained in the mantle, providing a record of the planet’s history. Analyzing these isotopes allows scientists to trace the building blocks of both Earth and Theia and infer their origins, as isotope ratios varied across the early Solar System.

Calculations suggest that Theia likely originated closer to the Sun than Earth, originating in the inner Solar System. While the early Earth’s building materials can be linked to known meteorite classes, Theia’s composition appears to involve previously unknown materials. This suggests Earth and Theia were neighbors in the early Solar System, providing insight into the conditions and materials present during the Moon’s formation approximately 4.5 billion years ago.

Inner Solar System Origins of Theia and Earth

New research, published November 20, 2025, in Science, suggests Theia—the celestial body that collided with Earth to form the Moon—originated in the inner Solar System. Researchers determined Theia’s possible composition by analyzing iron isotope ratios in 15 terrestrial and six lunar rocks brought back by Apollo missions. This analysis builds on earlier measurements of chromium, calcium, titanium, and zirconium isotope ratios, revealing similarities between Earth and Moon compositions. Understanding Theia’s “ingredients” is key to pinpointing its place of origin.

The research team utilized a “reverse engineering” approach, calculating potential Theia compositions and sizes that would result in the observed isotopic ratios in Earth and Moon rocks. They examined iron, chromium, molybdenum, and zirconium isotopes, recognizing that elements partition differently during planetary formation—like iron accumulating in cores while zirconium remains in the mantle. This allows insights into the history of building materials and identifies whether material originated before or after Earth’s core formation.

Calculations indicate that the building blocks of both Earth and Theia likely originated in the inner Solar System, suggesting they were neighbors closer to the Sun than Earth. While Earth’s composition aligns with known meteorite classes, Theia appears to have involved previously unknown material. By comparing meteorite compositions—representing different areas of the outer Solar System—researchers determined that Theia’s origin was closer to the Sun than our planet.