Theia’s Origin Revealed: Moon-Forming Impact Came From Inner Solar System

researchers led by the Max Planck Institute for Solar System Research (MPS) and the university of Chicago have figured out where Theia, the celestial body that crashed into Earth around 4.5 billion years ago, came from – the inner Solar System. Published on November 20, 2025, in the journal Science, the study used incredibly precise measurements of iron isotope ratios in 15 rocks from Earth and six samples collected during the Apollo missions. These ratios give us clues about how Theia formed and where it originated, suggesting it formed closer to the Sun than Earth did. this helps us understand how the Moon was born and what the early Solar System was like.

Theia’s Composition and Origin

Researchers persistent what Theia might have been made of by analyzing iron isotope ratios in 15 terrestrial and six lunar rocks from the Apollo missions. This analysis, published November 20, 2025, in Science, was designed to figure out where Theia came from. The team also looked at isotopes of chromium, molybdenum, and zirconium, thinking about how these elements end up in planetary mantles – explaining why some, like gold, are so rare. This ‘reverse engineering’ approach helps us understand what Earth and Theia were built from.

The study suggests Theia and Earth likely formed as “neighbors” in the inner Solar System, closer to the Sun than Earth is now. Calculations show that while Earth’s building blocks look like meteorite types from the outer Solar System, Theia’s composition might include materials we haven’t found yet. Researchers used different meteorite types as examples of what building materials were available when the planets were forming.

Figuring out where Theia came from isn’t easy because there are different ideas about where the Moon’s material came from. Some models say the Moon formed almost entirely from Theia, while others think Earth’s mantle contributed a lot, or that the materials mixed entirely.The isotopic composition of elements like iron, which settled into Earth’s core after it formed, gives us important clues about what theia brought with it, helping us narrow down the possibilities.

Isotopic Analysis of Earth and Moon

Researchers used isotopic analysis of iron, chromium, molybdenum, and zirconium in rocks from Earth and the Moon – specifically 15 terrestrial and 6 Apollo mission lunar samples – to figure out what Theia, the body that collided with Earth, was made of. The amazing precision of these measurements showed that Earth and the Moon have almost identical isotope ratios for these elements.But this similarity didn’t instantly reveal Theia’s origin, so they needed to use a “reverse engineering” approach to understand how planets form.