How the destroyed world ‘Theia’ became part of Earth and the Moon

A new wave of research is reshaping what we know about our own origins and the Moon’s.

Scientists now say Earth once had a “missing twin,” a planet-sized world called Theia that collided with the early Earth around 4.5 billion years ago. The impact was so violent that Theia was completely destroyed, blasting debris from both bodies into space.

That debris eventually fused into the Moon, explaining why lunar rocks match Earth’s chemical fingerprints almost perfectly.

The breakthrough comes from detailed analysis of ancient rocks collected during the Apollo missions and samples found on Earth. By studying their metal isotope ratios, chemical signatures that vary depending on where a planet formed, researchers concluded that Theia likely originated even closer to the Sun than the early Earth.

The collision mixed their materials so thoroughly that traces of Theia now survive in two places: deep inside Earth’s mantle and across the lunar surface.

This evidence strengthens the long-debated giant-impact theory, offering the clearest picture yet of how Earth grew, how the Moon formed, and how violent collisions shaped the early Solar System.