What is the new theory about the moon's origin involving Theia?

The new theory suggests Earth had a secret neighbor, a Mars-sized object named Theia, which collided with Earth 4.5 billion years ago to form the moon.

Where did the planet Theia form?

Scientists now believe Theia likely formed in the inner solar system, closer to the Sun than Earth, before its collision.

Why are Earth and the Moon so similar in composition according to the Theia theory?

The collision between Theia and Earth was so thorough that their materials mixed completely, resulting in the moon and Earth having identical isotopic compositions.

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Earth Had Secret Neighbor: New Moon Theory

21 Nov, 2025

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Summary

Scientists propose Earth had a secret neighbor, Theia, before the moon formed.
Theia was a Mars-sized object that collided with Earth 4.5 billion years ago.
New research suggests Theia formed in the inner solar system, closer to the Sun.

Earth Had Secret Neighbor: New Moon Theory

Scientists have unveiled a compelling new theory regarding the moon's formation, proposing that Earth once shared its cosmic neighborhood with a hidden companion. This Mars-sized object, dubbed Theia, is thought to have existed for the initial 100 million years of the solar system's history. Its stable orbit was believed to be closer to the Sun than Earth's current position.

The prevailing theory suggests Theia met its end approximately 4.5 billion years ago in a colossal collision with the early Earth. This cataclysmic event is credited with the creation of our moon, with Theia itself being completely obliterated and its remnants mixed into Earth's crust and the lunar body. Recent research focuses on analyzing isotopic ratios in Earth and moon rocks to pinpoint Theia's origins.

This latest analysis indicates that Theia likely formed within the inner solar system. The study of iron isotopes in lunar and terrestrial samples reveals a thorough mixing of material, making it difficult to ascertain the exact proportions of each body that contributed to the Earth-moon system. However, comparisons with meteorites suggest both Theia and proto-Earth were composed of rocky materials from the inner solar system.