[3][4] Theia is hypothesized to have been about the size of Mars, and may have formed in the outer Solar System and provided much of Earth's water, though this is debated.

[5] In Greek mythology, Theia was one of the Titans, the sister of Hyperion whom she later married, and the mother of Selene, the goddess of the Moon.

[12][13] Such accounts assumed that a head-on impact would have destroyed both planets, creating a short-lived second asteroid belt between the orbits of Venus and Mars.

In contrast, evidence published in January 2016 suggests that the impact was indeed a head-on collision and that Theia's remains are on Earth and the Moon.

However, a 2020 study showed that lunar rocks were more variable in oxygen isotope composition than previously thought, some differing more from Earth than others, with the more divergent values probably originating deeper in the lunar mantle suggested to be a more true reflection of Theia, and may suggest that Theia formed further away from the Sun than Earth.