Theia (planet)

Theia is the hypothetical planet that, according to the giant impact hypothesis of the Moon's formation, collided with Earth over four billion years ago.

Birth

Theia, like the other planets of the solar system, was born about 4.5 billion years ago. According to the Giant impact theory, Theia would have been approximately Mars-sized and developed in the Earth-Sun L₄ or L₅ position, similarly to Trojan asteroids. However, it would cease to be in a stable position due to its growing mass crossing the threshold at which it could no longer be considered negligible for a two-body Earth-Sun problem. It would begin to oscillate between approaching Earth and receding from Earth, before finally approaching close enough to fall along a path which would lead to a moderate-velocity impact (at a glancing angle) with Earth.

Destruction

This impact at a glancing angle would have destroyed Theia, its iron core burying itself within the primordial Earth. Volatiles in its mantle and surface would become massive quantities of ejecta thrown into orbit around Earth. From that material the Moon coalesced quickly, possibly within less than a month but in no more than a century. Earth would have gained significant amounts of angular momentum from such a collision, as well as an increase in total mass to its current level.

In Greek mythology, Theia was a Titan who gave birth to the Moon goddess Selene.

Other names often given to this hypothetical planet include Orpheus and Hephaestus.

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