Giant impact hypothesis

Theia

The name of the hypothesized protoplanet is derived from the mythical Greek goddess Theia, a Titan who gave birth to the Moon goddess Selene. According to the giant impact hypothesis, Theia formed alongside the other planet size bodies in the Solar System about 4.6 Ga (4.6 billion years ago), and was approximately the size of Mars.

One formation theory is that Theia materialized at the L4 or L5 Lagrangian points relative to Earth (in about the same orbit and about 60° ahead or behind),[1] similar to a trojan asteroid.[9] The stability of Theia's orbit was affected when its growing mass exceeded a threshold of about 10% of the Earth's mass.[1] Gravitational perturbations by planetesimals caused Theia to depart from its stable Lagrangian location, and subsequent interactions with proto-Earth caused the two bodies to collide.[1]

Astronomers think the collision between Earth and Theia happened about 4.53 Ga; about 30-50 million years after the rest of the Solar System formed. However, evidence presented in 2008 suggests that the collision may have occurred later, at about 4.48 Ga.[10]