Theia

The name of the hypothesized protoplanet is derived from the mythical Greek titan Theia, 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 coalesced at the L4 or L5 Lagrangian point relative to Earth (in about the same orbit and about 60° ahead or behind),similar to a trojan asteroid. The stability of Theia's orbit was affected when its growing mass exceeded a threshold of about 10% of the Earth's mass.

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. 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. In astronomical terms, the impact would have been of moderate velocity. Theia is thought to have struck the Earth at an oblique angle when the latter was nearly fully formed. Computer simulations of this "late-impact" scenario suggest an impact angle of about 45° and an initial impactor velocity below 4 km/s.Theia's iron core sank into the young Earth's core, as most of Theia's mantle and a significant portion of the Earth's mantle and crust were ejected into orbit around the Earth. This material quickly coalesced into the Moon (possibly within less than a month, but in no more than a century). Estimates based on computer simulations of such an event suggest that some two percent of the original mass of Theia ended up as an orbiting ring of debris, and about half of this matter coalesced into the Moon. The Earth would have gained significant amounts of angular momentum and mass from such a collision.

Regardless of the rotation and inclination the Earth had before the impact, it would have had a day some five hours long after the impact, and the Earth's equator would have shifted closer to the plane of the Moon's orbit. It has been suggested that other significant objects may have been created by the impact, which could have remained in orbit between the Earth and Moon, stuck in Lagrangian points. Such objects may have stayed within the Earth-Moon system for up to 100 million years, until the gravitational tugs of other planets destabilized the system enough to free the objects.It is thoght that the subsequent collision between the Moon and one of these smaller bodies caused the notable differences in physical characteristics between the two hemispheres of the Moon.This collision, simulations have supported, would have been at a low enough velocity so as not to form a crater; the material from the smaller body would instead have spread out across the Moon (in what would become its far side), adding a thick layer of highlands crust.