Theia (planet)

Theia (pronunciation: /θiːə/) is a hypothesized ancient planetary-mass object in the early Solar System that, according to the giant impact hypothesis, collided with another planetary-mass object, Gaia (the early Earth) around 4.51 billion years ago. According to the hypothesis, Theia was an Earth trojan about the size of Mars, with diameter of about 6,000 km (3,700 miles). Geologist Edward Young of the University of California, Los Angeles, drawing on an analysis of rocks collected by Apollo missions 12, 15, and 17, suggests that Theia collided head-on with Earth, in contrast to the previous theory that suggested a glancing impact. Models of the impact propose that Theia's debris gathered around Earth to form the early Moon.

Some scientists think that the material thrown into orbit originally formed two moons that later merged to form the single moon we know today. The Theia hypothesis also explains why Earth's core is larger than would be expected for a body its size: according to the hypothesis, Theia's core and mantle mixed with Earth's. Analysis of lunar rocks, published in 2016, suggests that the impact may have been a direct hit, causing a thorough mixing of both parent bodies.

Theia is thought to have orbited in the L4 or L5 configuration presented by the Earth–Sun system, where it would tend to remain. In that case, it would have grown, potentially to a size comparable to Mars. Gravitational perturbations by Venus could have eventually put it onto a collision course with the Earth.

...
Wikipedia