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,[6] a story that parallels the planet Theia's theorized role in creating the Moon.[7]
Orbit
Theia is hypothesized to have orbited in the
L4 or L5 configuration presented by the Earth–Sun system, where it would tend to remain. If this were the case it might have grown to a size comparable to
Mars, with a diameter of about 6,102 kilometres (3,792 miles).[citation needed]Gravitational perturbations by
Venus could have put it onto a collision course with the early Earth.[8]
Collision
According to the
giant impact hypothesis, Theia orbited the
Sun, nearly along the orbit of the
proto-Earth, by staying close to one or the other of the Sun-Earth system's two more stable
Lagrangian points (i.e., either L4 or L5).[8] Theia was eventually
perturbed away from that relationship by the gravitational influence of
Jupiter,
Venus, or both, resulting in a collision between Theia and Earth.[citation needed]
Initially, the hypothesis supposed that Theia had struck Earth with a glancing blow[9] and ejected many pieces of both the
proto-Earth and Theia, those pieces either forming one body that became the Moon or forming two moons that eventually merged to form the Moon.[10][11] 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.[12][13][14]
The lunar rock samples retrieved by
Apolloastronauts were found to be very similar in composition to Earth's crust, and so were likely removed from Earth in some violent event.[12][15][16]
By 2012, theories found Theia could explain why
Earth's core is larger than expected for a body its size; Theia's core and mantle could have fused with those of Earth.[17] It is also possible that the
large low-shear-velocity provinces detected deep in Earth's mantle may be fragments of Theia. It is possible that Theia was captured by the Earth.[18][19] In 2023, computer simulations reinforced that hypothesis.[4][20]