Theia // is a hypothesized ancient planet in the early Solar System that according to the giant impact hypothesis collided with the Early Earth around 4.533 billion years ago (BYa). Theia was about the size of Mars; if the impact were not glancing, it could have destroyed Earth. In the end, Theia's debris gathered together around Earth to form what was the early Moon. After the debris from the collision flew into space, some scientists think that it originally formed two moons which later merged to form the single Moon we know today. Others, however, believe that the two-moon hypothesis is not necessary to explain the difference in the faces of the near and far sides of the Moon. The theory also explains the reason that Earth's core is larger than it would be for a body its size, since according to the theory, Theia's core merged with Earth's.
Theia was thought to have orbited in the L4 or L5 spots in the Earth's orbit. It grew to a Mars-like size and, through gravitational perturbation from Venus, was moved around and eventually set on a collision course with Earth.
The name for Theia came from the Greek goddess, Theia. In mythology, Theia was the mother of Selene, the goddess of the moon, just as the planet Theia's collision with the early Earth is theorized to have created the Moon. An alternative name, Orpheus, has been used in the past but has largely been discarded.
Theia orbited the Sun at around the orbit of Earth at the L4 or L5 Lagrangian points, but was perturbed by Jupiter and Venus into a collision with the early Earth. Theia struck Earth with a glancing blow and ejected many pieces of both Earth and Theia. These pieces either formed one body that became the Moon, or formed two moons that eventually merged to form the Moon. Had Theia struck Earth head-on, it would have led to the destruction of both planets, creating a short-lived asteroid belt between the orbits of Venus and Mars.
From the beginning of modern astronomy, many hypotheses have been proposed for the origin of the Moon: that a single body somehow divided into Earth and Moon, that the Moon was captured by Earth's gravity (as most of the outer planets' smaller moons were captured), that Earth and Moon formed at the same time when the protoplanetary disk accreted, and the Theia scenario. The lunar rock samples retrieved by Apollo astronauts were found to be very similar in composition to Earth's crust, and so were likely removed from Earth in some violent event.
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- "Faceoff! The Moon's oddly different sides", Astronomy, August 2014, 44-49.
- "STEREO Hunts for Remains of an Ancient Planet near Earth". NASA. 2009-04-09. Retrieved 2013-11-13.
- "Moon Created by Giant Collision, Studies Confirm". History. 2012-10-18.