Theia // is a hypothesized ancient Planet in the Early Solar System that supposedly collided with the Early Earth around 4,533,000,000 (4.533 BYa.) Theia was about the size of Mars, and if it hadn't collided with Earth with a glancing blow, may have destroyed it. 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, scientists think that it originally formed 2 moons. The moons later merged to form the single Moon we know today.
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.
Theia orbited the Sun at around the orbit of Earth at the L4 or L5 Lagrangian points, but was disturbed by Venus and thrown towards the early Earth. Fortunately, Theia struck Earth with a glancing blow and ejected many pieces of both Earth and the early planet, which later formed 2 moons which collided and formed the Moon.
Scientists, ever since the 1800s, wondered how the Moon had formed, and many theories came out, such as the theory that somehow the Moon separated from Earth at some time, the theory that the Moon was captured by Earth's gravity and brought into orbit, the Moon was formed with the Earth from the beginning when the Protoplanetary disk accreted, and the theory that Theia hit Earth and formed the moon. The last theory stuck because Astronauts on the moon took back lunar rock samples which were very similar in composition to the Earth, and as it is highly improbable that a large chunk of Earth came off of it, the Theia collision theory stuck.
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- "Moon Created by Giant Collision, Studies Confirm". History. last updated 2012-10-18.