Theia (planet)

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For other uses, see Theia (disambiguation).

Theia /θə/ 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).[1] Theia was about the size of Mars, and, had it not collided with Earth with a glancing blow, 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[2][3] which later merged to form the single Moon we know today. Others,[3] 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.


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.[4]


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.


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. Fortunately, Theia struck Earth with a glancing blow[2] 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.


Main article: Origin of 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 has stuck because Apollo 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.[5]


  1. ^ "The Theia Hypothesis: New Evidence Emerges that Earth and Moon Were Once the Same". The Daily Galaxy. 2007-07-05. Retrieved 2013-11-13. 
  2. ^ a b "Earth used to have TWO moons... but one was destroyed in a giant lunar collision". Mail online (reliable source?). 2011-08-04. Retrieved 2013-11-13. 
  3. ^ a b "Faceoff! The Moon's oddly different sides", Astronomy, August 2014, 44-49.
  4. ^ "STEREO Hunts for Remains of an Ancient Planet near Earth". NASA. 2009-04-09. Retrieved 2013-11-13. 
  5. ^ "Moon Created by Giant Collision, Studies Confirm". History. 2012-10-18.