page_title: Theia (planet)
section_title: Collision
hierarchical_section_title: Theia (planet) / Collision
caption_attribution_description: The "Big Whack" theory, concerning the issue of the moon's origin.
context_page_description: Theia or Orpheus ) is a hypothesized ancient planet in the early Solar System that, according to the giant-impact hypothesis, collided with the early Earth around 4.5 billion years ago, with the resulting debris gathering to form the Moon.
  In addition to explaining Earth's large moon, the Theia hypothesis can also explain why Earth's core is larger than would be expected for a body its size; Theia's core and mantle mixed with Earth's.
  According to one version of the hypothesis, Theia was an Earth trojan about the size of Mars, with a diameter of about 6,102 km. Additional evidence published in 2019 suggests that Theia might have formed in the outer Solar System rather than the inner Solar System, and that much of Earth's water originated on Theia.
context_section_description: 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). 
  Theia was eventually perturbed away from that relationship by the gravitational influence of Jupiter and/or Venus, resulting in a collision between Theia and Earth.
  Computer simulations suggest that Theia was traveling no faster than 4 km/s (8,900 mph) when it struck Earth at an estimated 45 degree angle.
  Originally, the hypothesis supposed that Theia had struck Earth with a glancing blow 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. Such accounts assumed that if Theia had struck the proto-Earth head-on both planets would have been destroyed, 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 can be found in both the Earth and the Moon.
caption: Animation of collision event between Earth (blue) and Theia (black), forming the Moon (red and grey). Celestial bodies are not to scale.