Theia is a hypothesized ancient planet in the early Solar System that, according to the giant-impact hypothesis, collided with the early Earth around 4.5 billion years ago, with some of the resulting ejected debris gathering to form the Moon.
In addition to explaining Earth's large satellite, 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 core and mantle.
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 (3,792 miles). 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.
Theia was named for the titaness Theia, who in Greek mythology was the mother of Selene, the goddess of the Moon, which parallels the planet Theia's collision with the early Earth that is theorized to have created the Moon. In modern Greek, it has the same origin with the words "θείος" (theios) and "θεία" (theia) ('uncle' and 'aunt', also meaning 'divine' in Ancient Greek).
Theia is hypothesized to have orbited in the L4 or L5 configuration presented by the Earth–Sun system, where it would tend to remain. In that case, it would have grown, potentially to a size comparable to Mars. Gravitational perturbations by Venus could have eventually put it onto a collision course with the early Earth.
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.
From the beginning of modern astronomy, there have been at least four hypotheses for the origin of the Moon:
- A single body split into Earth and Moon
- The Moon was captured by Earth's gravity (as most of the outer planets' smaller moons were captured)
- The Earth and Moon formed at the same time when the protoplanetary disk accreted
- The Theia-impact scenario described above.
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