Theia // is a hypothesized ancient planetary-mass object in the early Solar System that according to the giant impact hypothesis collided with the early Earth around 4.31 billion years ago (bya). According to the hypothesis, Theia was an Earth trojan about the size of Mars, with diameter of about 6,000 km (3,700 miles). New research by UCLA based on rock analysis sourced from Apollo missions 12, 15, and 17 suggests that Theia collided head-on with Earth, in contrast to the previous theory that suggested a glancing impact. Models of the impact propose that Theia's debris gathered around Earth to form the early Moon. Some scientists think that the material thrown into orbit originally formed two moons that later merged to form the single moon we know today. However, the two-moon hypothesis is not necessary to explain the difference in the faces of the near and far sides of the Moon. The Theia theory also explains why Earth's core is larger than would be expected for a body its size: according to the hypothesis, Theia's core and mantle mixed with Earth's.
Theia is thought 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 Earth.
Theia was named for the titaness, Theia, who in Greek mythology was the mother of Selene, the goddess of the moon; that echoes the planet Theia's collision with the early Earth, theorized to have created the Moon.
An alternative name, Orpheus, had also been used, but has[when?] largely been discarded.
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), but
- was eventually perturbed away from that relationship by the gravitational influence of Jupiter and/or Venus, resulting in a collision between Theia and Earth.
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 Theia striking the proto-Earth head-on would have led to the destruction of both planets, 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 both in Earth and the Moon.
From the beginning of modern astronomy, there have been at least four hypotheses for the origin of the Moon:
- that a single body split into Earth and the Moon;
- that the Moon was captured by Earth's gravity (as most of the outer planets' smaller moons were captured);
- that Earth and the Moon formed at the same time when the protoplanetary disk accreted; and
- the Theia scenario.
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