Theia (hypothetical planet)
Theia is a hypothesized ancient planet in the early Solar System which, according to the giant-impact hypothesis, collided with the proto-Earth around 4.5 billion years ago, with some of the resulting ejected debris re-coalescing to form the Moon. Collision simulations support the idea that the two large low-shear-velocity provinces in the Earth's lower mantle may be remnants of Theia. Theia is hypothesized to have been about the size of Mars and likely formed at the L4 or L5 Lagrange points of the Earth's orbit, although some hypotheses debatably suggested it may have formed in the Outer Solar System and later migrated into the Earth's orbit, and might have provided much of Earth's water.
Name
In Greek mythology, Theia was one of the Titans, the sister of Hyperion whom she later married, and the mother of Selene, the goddess of the Moon. This story parallels the planet Theia's theorized role in creating the Moon.Orbit
Theia is hypothesized to have orbited in the L4 or L5 configuration presented by the Earth–Sun system, where it would tend to remain. If this were the case it might have grown to a size comparable to Mars. Gravitational perturbations by Venus could have put it onto a collision course with the early Earth.Later studies have investigated Inner Solar System orbits. This hypothesis is supported by isotope analysis of both lunar and terrestrial rocks.
Size of the planet
Theia is often suggested to be around the size of Mars, with a mass about 10% that of modern-day Earth. However, its size is not certain, with some authors suggesting that it may have been larger, perhaps 30% or even 40–45% the mass of Earth, making it nearly equal to the mass of the proto-Earth.Collision
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 Earth–Sun system's two more stable Lagrangian points. Theia was eventually perturbed away from that relationship, most likely by the gravitational influence of Jupiter, Venus, or both, resulting in a collision between Theia and Earth.Initially, 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 a head-on impact would have destroyed 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 are on Earth and the Moon.
Simulations suggest that Theia would be responsible for around 70–90% of the total mass of the Moon under a classic giant impact scenario where Theia is considerably smaller than proto-Earth.
Hypotheses
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
- The Earth and Moon formed at the same time when the protoplanetary disk accreted
- The Theia-impact scenario described above
It is possible that the large low-shear-velocity provinces detected deep in Earth's mantle may be fragments of Theia. In 2023, computer simulations reinforced that hypothesis.