Theia () is a hypothesized ancient

planet A planet is a large, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...

in the early Solar System which, according to the

giant-impact hypothesis The giant-impact hypothesis, sometimes called the Theia Impact, is an astrogeology hypothesis for the formation of the Moon first proposed in 1946 by Canadian geologist Reginald Daly. The hypothesis suggests that the Early Earth collided wi ...

, collided with the early Earth around 4.5 billion years ago, with some of the resulting ejected debris coalescing to form the

Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

. Collision simulations support the idea that the

large low-shear-velocity provinces Large low-shear-velocity provinces (LLSVPs), also called large low-velocity provinces (LLVPs) or superplumes, are characteristic structures of parts of the lowermost mantle, the region surrounding the outer core deep inside the Earth. These provi ...

in the lower mantle may be remnants of Theia. Theia is hypothesized to have been about the size of

Mars Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...

, and may have formed in the outer Solar System and provided much of Earth's water, though this is debated.

Name

Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of ancient Greek folklore, today absorbed alongside Roman mythology into the broader designation of classical mythology. These stories conc ...

Theia Theia (; , also rendered Thea or Thia), also called Euryphaessa (, "wide-shining"), is one of the twelve Titans, the children of the earth goddess Gaia and the sky god Uranus in Greek mythology. She is the Greek goddess of sight and vision, an ...

was one of the

Titans In Greek mythology, the Titans ( ; ) were the pre-Twelve Olympians, Olympian gods. According to the ''Theogony'' of Hesiod, they were the twelve children of the primordial parents Uranus (mythology), Uranus (Sky) and Gaia (Earth). The six male ...

, the sister of Hyperion whom she later married, and the mother of

Selene In ancient Greek mythology and Ancient Greek religion, religion, Selene (; , meaning "Moon")''A Greek–English Lexicon's.v. σελήνη is the goddess and personification of the Moon. Also known as Mene (), she is traditionally the daughter ...

, 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

, with a diameter of about . Gravitational perturbations by

Venus Venus is the second planet from the Sun. It is often called Earth's "twin" or "sister" planet for having almost the same size and mass, and the closest orbit to Earth's. While both are rocky planets, Venus has an atmosphere much thicker ...

could have put it onto a collision course with the early Earth.

Size

Theia is often suggested to be around the size of Mars, with a mass about 10% that of current Earth; however, its size is not definitively settled, with some authors suggesting that Theia may have been considerably larger, perhaps 30% or even 40-45% the mass of current Earth making it nearly equal to the mass of proto-Earth.

Collision

NASA supercomputer simulation on how the Moon formed

According to the

giant impact hypothesis The giant-impact hypothesis, sometimes called the Theia Impact, is an astrogeology hypothesis for the formation of the Moon first proposed in 1946 by Canadian geologist Reginald Daly. The hypothesis suggests that the Early Earth collided wi ...

, Theia orbited the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

, 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 point In celestial mechanics, the Lagrange points (; also Lagrangian points or libration points) are points of equilibrium (mechanics), equilibrium for small-mass objects under the gravity, gravitational influence of two massive orbit, orbiting b ...

s (''i.e.'', either L₄ or L₅). Theia was eventually perturbed away from that relationship, most likely by the gravitational influence of

Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...

, 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 The asteroid belt is a torus-shaped region in the Solar System, centered on the Sun and roughly spanning the space between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies called asteroids ...

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 (as most of the

outer planet The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

s' smaller moons were captured) # The Earth and Moon formed at the same time when the

protoplanetary disk A protoplanetary disk is a rotating circumstellar disc of dense gas and dust surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may not be considered an accretion disk; while the two are sim ...

accreted # The Theia-impact scenario described above The lunar rock samples retrieved by

Apollo Apollo is one of the Twelve Olympians, Olympian deities in Ancient Greek religion, ancient Greek and Ancient Roman religion, Roman religion and Greek mythology, Greek and Roman mythology. Apollo has been recognized as a god of archery, mu ...

astronauts An astronaut (from the Ancient Greek (), meaning 'star', and (), meaning 'sailor') is a person trained, equipped, and deployed by a List of human spaceflight programs, human spaceflight program to serve as a commander or crew member of a spa ...

were found to be very similar in composition to Earth's crust, and so were likely removed from Earth in some violent event. It is possible that the

detected deep in Earth's mantle may be fragments of Theia. In 2023, computer simulations reinforced that hypothesis.

Composition

The composition of Theia and how different it was from Earth is disputed and subject to debate. It is considered unlikely that Theia had an exactly similar isotopic composition to proto-Earth. A key constraint has been that the many isotope ratios of retrieved rocks from the Moon are nearly identical to those from Earth, either implying that the two bodies were extensively homogenized by the collision, or that the isotopic composition of Theia was very similar to Earth. However, a 2020 study showed that lunar rocks were more variable in oxygen isotope composition than previously thought, some differing more from Earth than others, with the more divergent values probably originating deeper in the lunar mantle suggested to be a more true reflection of Theia, and may suggest that Theia formed further away from the Sun than Earth.

References

{{Portal bar, Stars, Spaceflight, Outer space Lunar science Hypothetical impact events Hypothetical bodies of the Solar System Hypothetical planets Water Space Solar System Hypothetical astronomical objects