Theia (planet)

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An artist's depiction of the hypothetical impact of a planet like Theia and the Earth

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.[1][2] Theia could explain why Earth's core is larger than expected for a body its size, with Theia's core and mantle fusing with those of Earth. Theia is hypothesized to have been about the size of Mars. Theia may have formed in the outer Solar System and provided much of Earth's water.


Theia was named after Theia, one of the Titans, who in Greek mythology was the mother of Selene, the goddess of the Moon,[3] which parallels the planet Theia's collision with the early Earth that is theorized to have created the Moon.[4] In modern Greek, it has the same origin as 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, with a diameter of about 6,102 kilometres (3,792 miles).[citation needed] Gravitational perturbations by Venus could have eventually put it onto a collision course with the early Earth.[5]


Animation of collision between Earth (blue) and Theia (black), forming the Moon (red and gray). Bodies are not to scale.

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).[5] Theia was eventually perturbed away from that relationship by the gravitational influence of Jupiter, Venus, or both, resulting in a collision between Theia and Earth.[citation needed]

Computer simulations suggest that Theia was traveling no faster than 4 km/s (14,000 km/h) when it struck Earth at an estimated 45-degree angle.

Initially, the hypothesis supposed that Theia had struck Earth with a glancing blow[6] 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.[7][8] 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.[9][10][11]


From the beginning of modern astronomy, there have been at least four hypotheses for the origin of the Moon:

  1. A single body split into Earth and Moon
  2. The Moon was captured by Earth's gravity (as most of the outer planets' smaller moons were captured)
  3. The Earth and Moon formed at the same time when the protoplanetary disk accreted
  4. The Theia-impact scenario described above.

The lunar rock samples retrieved by Apollo astronauts were found to be very similar in composition to Earth's crust, and so were likely removed from Earth in some violent event.[9][12][13]

By 2012, Matija Ćuk and Sarah Stewart theorized that Theia could explain why Earth's core is larger than expected for a body its size; Theia's core and mantle could have fused with those of Earth.[14]

Evidence published in 2019 suggests that Theia might have formed in the outer Solar System, and that much of Earth's water originated on Theia.[15]

See also[edit]


  1. ^ Wolpert, Stuart (January 12, 2017). "UCLA Study Shows the Moon is Older Than Previously Thought". UCLA. Retrieved 23 March 2022.{{cite web}}: CS1 maint: url-status (link)
  2. ^ "The Theia Hypothesis: New Evidence Emerges that Earth and Moon Were Once the Same". The Daily Galaxy. 2007-07-05. Retrieved 2013-11-13.
  3. ^ Murdin, Paul (2016). Rock Legends: The Asteroids and Their Discoverers. Springer. p. 178. doi:10.1007/978-3-319-31836-3. ISBN 9783319318363.
  4. ^ "Selene | Origin and meaning of selene by Online Etymology Dictionary".
  5. ^ a b "STEREO Hunts for Remains of an Ancient Planet near Earth". NASA. 2009-04-09. Archived from the original on 2013-11-13. Retrieved 2013-11-13.
  6. ^ Reufer, Andreas; Meier, Matthias M. M.; Benz, Willy; Wieler, Rainer (2012). "A hit-and-run giant impact scenario". Icarus. 221 (1): 296–299. arXiv:1207.5224. Bibcode:2012Icar..221..296R. doi:10.1016/j.icarus.2012.07.021. S2CID 118421530.
  7. ^ Jutzi, M.; Asphaug, E. (2011). "Forming the lunar farside highlands by accretion of a companion moon". Nature. 476 (7358): 69–72. Bibcode:2011Natur.476...69J. doi:10.1038/nature10289. PMID 21814278. S2CID 84558.
  8. ^ "Faceoff! The Moon's oddly different sides", Astronomy, August 2014, 44–49.
  9. ^ a b Nace, Trevor (2016-01-30). "New Evidence For 4.5 Billion Year Old Impact Formed Our Moon". Forbes. Retrieved 2016-01-30.
  10. ^ Young, E. D.; Kohl, I. E.; Warren, P. H.; Rubie, D. C.; Jacobson, S. A.; Morbidelli, A. (28 January 2016). "Oxygen isotopic evidence for vigorous mixing during the Moon-forming giant impact". Science. 351 (6272): 493–496. arXiv:1603.04536. Bibcode:2016Sci...351..493Y. doi:10.1126/science.aad0525. PMID 26823426. S2CID 6548599.
  11. ^ Wolpert, Stuart (January 28, 2016). "Moon was produced by a head-on collision between Earth and a forming planet". UCLA newsroom. UCLA.
  12. ^ Herwartz, D.; Pack, A.; Friedrichs, B.; Bischoff, A. (2014). "Identification of the giant impactor Theia in lunar rocks". Science. 344 (6188): 1146–1150. Bibcode:2014Sci...344.1146H. doi:10.1126/science.1251117. PMID 24904162. S2CID 30903580.
  13. ^ Meier, M. M. M.; Reufer, A.; Wieler, R. (2014). "On the origin and composition of Theia: Constraints from new models of the Giant Impact". Icarus. 242: 316–328. arXiv:1410.3819. Bibcode:2014Icar..242..316M. doi:10.1016/j.icarus.2014.08.003. S2CID 119226112.
  14. ^ "A New Model for the Origin of the Moon". SETI Institute.
  15. ^ Budde, Gerrit; Burkhardt, Christoph; Kleine, Thorsten (2019-05-20). "Molybdenum isotopic evidence for the late accretion of outer Solar System material to Earth". Nature Astronomy. 3 (8): 736–741. Bibcode:2019NatAs...3..736B. doi:10.1038/s41550-019-0779-y. ISSN 2397-3366. S2CID 181460133.