Astronomy:Theia (planet)

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Short description: Planet hypothesized to have impacted Earth and created the Moon
An artist's depiction of the hypothetical impact of a planet like Theia and the Earth

Theia (/ˈθə/) 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] Such a collision, with the two planets' cores and mantles fusing, could explain why Earth's core is larger than expected for a body its size. Collision simulations support the idea that the large low-shear-velocity provinces in the lower mantle may be remnants of Theia.[3][4] Theia is hypothesized to have been about the size of Mars, and may have formed in the outer Solar System and provided much of Earth's water.


In Greek mythology, Theia was one of the Titans. She was the mother of Selene, the goddess of the Moon,[5] a story that parallels the planet Theia's theorized role in creating the Moon.[6]


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, with a diameter of about 6,102 kilometres (3,792 miles).[citation needed] Gravitational perturbations by Venus could have put it onto a collision course with the early Earth.[7]


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).[7] 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]

Initially, the hypothesis supposed that Theia had struck Earth with a glancing blow[8] 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.[9][10] 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.[11][12][13]


Main page: Astronomy:Origin of the Moon

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.[11][14][15]

By 2012, theories found 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.[16] It is also possible that the large low-shear-velocity provinces detected deep in Earth's mantle may be fragments of Theia. It is possible that Theia was captured by the Earth.[17][18] In 2023, computer simulations reinforced that hypothesis.[4][19] Evidence published in 2019 suggests that Theia might have formed in what is now the asteroid belt and that much of Earth's water originated on Theia.[20]

See also


  1. Wolpert, Stuart (January 12, 2017). "UCLA Study Shows the Moon is Older Than Previously Thought". UCLA. 
  2. "The Theia Hypothesis: New Evidence Emerges that Earth and Moon Were Once the Same". The Daily Galaxy. 2007-07-05. 
  3. Sample, Ian (November 1, 2023). "Blobs near Earth's core are remnants of collision with another planet, study says". The Guardian. 
  4. 4.0 4.1 Chang, Kenneth (1 November 2023). "A 'Big Whack' Formed the Moon and Left Traces Deep in Earth, a Study Suggests - Two enormous blobs deep inside Earth could be remnants of the birth of the moon.". The New York Times. Archived from the original on 1 November 2023. Retrieved 2 November 2023. 
  5. Murdin, Paul (2016). Rock Legends: The Asteroids and Their Discoverers. Springer. p. 178. doi:10.1007/978-3-319-31836-3. ISBN 9783319318363. 
  6. "Selene | Origin and meaning of selene by Online Etymology Dictionary". 
  7. 7.0 7.1 "STEREO Hunts for Remains of an Ancient Planet near Earth". NASA. 2009-04-09. 
  8. Reufer, Andreas; Meier, Matthias M. M.; Benz, Willy; Wieler, Rainer (2012). "A hit-and-run giant impact scenario". Icarus 221 (1): 296–299. doi:10.1016/j.icarus.2012.07.021. Bibcode2012Icar..221..296R. 
  9. Jutzi, M.; Asphaug, E. (2011). "Forming the lunar farside highlands by accretion of a companion moon". Nature 476 (7358): 69–72. doi:10.1038/nature10289. PMID 21814278. Bibcode2011Natur.476...69J. 
  10. "Faceoff! The Moon's oddly different sides", Astronomy, August 2014, 44–49.
  11. 11.0 11.1 Nace, Trevor (2016-01-30). "New Evidence For 4.5 Billion Year Old Impact Formed Our Moon". Forbes. 
  12. 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. doi:10.1126/science.aad0525. PMID 26823426. Bibcode2016Sci...351..493Y. 
  13. Wolpert, Stuart (January 28, 2016). "Moon was produced by a head-on collision between Earth and a forming planet". UCLA. 
  14. Herwartz, D.; Pack, A.; Friedrichs, B.; Bischoff, A. (2014). "Identification of the giant impactor Theia in lunar rocks". Science 344 (6188): 1146–1150. doi:10.1126/science.1251117. PMID 24904162. Bibcode2014Sci...344.1146H. 
  15. 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. doi:10.1016/j.icarus.2014.08.003. Bibcode2014Icar..242..316M. 
  16. "A New Model for the Origin of the Moon". SETI Institute.
  17. Yuan, Qian; Li, Mingming; Desch, Steven J.; Ko, Byeongkwan (2021). "Giant impact origin for the large low shear velocity provinces". 52nd Lunar and Planetary Science Conference. Retrieved 27 March 2021. 
  18. Why are there continent-sized 'blobs' in the deep Earth?, BBC Future, by Zaria Gorvett, 12 May 2022
  19. Yuan, Qian (1 November 2023). "Moon-forming impactor as a source of Earth's basal mantle anomalies". Nature 623 (7985): 95–99. doi:10.1038/s41586-023-06589-1. PMID 37914947. Bibcode2023Natur.623...95Y. Archived from the original on 2 November 2023. Retrieved 2 November 2023. 
  20. Budde, Gerrit; Burkhardt, Christoph; Kleine, Thorsten (2019-05-20). "Molybdenum isotopic evidence for the late accretion of outer Solar System material to Earth" (in en). Nature Astronomy 3 (8): 736–741. doi:10.1038/s41550-019-0779-y. ISSN 2397-3366. Bibcode2019NatAs...3..736B.