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Theia (planet)

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 Gaia (the early Earth) around 4.5 billion years ago.[1][2] According to the hypothesis, Theia was an Earth trojan about the size of Mars, with a diameter of about 6,102 km (3,792 miles). Geologist Edward Young of the University of California, Los Angeles, drawing on an analysis of rocks collected by Apollo missions 12, 15, and 17, proposes that Theia collided head-on with Earth,[3] in contrast to the previous theory that suggested a glancing impact. Models of the impact indicate that Theia's debris gathered around Earth to form the early Moon.

Some scientists think the material thrown into orbit originally formed two moons[4][5] that later merged to form the single moon we know today. The Theia hypothesis 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.[6]

Additional evidence published in 2019 suggests that Theia may have formed in the outer solar system rather than the inner solar system, thus making it analogous to a Kuiper-Belt object like Pluto, and that much of Earth's water originated on Theia.[7]


Theia was named for the titaness Theia, who in Greek mythology was the Titaness of Clear Sight and she gave gold its lustre and diamonds their sparkle and she was also the mother of Selene, the goddess of the moon, and Helios was the God of the Sun.[8] which parallels the planet Theia's collision with the early Earth that is theorized to have created the Moon.[9]

An alternative name, Orpheus, is also used.[10]


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 the early Earth. [11]


Animation of collision event between Earth (blue) and Theia (black), forming the Moon (red->grey). Celestial 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).[11] Theia was eventually perturbed away from that relationship by the gravitational influence of Jupiter and/or Venus, resulting in a collision between Theia and Earth.[citation needed]

Computer simulations suggest that Theia was traveling no faster than 4 km/s (8,900 mph) when it struck Earth at an estimated 45 degree angle.

Originally, the hypothesis supposed that Theia had struck Earth with a glancing blow[12] 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.[4] Such accounts assumed that if Theia had struck the proto-Earth head-on both planets would have been destroyed, 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 in both the Earth and the Moon.[13][14]


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

  1. A single body split into Earth and Moon
  2. Moon was captured by Earth's gravity (as most of the outer planets' smaller moons were captured)
  3. Earth and Moon formed at the same time when the protoplanetary disk accreted
  4. Theia scenario described above.
  5. A massive asteroid hit Earth, broke up in pieces and eventually accreted to formed the Moon.

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.[3][15][16]

See also


  1. ^ Wolpert, Stuart (2017-01-12). "UCLA study shows the moon is older than previously thought". Retrieved 14 January 2017.
  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. ^ a b Nace, Trevor (2016-01-30). "New Evidence For 4.5 Billion Year Old Impact Formed Our Moon". Forbes. Retrieved 2016-01-30.
  4. ^ a b 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.
  5. ^ "Faceoff! The Moon's oddly different sides", Astronomy, August 2014, 44–49.
  6. ^ "A New Model for the Origin of the Moon". SETI Institute.
  7. ^ 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. doi:10.1038/s41550-019-0779-y. ISSN 2397-3366.
  8. ^ Murdin, Paul (2016). Rock Legends: The Asteroids and Their Discoverers. Springer. p. 178. doi:10.1007/978-3-319-31836-3. ISBN 9783319318363.
  9. ^ "Selene | Origin and meaning of selene by Online Etymology Dictionary".
  10. ^ Byrne, Charles (2007). The Far Side of the Moon: A Photographic Guide. Springer. p. 202. ISBN 9780387732060.
  11. ^ 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.
  12. ^ 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.
  13. ^ 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. doi:10.1126/science.aad0525. PMID 26823426.
  14. ^ Wolpert, Stuart (January 28, 2016). "Moon was produced by a head-on collision between Earth and a forming planet". UCLA newsroom. UCLA.
  15. ^ 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.
  16. ^ 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.