Astronomy:Earth trojan

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Short description: Asteroid with which Earth shares its orbit around the Sun
The orbit of 2010 TK7, the first Earth trojan to be discovered (left). Lagrangian points L4 and L5. Lines around the blue triangles represent tadpole orbits (right)

An Earth trojan is an asteroid that orbits the Sun in the vicinity of the Earth–Sun Lagrangian points L4 (leading 60°) or L5 (trailing 60°), thus having an orbit similar to Earth's. Only two Earth trojans have so far been discovered. The name "trojan" was first used in 1906 for the Jupiter trojans, the asteroids that were observed near the Lagrangian points of Jupiter's orbit.


2010 TK7, one of the two known Earth trojans, is located at the lower right, circled by a small green ring.

L4 (leading)

L5 (trailing)

  • No known objects are currently thought to be L5 trojans of Earth.


An Earth-based search for L5 objects was conducted in 1994, covering 0.35 square degrees of sky, under poor observing conditions.[5] That search failed to detect any objects:

"The limiting sensitivity of this search was magnitude ~22.8, corresponding to C-type asteroids ~350 m in diameter, or S-type asteroids ~175 m in diameter."[5]

In February 2017, the OSIRIS-REx spacecraft performed a search from within the L4 region on its way to asteroid Bennu.[6] No additional Earth trojans were discovered.[7]

In April 2017, the Hayabusa2 spacecraft searched the L5 region while proceeding to asteroid Ryugu,[8] but did not find any asteroids there.[9]


The orbits of any Earth trojans could make them less energetically costly to reach than the Moon, even though they will be hundreds of times more distant. Such asteroids could one day be useful as sources of elements that are rare near Earth's surface. On Earth, siderophiles such as iridium are difficult to find, having largely sunk to the core of the planet shortly after its formation.

A small asteroid could be a rich source of such elements even if its overall composition is similar to Earth's; because of their small size, such bodies would lose heat much more rapidly than a planet once they had formed, and so would not have melted, a prerequisite for differentiation (even if they differentiated, the core would still be within reach). Their weak gravitational fields also would have inhibited significant separation of denser and lighter material; a mass the size of 2010 TK7 would exert a surface gravitational force of less than 0.00005 times that of Earth (although the asteroid's rotation could cause separation).

Giant-impact hypothesis

A hypothetical planet-sized Earth trojan the size of Mars, given the name Theia, is thought by proponents of the giant-impact hypothesis to be the origin of the Moon. The hypothesis states that the Moon formed after Earth and Theia collided,[10] showering material from the two planets into space. This material eventually accreted around Earth and into a single orbiting body, the Moon.[11]

At the same time, material from Theia mixed and combined with Earth's mantle and core. Supporters of the giant-impact hypothesis theorise that Earth's large core in relation to its overall volume is as a result of this combination.

Continuing interest in near-Earth asteroids

Astronomy continues to retain interest in the subject. A publication[12] describes these reasons thus:

The survival to the present day of an ancient [Earth Trojan] population is reasonably assured provided Earth's orbit itself was not strongly perturbed since its formation. It is therefore pertinent to consider that modern theoretical models of planet formation find strongly chaotic orbital evolution during the final stages of assembly of the terrestrial planets and the Earth–Moon system.

Such chaotic evolution may at first sight appear unfavorable to the survival of a primordial population of [Earth trojans]. However, during and after the chaotic assembly of the terrestrial planets, it is likely that a residual planetesimal population, of a few percent of Earth's mass, was present and helped to damp the orbital eccentricities and inclinations of the terrestrial planets to their observed low values, as well as to provide the so-called "late veneer" of accreting planetesimals to account for the abundance patterns of the highly siderophile elements in Earth's mantle.

Such a residual planetesimal population would also naturally lead to a small fraction trapped in the Earth's Trojan zones as Earth's orbit circularized. In addition to potentially hosting an ancient, long-term stable population of asteroids, Earth's Trojan regions also provide transient traps for NEOs that originate from more distal reservoirs of small bodies in the solar system like the main asteroid belt.

Other companions of Earth

Several other small objects have been found on an orbital path associated with Earth. Although these objects are in 1:1 orbital resonance, they are not Earth trojans, because they do not librate around a definite Sun–Earth Lagrangian point, neither L4 nor L5.

Earth has another noted companion, asteroid 3753 Cruithne. About 5 km across, it has a peculiar type of orbital resonance called an overlapping horseshoe, and is probably only a temporary liaison.[13]

469219 Kamoʻoalewa, an asteroid discovered on 27 April 2016, is possibly the most stable quasi-satellite of Earth.[14]

See also


  1. Reilly, M. (27 July 2011). "Earth stalker found in eternal twilight". New Scientist. Retrieved 2014-02-21. 
  2. Choi, C.Q. (27 July 2011). "First asteroid companion of Earth discovered at last". 
  3. "OSIRIS-REx searches for Earth-trojan asteroids" (Press release). NASA. 9 February 2017.
  4. Hui, Man-To; Wiegert, Paul A.; Tholen, David J.; Föhring, Dora (November 2021). "The second Earth trojan 2020 XL5". The Astrophysical Journal Letters 922 (2): L25. doi:10.3847/2041-8213/ac37bf. Bibcode2021ApJ...922L..25H. 
  5. 5.0 5.1 Whiteley, Robert J.; Tholen, David J. (1998). "CCD search for Lagrangian asteroids of the Earth–Sun system". Icarus 136: 154–167. article no. IS985995A.  Received 24 November 1997; revised 13 April 1998.
  6. "NASA mission to search for rare asteroids" (Press release). NASA. Retrieved 2017-03-01.
  7. "OSIRIS-REx asteroid search tests instruments". NASA. 
  8. "太陽−地球系のL5点付近の観測について". JAXA. 2017-04-11. 
  9. "Mission status of Hayabusa2". 49th Lunar and Planetary Science Conference 2018. Retrieved 2018-08-10. 
  10. Knapton, Sarah (29 January 2016). "Earth is actually two planets, scientists conclude". The Telegraph. 
  11. "The Theia hypothesis: New evidence emerges that Earth and Moon were once the same". 2007-07-05. 
  12. Malhotra, Renu (February 18, 2019). "The case for a deep search for Earth's Trojan asteroids". Nature Astronomy 3 (3): 193–194. doi:10.1038/s41550-019-0697-z. Bibcode2019NatAs...3..193M. 
  13. Murray, C. (1997). "The Earth's secret companion". Nature 387 (6634): 651–652. doi:10.1038/42585. Bibcode1997Natur.387..651M. 
  14. Agle, D.C.; Brown, Dwayne; Cantillo, Laurie (15 June 2016). "Small asteroid is Earth's constant companion". NASA / JPL.