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 one Earth trojan has 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: A 150 to 500-metre-diameter asteroid, discovered using the Wide-field Infrared Survey Explorer (WISE) satellite, is the only confirmed Earth trojan as of 2017.
- No known objects are currently thought to be L5 trojans of Earth. A search was conducted in 1994 covering 0.35°2 of sky under poor observing conditions  which failed to detect any objects "The limiting sensitivity of this search was magnitude ~22.8, corresponding to C-type asteroids ~350m in diameter or S-type asteroids ~175m in diameter."
This section does not cite any sources. (July 2013) (Learn how and when to remove this template message)
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).
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, showering material from the two planets into space. This material eventually accreted around Earth and into a single orbiting body, the Moon.
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.
Astronomy continues to retain interest in the subject. A publication 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, either L4 or 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.
- Reilly, M. (27 July 2011). "Earth Stalker Found in Eternal Twilight". New Scientist. Retrieved 2014-02-21.
- Choi, C. Q. (27 July 2011). "First Asteroid Companion of Earth Discovered at Last". Space.com. Retrieved 2011-07-27.
- Robert J. Whiteley & David J. Tholen, 1998, in Icarus vol.136, pages 154–167 (1998) Article No.IS985995A "CCD Search for Lagrangian Asteroids of the Earth–Sun System", Received November 24, 1997; revised April 13, 1998.
- "NASA mission to search for rare asteroids". NASA. Retrieved 2017-03-01.
- "OSIRIS-REx Asteroid Search Tests Instruments". NASA. Retrieved 2017-03-24.
- "太陽−地球系のL5点付近の観測について". JAXA. Retrieved 2017-04-18.
- "Mission Status of Hayabusa2" (PDF). 49th Lunar and Planetary Science Conference 2018. Retrieved 2018-08-10.
- "Earth is actually two planets, scientists conclude"
- Malhotra, Renu (February 18, 2019). "The case for a deep search for Earth's Trojan asteroids". Nature Astronomy. 3 (3): 193–194. arXiv:1903.01922. doi:10.1038/s41550-019-0697-z.
- Murray, C. (1997). "The Earth's secret companion". Nature. 387 (6634): 651–652. Bibcode:1997Natur.387..651M. doi:10.1038/42585.
- Agle, DC; Brown, Dwayne; Cantillo, Laurie (15 June 2016). "Small Asteroid Is Earth's Constant Companion". NASA. Retrieved 15 June 2016.
- "The Theia Hypothesis: New Evidence Emerges that Earth and Moon Were Once the Same". The Daily Galaxy. 2007-07-05. Retrieved 2013-11-13.