Astronomers have just confirmed the existence of only the second-known Earth Trojan. This isn’t some mythical wooden horse or pesky computer virus, but actually a massive, 1-km-wide asteroid that shares an orbit with the planet, clustered around special gravitationally balanced areas known as Lagrange points.

Trojan asteroids trail ahead or behind the orbit of a planet at approximately 60°, in the Lagrange points L4 and L5. These are gravitational “sweet spots” where the influence of two large bodies, such as the Sun and a planet, cancel each other out, so a relatively tiny body isn’t drawn towards any particular object. Instead, objects stay put in the same orbital point relative to the two large bodies, which is why NASA recently sent Hubble’s successor, the powerful James Webb Telescope, to L2, where it remains in a stable orbit with its back constantly facing the sun in order to perform infrared observations of some of the most distant objects in the universe with minimal disturbances.

The first official Trojan was discovered in a Lagrange point around Jupiter on February 22, 1906, by German astronomer Max Wolf. Two other Trojans were found quickly after, and the three were named Achilles, Patroclus, and Hektor. By 2017, more than 6,400 Trojans had been spotted:  4,184 at Jupiter’s L4 point and 2,326 at L5. In order to track them, Austrian astronomer Johann Palisa, a prolific discoverer of asteroids, came up with the naming convention where asteroids near the L4 point were named for Greek heroes from Homer’s Iliad (The Achean camp) and those near L5 for Trojan heroes (the Trojan camp). However, 617 Patroclus (at L5) and 624 Hektor (at L4) were named before this convention took root, so each camp has a “spy” in its midst!

Although Jupiter and its swarm of Trojans comprise by far the lion’s share of Lagrangian asteroids in the solar system, astronomers have identified Trojans near other worlds, such as Mars (4 to date, 1 at L4 and 3 at L5) and Neptune (8 Trojans, 6 at L4 and 2 at L5) and even Earth.

The first Earth Trojan, called 2010 TK₇, was found a decade ago. It’s estimated to be less than 400 meters across. The second, newly found Trojan, known as 2020 XL₅, is nearly three times larger, with an estimated diameter of 1.2 kilometers (0.7 miles). It was discovered on 12 December 2020 by the Pan-STARRS1 telescope in Hawai‘i during a routine survey of the sky. A preliminary analysis suggests the asteroid’s orbit may be compatible with L4 and after some convincing work by researcher Toni Santana-Ros, the director of the 4.1-meter SOAR (Southern Astrophysical Research) Telescope on Cerro Pachón in Chile was persuaded to allocate more observation time to confirm this hypothesis.

Armed with new precise measurements of 2020 XL₅ of movements in the sky, the astronomers could then access archival images taken since 2012 by the Víctor M. Blanco 4-meter Telescope located at the Cerro Tololo Inter-American Observatory (CTIO), in Chile.

“The day we discovered the precovery data was an explosion of emotions. Suddenly, out of the blue, we had 10 years of observations of our object! Santana-Ros,” an astronomer with the Institut de Ciències del Cosmos (ICCUB) at the Universitat de Barcelona and lead author of the new study, told ZME Science.

This second Earth Trojan is likely a C-complex type asteroid, a designation for asteroids predominantly composed of carbon. Based on its orbital analysis, 2020 XL₅ will remain in its orbit for at least 4,000 years. The asteroid could have been ejected from the main asteroid belt between Mars and Jupiter, following an interaction with Jupiter.

Further research would be needed to confirm the origins of 2020 XL_5. What’s certain is that both Earth Trojans were captured after the planet formed, unlike primordial Jupiter Trojans that orbit L4 and L5 points since the time of the gas giant’s formation. That’s why Jupiter Trojans are much more important and interesting to study, as they may lock secrets pertaining to the formation of Jupiter and the solar system as a whole. In late 2021, NASA launched the Lucy spacecraft, which is now on route to rendezvous with 3548 Eurybates, 15094 Polymele, 11351 Leucus, and 21900 Orus in the L4 Greek Camp, plus 617 Patroclus and its binary companion, Menoetius, in the L5 Trojan Camp. During its 12-year mission, Lucy is tasked with gathering data on the surface composition, surface geology, and the interior and bulk properties of the Trojan targets.

“Primordial Trojan asteroids (i.e. those orbiting the L4/L5 points of a planet from the time of its formation) can provide us information about the formation of its host planet and, in turn, keys to better understand the evolution of the Solar System by adding constraints to its evolution models. We have studied the primordial Jupiter Trojans for several years and we will soon have the opportunity to investigate them with in situ observations taken by NASA’s space mission Lucy,” Santana-Ros said.

“Unfortunately, both Earth Trojans known have been confirmed to be transient objects, meaning that they have been captured in the L4 stability point many years after the Earth formation (actually quite recently! Only 600 years ago for 2020 XL5). Nevertheless, the discovery of 2020 XL5 as an Earth Trojan, confirms that 2010 TK7 is not a rare exception and that there are probably more bodies populating L4 and probably L5 of the Earth-Sun system. This encourages us to keep enhancing our survey strategies to find, if it exists, the first primordial Earth Trojan,” he added.

Due to its huge mass, Jupiter has cleared its neighboring region of objects, gathering 79 moons and a swarm of Trojans. Earth and other rocky planets in the solar system have more delicate environments, hence they have far fewer Trojans. Even so, the researchers estimate that Earth probably has tens or hundreds — but certainly not thousands — of Trojans waiting to be discovered. But it won’t be easy.

“It is a pain for astronomers to point to the L4 and L5 points of the Sun-Earth system while being on our planet! Any asteroid orbiting around these points will only be visible during a short time window close to twilight, at very low elevations above the horizon,” Santana-Ros said.

The findings appeared in a study published today in the journal Nature Communications.