Astronomers have confirmed the existence of a second known Earth trojan asteroid. Called 2020 XL₅, the space rock is located in one of the gravitational sweet spots in the Earth-Sun system, roughly 60 degrees ahead of our planet in its orbit. Sharing the same stable orbit as Earth makes it a great candidate for future flyby missions, which we can do for the next 4,000 years, according to astronomers who say that's at least how long it will remain in position.

"Trojan asteroids" are asteroids that occupy a stable Lagrangian point in a planet's orbit around the Sun. Jupiter's Trojans are the originals and most famous (around 9,800 known so far), but there are Mars trojans (nine), Neptune trojans (28), and Uranus ones (two). Now, Earth's second-ever trojan has been described in the journal Nature Communications.

The asteroid was discovered by the Pan-STARRS1 survey telescope in Hawai'i in December 2020. Astronomers had data to suggest that this could be a second trojan asteroid after 2010 TK₇, which was discovered back in 2010. Follow-up observations were vital but far from easy, given how close the object appears near the Sun in the sky.

Crucial to this were the 4.3-meter Lowell Discovery Telescope, the 4.1-meter SOAR (Southern Astrophysical Research) Telescope on Cerro Pachón in Chile, and the European Space Agency’s 1-meter Optical Ground Station in Tenerife in the Canary Islands. They were able to point very low on the horizon just before sunrise and observe this very faint celestial body.

This was combined with archival data. Given the size of many surveys, asteroids can be observed before they are recognized as objects of interest. In the end, everything combined provided an intriguing picture.

“With this data at hand, we increased our knowledge of the asteroid’s orbit, and then we could confirm that this has to be the second Earth trojan object,” lead author Toni Santana-Ros of the University of Alicante and the Institute of Cosmos Sciences of the University of Barcelona told IFLScience. “In addition, we took some measurements of the brightness of the object so we could calculate the size of the objects. It is expected to be larger than one kilometer.”

2020 XL₅ is about 1.2 kilometers (0.73 miles) in size, about three times the size of the first Earth trojan. Both the trojans are located at the Lagrangian point 4 or simply L4, one of the five special equilibrium points. L1 is located between the Earth and the Sun. L2 is where the JWST is now located and from where it operates. L3 is on the opposite side of the Sun on the Earth’s orbital plane. L4 and L5 are respectively leading and following our planet.

Earth’s trojans are rare compared to the vast population that a planet like Jupiter has, although Santana-Ros explains that there is certainly an observational bias given how difficult it is to study regions like L4 and L5. And not just from Earth. NASA’s OSIRIS-REx passed there on its way to Bennu and did not see any trojans.

But even without the difficulty in spotting them, there is also a stability issue. Objects that get in L4 or L5 might not stay there for long. The researchers estimate that 2020 XL₅ will be a trojan for at least the next 4,000 years but its future is far from certain.

“Jupiter is a huge object, it’s a giant that has cleaned a lot of its neighborhood. It has no other competitor nearby. So it gathers a lot of objects around its stability point, L4 and L5, and it is very hard for them to escape from these points,” Santana-Ros told IFLScience.

“In Earth’s case, we are much closer to the Sun and in addition, you have Venus [which is roughly the same mass as Earth], you have Mars, you even have the Moon,” he continued. “that means that these points are not that stable as it would be for Jupiter. It is easier for an object in these points to escape due to some perturbation.”

Estimations put the Earth’s trojan population to be in the hundreds but the researchers are confident that they are most likely much smaller than 2020 XL₅. If something is of comparable size it will have to be darker, reflecting very little sunlight, or on a very difficult orbit to observe.

Given that this object is a captured asteroid, possibly ejected from the main asteroid belt, and not something that has been with Earth from the beginning, Santana-Ros doesn’t believe it warrants a spacecraft mission — yet. As software and observational strategies continue to improve, more trojans are expected to be discovered.   

“At some point maybe we discover, two, three, four five Earth trojans and then it would make sense to place a mission there to study all of them in one go,” Santana-Ros told IFLScience.

A similar mission called Lucy has been sent by NASA to explore seven of the 7,000 large asteroids (over 1 km) that make up Jupiter’s Trojans.