And the space rock will accompany our planet for at least another 4,000 years.

In 2020, scientists will spot a new asteroid: 2020 XL5. And soon the suspicion arose that this space rock was special. It would be a terrestrial Trojan. “Trojans are objects that share an orbit with a planet,” said researcher Cesar Briceño. The objects are located in the Lagrange points L4 or L5 and move with the planet at 60 degrees arc distance. Jupiter carries thousands of these Trojans with it. And some of these Trojans have also been found on other planets – such as Mars and Neptune. The search for terrestrial Trojans yielded little for years. Until 2011. Then it was discovered that the Earth also contains a Trojan: 2010 TK7. It stayed that way for a long time. Until the asteroid 2020 XL5 was encountered in 2020. Immediately the suspicion arose that this too was an earthly Trojan. And scientists have now confirmed that suspicion. This means that the earth is richer by a second Trojan. And that Trojan will accompany our planet for thousands of years, the researchers predict in the magazine Nature Communications. The second terrestrial Trojan will also go straight into the books as the largest terrestrial Trojan discovered to date. Because with an estimated size of 1.2 kilometers, it is no less than three times larger than 2010 TK7.

New findings

And so the new research reveals many secrets of 2020 XL5. After all, we still knew very little about the asteroid that was first spotted through a telescope in Hawaii in December 2020. Previous studies had suggested it was a terrestrial Trojan, but never before has it been so convincingly demonstrated as in this new study. In addition, the study also reveals the size of 2020 XL5 and what type of asteroid we are dealing with.


In the new study, the scientists first try to accurately pinpoint the orbit of 2020 XL5. To this end, they use observations from the Southern Astrophysical Research Telescope (SOAR) in Chile. The very accurate measurements make it possible to make a reasonably accurate estimate of the orbit of the space rock. And that will then enable the researchers to target 2020 XL5 on images captured by the Dark Energy Camera of the sky between 2012 and 2019. Based on those images, the track could then be further tightened.

Here you see the five Lagrange points. They are places in space where the gravitational forces of heavy objects – such as the Sun and Earth – are in equilibrium, making it easier for small-mass objects (such as an asteroid) to maintain a stable orbit. Image: NOIRLab/NSF/AURA/J. da Silva.

At least 4000 years in L4

The highly accurate orbital system not only confirms that 2020 XL5 is a terrestrial Trojan, but also makes it possible to predict the future of 2020 XL5. Because how would the space rock fare? Would it accompany our planet for a long time to come or would its stable orbit around L4 soon become unstable, after which it will continue as a ‘regular’ asteroid? To find out, the scientists produced 800 ‘near-clones’ of the 2020 orbit XL5. “It means we produced orbits that were very similar to that nominal orbit, but had some very slight variations,” explains researcher Toni Santana-Ros. from. We then looked at how those 800 jobs developed over time. “And we can conclude that all 800 orbits will last at least 4000 years around L4. It means that during that time the object will share the Earth’s orbit, but then move 60 degrees out in front of the Earth. After that period of more than 4,000 years, some of the cloned orbits are slightly off, meaning that from that point on we cannot say with certainty where 2020 XL5 will be based on current data.”

Minor disturbances

In any case, it is certain that 2020 XL5 will eventually detach itself from the orbit around L4, according to the researchers. “While the orbit of 2020 XL5 is mainly influenced by the sun and Earth, any other massive object approaching 2020 XL5 – such as Venus, for example – could affect its orbit. According to our calculations, those small perturbations are eventually enough to take the object out of that stable orbit around L4.” It is unclear where 2020 XL5 will go.

Size and type

SOAR’s observations also provide a little more insight into the appearance of 2020 XL5. For example, the observations reveal that the asteroid with a size of 1.2 kilometers is quite large. Much sturdier than 2010 TK7: the first terrestrial Trojan discovered years ago. In addition, the observations indicate that 2020 XL5 is most likely a C-type asteroid. These are dark asteroids that contain a lot of carbon and are common in our solar system.

More Trojans?

This second Trojan is curious in several ways. For example, the fact that two terrestrial Trojans have now been discovered in just over 10 years raises the question of whether there might be more pending discovery. Santana-Ros thinks so. “We know that Jupiter has thousands of Trojans. Should we expect the same numbers near Earth? No, not that. Jupiter is gigantic and has swept the surrounding area with its mass, collecting thousands of objects in L4 and L5 along the way. Earth’s environment is much more delicate, with nearby heavy objects such as Venus, Mars and even the Moon. Therefore, we expect that the number of terrestrial Trojans is likely to be in the tens or hundreds, but certainly not in the thousands.” The fact that only two have been discovered to date is mainly due to the fact that they cannot be spotted so easily. “The Lagrange points L4 and L5 can only be seen shortly after sunset and shortly before sunrise. It means that the chances of seeing an asteroid orbiting one of these points are very slim.” To still stand a chance of success, researchers are forced to aim their telescopes at the area just above the horizon. “Where the conditions to perceive something are also very bad.”

So it is not easy to find terrestrial Trojans. But that doesn’t stop the researchers from looking for it anyway. The holy grail in that regard is to find a primal Earth Trojan: a Trojan that has been found in L4 or L5 since the creation of the earth. “Such Trojans can tell us more about the formation of our planet and also increase our understanding of the evolution of our solar system.” 2020 XL5 is not such a primal Trojan. The same applies to 2010 TK7. “Both Trojans didn’t settle into L4 until years after Earth was formed (2020 XL5 didn’t do that until about 600 years ago!).” Nevertheless, researchers are happy with the discovery of 2020 XL5. “Because the discovery of this Trojan reveals that 2010 TK7 is not an odd exception and that there are likely to be many more objects around L4 and L5.” And with that, researchers are also in good spirits that a primal Trojan is still waiting to be discovered in one of the two Lagrange points. “Actually, a mission should be set up to L4,” Santana-Ros says. “To search for more terrestrial Trojans and thus achieve the ultimate goal – finding the first terrestrial primordial Trojan.”