One of the latest missions we have been covering has been the Lucy mission to Jupiter’s Trojan asteroids – those asteroids that orbit about 60 degrees in front of and behind the gas giant. Jupiter, it turns out, is not the only planet with Trojan asteroids. Mars, Neptune, and Uranus all have Trojans as well, although not nearly as many as Jupiter. And Earth, up until this week, was known to have one Trojan. But now, NOIRLab has announced the discovery of a second, larger Earth Trojan.

In a new paper published in Nature Communications, scientists detail the discovery of 2020 XL5 which was confirmed using the Southern Astrophysical Research (SOAR) telescope in Chile. The asteroid was originally found with the Pan-STARRS1 survey telescope in Hawai’i back in December 2020. It’s about 1.2 kilometers in diameter, which makes it three times wider than Earth’s other known Trojan asteroid, 2010 TK7.

The team not only observed 2020 XL5 with the SOAR telescope but went back and looked through precovery data from the Dark Energy Survey to put together nearly ten years of data and images. This meant that they could determine the size, orbit, and even type of asteroid our largest (so far) Trojan is. Lead author Toni Santana-Ros notes: SOAR’s data allowed us to make a first photometric analysis of the object, revealing that 2020 XL5 is likely a C-type asteroid, with a size larger than one kilometer.

Now, 2020 XL5 will not always be a Trojan asteroid of Earth. Its orbit is predicted to only be stable for the next 4,000 years or so before gravitational perturbations will nudge it away and out into deeper space.

It should be noted that while we may have only confirmed two Earth Trojans, it’s expected that there are many more. They are hard to observe, however, because they end up being close to the Sun in the sky. You have to look for them close to sunrise or sunset, and you have to aim your telescope near the horizon, where the atmosphere is thickest. That means, as all you astrophotographers know, the seeing is the worst. SOAR can be pointed just 16 degrees above the horizon, which is lower than most 4-meter and larger telescopes can aim, making it the best one for the challenge.

More Information

NOIRLab press release

“Orbital stability analysis and photometric characterization of the second Earth Trojan asteroid 2020 XL5,” T. Santana-Ros et al., 2022 February 1, Nature Communications