Earth also appeared to have no Trojan asteroids until astronomers found one in 2010 at the L4 Lagrange point, 60 degrees ahead of Earth. Subsequent searches came up empty until Pan-Starrs, an automated aerial survey in Hawaii, turned up an intriguing object, 2020 XL5, that appeared to be trapped around L4 as well.
But the initial observations were insufficient to definitively establish the object’s orbit. In 2021, an international team of astronomers, including Dr. Santana-Ros, additional observations of 2020 XL5 using three ground-based telescopes. The team was then able to search images dating back to 2012 — where the asteroid did indeed appear, though no one had recognized it as such.
A decade of data was finally enough to firmly map the 2020 XL5’s elliptical orbit. “We were 100 percent sure this was an Earth Trojan,” said Dr. Santana-Ros.
While 2020 XL5 is stuck in orbit around a stable Lagrange point, it isn’t particularly close to L4. Its elliptical orbit, tilted nearly 14 degrees from the orbits of the planets, brings it closer to the sun than Venus and nearly as far away as Mars.
This makes it vulnerable to gravitational vibrations from other planets, especially Venus.
The researchers ran computer simulations of 2020 XL5’s orbit, which was modified 800 times. Sometimes the asteroid escaped from the Lagrange point within 3,500 years; sometimes it hung around for 5000 years or more. But it seems unlikely that the orbit will remain stable for much longer
It appears that the 2020 XL5 is a dark, carbon-rich body, perhaps an invader of the main asteroid belt between Mars and Jupiter. The researchers estimated its diameter to be about three-quarters of a mile, much larger than the Earth Trojan discovered in 2010, which was estimated to be about a quarter of a mile in diameter and is also located at the L4 Lagrange point.
While the two known Earth Trojans appear to be temporary additions to our orbital environment, other bodies orbiting closer to the stable Lagrange points may remain in place indefinitely, raising the possibility that some of Earth’s original building blocks are still found there.