Earth also appeared to lack 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 sky survey in Hawaii, turned up an intriguing object, 2020 XL5, that seemed like it might also be trapped around L4.

But the initial observations were insufficient to definitively pin down the object’s orbit. In 2021, an international team of astronomers including Dr. Santana-Ros made additional sightings of 2020 XL5 using three ground-based telescopes. The team was then able to search through images dating back to 2012 — where the asteroid indeed appeared, although no one had recognized it as one.

A decade of data was finally enough to firmly chart 2020 XL5’s elliptical orbit. “We were 100 percent sure this was an Earth Trojan,” Dr. Santana-Ros said.

Although 2020 XL5 is trapped in an orbit around a stable Lagrange point, it is not particularly close to L4. Its elliptical orbit, tilted nearly 14 degrees to the orbit of the planets, sweeps it closer to the sun than Venus and almost as far out as Mars.

This makes it vulnerable to gravitational buffeting from other planets, especially Venus.

The researchers ran computer simulations of the orbit of 2020 XL5, tweaking it 800 times. Sometimes the asteroid escaped from the Lagrange point within 3,500 years; sometimes it loitered 5,000 years or more. But the orbit appears unlikely to remain stable for much longer than that

From appearances, 2020 XL5 is a dark, carbon-rich body, perhaps an interloper from the main asteroid belt between Mars and Jupiter. The researchers estimate its diameter at about three-quarters of a mile, much larger than the Earth Trojan discovered in 2010, which was estimated to be about a quarter-mile in diameter and is also located at the L4 Lagrange point.

Whereas the two known Earth Trojans appear to be transitory additions to our orbital neighborhood, other bodies that hover closer to the stable Lagrange points could remain in place indefinitely, raising the possibility that some of the primordial building blocks of Earth might still be found there.