Astronomers have discovered a captive asteroid shadowing Earth in its orbit.
The asteroid, known as 2020 XL5, is only the second of its type ever seen, shepherded by Earth’s gravity into an orbit that is locked in synchrony with our planet’s. It has not shared our orbit for long — a few centuries, probably. And it will not be there in the far future. Simulations indicate that 2020 XL5 will slip out of Earth’s grasp within 4,000 years and head into the wider solar system.
But its presence offers a tantalizing glimpse of what else might be out there in the local gravitational whirlpools. Some bits might date back to the beginning of the solar system — shades of the building blocks that coalesced into our planet.
“These objects are not as exotic as we think,” said Toni Santana-Ros, a postdoctoral researcher at the University of Barcelona in Spain and an author of a paper describing the discovery, which was published on Tuesday in the journal Nature Communications. (A separate team of astronomers came to a similar conclusion in December.)
When two objects orbit each other, there are five points, known as Lagrange points, where the gravity of the two essentially balance, and a much smaller mass can sit there in equilibrium. Two of the five Lagrange points, known as L4 and L5, are stable: If a small body there is nudged slightly, it remains at that point. At the three unstable Lagrange points — L1 through L3 — a nudge will push the small body away for good.
(The unstable Lagrange points can still be very useful. The newly launched James Webb Space Telescope is about 900,000 miles away at the second Lagrange point, or L2, of the sun-Earth system. The instrument relies on thrusters to keep it from drifting away.)
Back in 1906, Max Wolf, a German astronomer who developed a photographic technique to discover asteroids, found an asteroid in the same orbit as Jupiter, always ahead of the planet by 60 degrees, or one-sixth of an orbit. Later observers found other asteroids in about the same place as well as asteroids that lagged Jupiter by the same margin.
These fit a prediction made more than a century earlier by mathematician Jose-Louis Lagrange. Astronomers started naming these objects after heroes of the Trojan War, and the bodies became known as the Trojan asteroids.
Many of Jupiter’s more than 11,000 Trojans do appear to be primordial remnants from the formation of the solar system. In October, NASA launched a probe named Lucy, after the fossilized skeleton of an early hominid ancestor, to visit several of them.
Similar asteroids appear to be trapped in the orbits of Venus, Mars, Uranus and Neptune. None have been found sharing Saturn’s orbit — the disruptive gravity of Jupiter is thought to be at fault — or Mercury’s, where any tiny Trojans would be all but impossible to find in the glare of the sun.
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.
“All the formation models gives you the idea that planets should have these families going around their L4 and L5 points from the beginning,” said Federica Spoto, an astronomer at the Center for Astrophysics in Cambridge, Mass., who was not involved with the research.
She added, “If we were able to find something like that, we might be able to understand what happened at the beginning, basically.”
