At the end of 2020, astronomers spotted a large boulder, a kind of mini-planet, which they suspected was moving in the exact same orbit around the sun as Earth. That suspicion has now been confirmed by a team of international astronomers using calculations on the orbit of the boulder. The boulder was previously named 2020 XL5 and moves millions of miles ahead of the Earth.
Cosmic boulders are called asteroids and are mostly remnants of the formation of the solar system. Asteroids that orbit in the same orbit as a planet at the same speed (ie without colliding), astronomers call trojans.
In 2010, astronomers already found a small terrestrial trojan. With 2020 XL5 the earth has a second one. The new trojan may be a candidate to visit on future space missions, the researchers wrote.
Trojans in the solar system usually come from the asteroid belt. That’s a region chock-full of asteroids somewhere between the orbits of neighboring planets Mars and Jupiter. Every now and then Jupiter marbles a boulder out of the belt with its gravity. When such a boulder crosses a planet’s orbit, a planet can trap that boulder in its orbit.
at a safe distance
That capture happens when the asteroid enters the orbit of the planet exactly where the gravity of the sun and that of the planet cancel each other out. The result of pulling is that the boulder and the planet circle around the sun, at a safe distance from the planet. These spots are called Lagrangian points. Each planet has five of those Lagrangian points, and two of them lie on the planet’s orbit: one in front of it and one behind it. In those two points, planets can catch multiple trojans. Mars has one trojan ahead of it and nine behind it. Jupiter has a swarm of thousands of Trojans both in front and behind. The swarm moving in front of the planet is also known as the Greek camp. The one after it moves the Trojan.
The first terrestrial trojan was discovered in 2010 with NASA’s WISE space telescope. That trojan was named 2010 TK7 and has a diameter of about three hundred meters and moves about eighty million kilometers in front of the earth.
In December 2020, astronomers spotted another asteroid in Earth’s orbit with the Pan-Starrs telescopes in Hawaii. However, due to a lack of observations, it remained unclear whether it was a second terrestrial trojan.
Now the team of astronomers controlled the orbit of the boulder with archive data from 2012 to 2019 and they made new observations in 2021. They published their results this week in magazine Nature Communications. She concluded that the boulder will accompany the earth as a real trojan for at least another four thousand years. “The boulder disappears from the Lagrangian point on its own, because the small mass of the Earth makes the point unstable, says Simon Portegies Zwart. He is professor of stellar dynamics at Leiden University. Jupiter is much heavier than Earth. Trojans linger there for billions of years.”
“Clever find,” responds Vincent Icke, professor of theoretical astronomy at Leiden University and not involved in the study. “When we study Jupiter’s trojans, we look with the sun. To see the terrestrial trojans, we just look a little bit into the sun. That makes terrestrial Trojans difficult to find.”
Asteroids are interesting to visit
Carsten Dominik astronomer
“The composition of the terrestrial trojan is not special,” says Icke. The trojan mainly consists of carbon, according to a color analysis. “Like most asteroids in the asteroid belt.”
With a diameter of 1.8 kilometers, the new terrestrial trojan is larger than the terrestrial trojan found in 2010. That makes it an easier target to travel to, the researchers write. It wouldn’t be the first time scientists land a probe on an asteroid. In 2018, the Japanese space probe Hayabusa2 visited the asteroid Ryugu to return samples to Earth.
“Planetoids are interesting to visit, because these remnants of the formation of the planets can reveal something about the history of the solar system,” says astronomy professor Carsten Dominik of the University of Amsterdam, who was also not involved in the study. “In addition, it may be interesting to get rare raw materials there. That may sound futuristic, but we still have 4,000 years to make it happen.” Icke does not see that happening any time soon: “It remains easier to build up a raw material here on earth atom by atom than to travel to another celestial body and start mining there.”