In 2020, astronomers thought they had unearthed the second terrestrial Trojan asteroid on record. A new team has just confirmed its presence around the point of Lagrange 4. According to these analyzes, this small additional "moon" should accompany the Earth for approximately 4000 years.
Trojan asteroids are small rocks that share their orbit with a planet. Most of them evolve upstream and downstream of Jupiter. So far, only one such object, called 2010 TK7, has been confirmed on the same trajectory as Earth. In a new study published in Nature, researchers confirm that a second such object called 2020 XL5 and identified the same year was indeed a terrestrial Trojan. Think of it as Earth's extra moon, albeit very small and only passing through.
In December 2020, 2020 XL5 was spotted thanks to the Pan-STARRS 1 telescope, located in Hawaii. Follow-up studies then made it possible to estimate its position around L4, the fourth point of Lagrange Earth-Sun. 2010 TK7, the first confirmed terrestrial Trojan asteroid, also evolves around L4.
As a reminder, a Lagrange point is a position where the gravity fields of two bodies in orbital motion around each other provide the centripetal force required for this point of space simultaneously accompanies the orbital motion of the two bodies. Simply put, a Lagrange point is a relatively stable "parking spot" in space.
Point L4 is ahead in the orbit of the smaller of the masses (therefore the Earth), in its orbit around the large one (the Sun), as you can see it below.
For these new analyses, the researchers relied on the SOAR (Southern Astrophysical Research) telescope in Chile, as well as on the Lowell Discovery telescope (Arizona) and the ESA optical ground station in Tenerife (Canary Islands). These new data confirm the positioning of 2020 XL5 around L4 .
Doing this type of follow-up is no small feat. It is indeed difficult for astronomers to point to points L4 and L5 of the Sun-Earth system from our planet, because any asteroid orbiting these points will only be visible for a short window of time near twilight and at very low altitude.
According to the photometrics, the object would probably be of type C (rich in carbon and dark), the most common in the Solar System. The study also found that 2020 XL5 is much larger than the first terrestrial Trojan discovered, measuring approximately 1.2 km in diameter (compared to 0.4 km for 2010 TK7).
Finally, the team points out that 2020 XL5 will not remain an Earth Trojan asteroid forever. According to analyses, will remain in its position forapproximately 4,000 years , before eventually escaping the gravitational force holding it in place.
