Bengaluru: Astronomers have confirmed the existence of a second Earth Trojan, an asteroid that shares the same orbit as our planet, going around the Sun. The trojan was detected in 2020 and is named 2020 XL5. It is a near-earth asteroid (NEO) that is expected to stay in orbit for the next 4,000 years before deviating away.
The asteroid was discovered by the Pan-STARRS S1 telescope survey on 12 December 2020 and is estimated to be about 1.18 km wide. The first known Earth Trojan asteroid was 2010 TK7, just about 0.3 km wide, and discovered in 2010.
2020 XL5 is a C-type or carbonaceous asteroid, the most common variety of asteroid, consisting of large amounts of carbon. They are found in large numbers in the outer edge of the asteroid belt. Astronomers guess that this asteroid was kicked out of the asteroid belt by Jupiter’s gravity before getting captured here.
The findings were published in Nature this week.
Trojans are asteroids that share orbit with a planet. They can do so because they tend to be present at one of the stable Lagrange Points in the Earth-Sun system. Lagrange points are five different points in a two-body system where the gravitational forces from the two bodies and the centrifugal forces balance each other, enabling another smaller body or a satellite to orbit stably here. Both earth trojans have been discovered in the L4 point, which is located 60 degrees in orbit ahead of Earth.
No trojans have been discovered in the L5 point, which trails earth.
Trojans have been discovered orbiting along with other planets too. There are four Mars trojans, one Venus trojan, two Uranus trojans, 28 Neptune trojans, and over 500,000 Jupiter trojans. The largest Jupiter trojan, 624 Hector, is over 200 km wide. The Venusian trojan, trojans of asteroids Vesta and Ceres, and the newly discovered earth trojan are temporary trojans that will eventually exit the orbit.
Detecting Earth trojans is difficult because they are located on the same plane as the Earth and the Sun. The L4 and L5 points are located 60 degrees from Earth, making them appear very close to the Sun. Thus, they are observable only during a short window at twilight, and are better visible from space.
The Pan-STARRS or Panoramic Survey Telescope and Rapid Response System is a ground-based telescope used for astronomical imaging, located at Haleakala Observatory in Hawaii. It is operated by the Institute for Astronomy at the University of Hawaii, and is funded by the US Air Force. The operational telescope, called PS1, has been fully functional since 2010.
The telescope takes continuous images as a part of large scale surveys. The second data set was released in January 2019, and consisted of 1.6 petabytes of data, making it the largest amount of astronomical data ever released.
To confirm that 2020 XL5 was a trojan, follow-up observations were made with telescopes in Chile, Arizona, and Canary Islands. The team further dug through archival surveys and used 10 years worth of data to understand the trojan better.
The discovery has made astronomers confident that more earth trojans would be discovered soon. To understand trojans better, NASA’s Lucy asteroid mission is on its way to visit seven Jupiter trojans over the next 12 years, to study their characteristics and properties.
(Edited by Poulomi Banerjee)