天文学：2つ目の地球トロヤ群小惑星が特定された

The recently discovered 2020 XL5 asteroid is confirmed as the second transient Earth Trojan asteroid to be identified, reports a study in Nature Communications. The findings suggest it will remain in its orbit for at least 4,000 years.

Trojan asteroids are small bodies sharing an orbit with a planet, which remain in a stable orbit approximately 60° ahead of or behind the main body. Such peculiar orbits provide important constraints for models of the evolution of the Solar System. Although Trojan asteroids have been observed around other planets in the Solar System, there has only been one Earth Trojan asteroid identified so far (called 2010 TK7). 2020 XL5 was discovered on the 12th December 2020 and early observations suggested that it could be an Earth Trojan asteroid, but due to low observational coverage, orbital uncertainties were too large for confirmation.

Toni Santana-Ros and colleagues studied the orbit of 2020 XL5 by combining archival data with observations they performed from three ground-based observatories. Using these analyses, they confirm that 2020 XL5 is an Earth Trojan asteroid. They propose it is a C-complex type asteroid, which is predominately composed of carbon. The authors conducted an orbital stability analysis and suggest that 2020 XL5 will remain in its orbit for at least 4,000 years. They suggest the asteroid could have been ejected from the main asteroid belt, following an interaction with Jupiter, however further research would be needed to confirm the origins of 2020 XL5.

The authors note that 2020 XL5 is a larger asteroid than 2010 TK7 and may be a better candidate for a future fly-by mission.