That's only about 300 meters across, so 2020 XL5 considerably outmasses it at about 1.2km across. The first, 2010 TK7, was discovered in 2010 (opens in new tab). But 2020 XL5 is only the second Trojan companion of Earth to have been found. Small numbers of Trojan asteroids have now been discovered associated with Neptune (23), Uranus (1) and Mars (9). (Image credit: Mdf at English Wikipedia, Public domain, via Wikimedia Commons)

Collectively, whether at L4 or L5 they are all referred to as Trojans.Īsteroid positions, with Jupiter’s Trojans in green. Only those trailing Jupiter (clustered at the sun-Jupiter L5 position) are given Trojan names, such as Hektor, whereas those ahead of Jupiter (at L4) are give Greek names, such as Achilles.

The first of those, discovered in 1906, was named Achilles after a central character at the siege of Troy (opens in new tab) in Homer's Iliad.Ī convention developed to name each one after a hero from the same story. Jupiter shares its orbit with nearly ten thousand known asteroids, half of them ahead of Jupiter, and half behind. XL5, but no fireballĢ020 XL5 is being called a Trojan companion to the Earth by analogy with Jupiter's Trojan asteroids. If a small-mass object is perturbed so as to move away from L4 or L5, the combined gravity of the sun and Earth draws it back - bending its path into a stable orbit around the Lagrange point that looks kidney bean shaped relative to Earth. These points are 60 degrees ahead of and 60 degrees behind the Earth, and because 60 degrees (see image above) is one-sixth of the Earth's orbit this amounts to two months separation. Here, a small-mass object making an equilateral triangle with sun and Earth is in a stable equilibrium. Two other points, L4 and L5, were discovered in 1772 by Euler's student Joseph-Louis Lagrange (opens in new tab). A diagram shows the five Earth-sun Lagrange points.

Earth orbit