What We Know About Huge Trojan Asteroids Following Earth As New One Is Found

Astronomers have discovered that a 1.2-kilometer-wide asteroid is accompanying Earth on its journey around the sun. It is our planet's second so-called Trojan asteroid to be identified and more than three times larger than the first.

A team of scientists used the Southern Astrophysical Research Telescope in Chile, part of Cerro-Tololo Inter-American Observatory, to observe the object, which has been designated 2020 XL5 because it was first spotted in 2020.The astronomers were able to determine its size as around 0.75 miles wide as well as its orbit, confirming that it is a Trojan asteroid.

Earth's first known Trojan asteroid, 2010 TK7, was discovered by NASA in 2010 lying ahead of Earth in its orbit around the sun. This asteroid was determined to be about a third of a kilometer or 0.2 miles wide. A Trojan asteroid is a rocky body that shares its orbit with a planet also circling that world's host star. The orbits of these objects are stable but can eventually be disturbed.

Cesar Briceño of the National Science Foundation's NOIRLab is part of the team that made the observation and has co-written a paper about the discovery, published in Nature Communications on Tuesday. He explained in a NOIRLab statement: "Trojans are objects sharing an orbit with a planet, clustered around one of two special gravitationally balanced areas along the orbit of the planet known as Lagrange points."

The astronomers were able to determine that 2020 XL5 is a Trojan asteroid now but won't remain so forever. Its orbit will remain stable for at least another 4,000 years but will eventually deteriorate as a result of gravitational perturbations caused by the solar system's other bodies.

When this happens, 2020 XL5 will escape its Earth-like orbit to wander space alone.

The team was also able to assess the composition of the object, determining it to be a C-type asteroid. These contain a lot of carbon and are therefore quite dark and difficult to spot.

These objects formed in the earliest era of the solar system and the materials they contain give us a clue as to the building blocks of the planets. Researchers believe Earth shares its orbit with many more Trojan asteroids. Evidence for this seems to be provided by the solar system's other planets.

Which Other Planets Have Trojan Asteroids?

The asteroid 2010 TK7 wasn't the first Trojan asteroid discovered in our solar system. Their existence was predicted as early as 1772 by astronomer Joseph-Louis Lagrange. The scientist, who would eventually lend his name to the stable gravitational points between planets and their stars, suggested the existence and location of two clusters of small bodies located at gravitationally stable points along the orbit of Jupiter.

The first of these predicted objects was discovered in 1906 by German astronomer Max Wolf. Two more were discovered within a year. These objects are located around the gravitationally stable Lagrange point 4 (L4) and were named after participants in the Trojan War as described in Homer's Iliad.

By 2020 over 7,000 Trojan asteroids had been discovered around Jupiter with two-thirds located around L4 and a third around L5. Astronomers estimate that between 1,800–2,200 of them have widths larger than 10 miles.

Until 1990 it was suspected that the smaller planets such as Mars and Earth would not be able to hang on to Trojan asteroids, with the gravitational influence of the major planets being too overwhelming.

This belief was overturned by the discovery of the Mars Trojan 5261 Eureka in that year. Astronomers have since discovered a further eight Trojans associated with the red planet and 24 that share an orbit with Neptune.

Spotting more Trojans sharing an orbit with Earth may not be an easy task. Searches for Earth Trojans have to be conducted at close to sunrise or sunset, with the telescope pointing near the horizon, through the thickest part of the atmosphere—which results in poor seeing conditions.

Briceño said: "If we are able to discover more Earth Trojans, and if some of them can have orbits with lower inclinations, they might become cheaper to reach than our moon.

"So, they might become ideal bases for an advanced exploration of the solar system, or they could even be a source of resources."

Earth Trojan Asteroid
An illustration depicting an Earth Trojan asteroid. Astronomers have discovered a second asteroid that shares our planet's orbit around the sun. NOIRLab/AURA/J. da Silva/Spaceengine Acknowledgment: M. Zamani NSF’s NOIRLab/NSF