Scientists have confirmed that the newly-discovered asteroid, called 2020 XL5, is the second Trojan asteroid known for Earth. And, at around 1.2km, it’s around three times larger than the only other known Earth Trojan, 2010 TK7.


What is a Trojan asteroid?

A Trojan is a small body, usually an asteroid, that shares an orbit with a planet. We now know of two Trojans for Earth (simply called 'Earth Trojans') that orbit the Sun with the Earth. They are often thought of as asteroid companions to a planet (so there’s no danger of them colliding with Earth).

Other planets also have Trojan asteroids, and so far, astronomers have found them for Venus (1), Mars (9), Jupiter (7,000+), Uranus (two), and Neptune (28). The first Earth Trojan, 2010 TK7, was discovered relatively recently in 2011.

Earth has a new companion

When asteroid 2020 XL5 was first discovered in late 2020 by the Pan-STARRS 1 survey telescope in Hawaii, scientists were uncertain whether it was a near-Earth asteroid (an asteroid with an orbit that brings it close to Earth) or a true Trojan (an asteroid that shares Earth's orbit).

Subsequent measurements from the 4.1m Southern Astrophysical Research (SOAR) telescope in Chile allowed researchers to investigate its physical characteristics and search for the asteroid in archival images.

By scouring images taken between 2012 and 2019 as part of the Dark Energy Survey, the team located the asteroid and were able to study its orbit, publishing the results in Nature Communications. Together with data taken from three ground-based observatories in 2021, they determined that 2020 XL5 is a true Trojan and will remain as a companion to Earth for at least 4,000 years. Subsequent photometric analysis revealed that it's likely a C-type asteroid, around 1.2km wide.

C-type (carbonaceous) asteroids are the most abundant type in our Solar System. They’re also among some of the darkest objects as they contain a large amount of carbon, making them too dim to see without a telescope.

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Why are Earth Trojans difficult to find?

The difficulty in finding Earth Trojans, is often due to their small size, low albedo (i.e. they are dark), and the unfavourable viewing geometry of their home - Lagrange point L4. Lagrange points are gravitationally stable areas of space that remain fixed in relation to the two bodies that create them - in our case, the Earth and the Sun.

There are five Lagrange points, and both the Earth Trojans are at Lagrange point L4, which is positioned 60° ahead of the Earth, i.e., it leads the Earth in our orbit around the Sun - it's 'Earth-leading'.

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However, it's difficult to see objects at L4 because they appear close to the Sun at sunrise. (Similarly, any objects at Earth-trailing L5 would appear near the sunset.) Looking towards the horizon, means you’re also looking through a thicker blanket of Earth’s atmosphere, as opposed to when you look directly overhead.

“There have been many previous attempts to find Earth Trojans, including in situ surveys such as the search within the L4 region, carried out by NASA’s OSIRIS-Rex spacecraft, or the search within the L5 region, conducted by JAXA’s Hayabusa-2 mission,” said lead author Dr Toni Santana-Ros of the University of Alicante and the Institute of Cosmos Sciences of the University of Barcelona.

Where is the new Trojan asteroid?

Both the Earth Trojans are caught in orbit around the Earth-leading L4, one of the two stable Lagrange points in the Earth-Sun system. As it orbits L4, 2020 XL5 follows an elliptical path that sends it closer to the Sun than Venus, and almost as far out as Mars. But it’s the repeated close encounters with Venus that will likely eventually destabilise its orbit in around 4,000 years time.

It’s very difficult to see 2020 XL5 from Earth, except with very large telescopes – and even then, it was a challenge. It required the SOAR telescope to point down to just 16° above the horizon, and many 4-metre telescopes are just not able to do this.

“These were very challenging observations, requiring the telescope to track correctly at its lowest elevation limit, as the object was very low on the western horizon at dawn,” said co-author Cesar Briceño, a researcher at the National Science Foundation’s National Optical-Infrared Astronomy Research Laboratory (NOIRLab).

But the presence of 2020 XL5 could offer a tantalising glimpse of what else could be lurking in these cosmic sweet spots.


“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,” says Briceño. “So, they might become ideal bases for an advanced exploration of the Solar System, or they could even be a source of resources.”


Holly SpannerStaff Writer, BBC Science Focus

Holly is the staff writer at BBC Science Focus, and specialises in astronomy. Before joining the team she was a geoenvironmental consultant and holds an MSc in Geoscience (distinction) from UCL.