Astronomers have confirmed Earth has a second Trojan asteroid travelling ahead of it in its orbit around the Sun.
Hints of the space rock were first detected in December 2020 in a survey of the sky by the Pan-STARRS telescope in Hawaii.
But there wasn't enough data at the time to confirm the asteroid, dubbed 2020 XL5, really was a Trojan, said Toni Santana-Ros of the University of Barcelona.
So Dr Santana-Ros and an international team of astronomers scanned the sky between February and March last year with three different telescopes to see if they could detect the asteroid.
They also went back through 10 years of archives from sky surveys, discovering a faint speck buried in data going back to 2012.
Piecing together the information, they worked out the space rock's path and size.
"When you look at [a single image], you can have some doubts that this is really an object; it could be some kind of dust in your camera," Dr Santana-Ros said.
"But when you link all the images from different years, you realise they are following an orbit.
And it's likely to be our celestial companion for the next 4,000 years or so, according to a new study published today in Nature Communications.
What are Trojans?
Trojan asteroids travel either in front or behind a planet as it orbits the Sun.
They orbit a sweet spot, known as a Lagrange point, in the planet's orbit.
There are five Lagrange points, but only L4 and L5 are able to draw in something wandering past and keep it captive for a while.
That's because the gravity of the planet and Sun cancel each other out at these points, so they are relatively stable.
Whole swarms of Trojan asteroids (complete with names from Greek mythology such as Achilles and Menoetius) have been discovered around Jupiter.
Neptune has 22 companions, Mars has nine, and Venus and Uranus have one or two.
But Earth was a loner until the first Trojan asteroid was discovered in 2011 orbiting the L4 point. Unlike its Jovian cousins, the space rock was given the less imaginative moniker of 2010 TK7.
The discovery of the first Trojan raised hopes Earth might have more companions.
But surveys of both Lagrangian points by NASA's OSIRIS-Rex spacecraft and the Japanese Hayabusa-2 spacecraft on their way out to other near-Earth asteroids drew a blank.
What does this one look like?
While we don't know anything about the shape of the object or if it is spinning, measurements of its brightness show it is very dark, which suggests it could be a C-type asteroid —one of the most common types of asteroids packed with carbon.
Its colour also indicates it is about 1km in diameter.
"When the albedo is low it means that it is probably larger than we thought," Dr Santana-Ros said.
While that makes it more than three times larger than the first Trojan discovered, it's pretty run of the mill as far as near-Earth objects go.
Dr Santana-Ros said the discovery of Earth's second Trojan asteroid orbiting the L4 point suggested there were probably many more out there to be found.
"This discovery confirms that the one we discovered 10 years ago is not the exception, it's not something weird," he said.
"It is one more of the family of objects that we are going to discover in the next few years."
Why are we only just discovering them?
Hunting for Earth Trojans is extremely challenging, Dr Santana-Ros said.
"From the Earth, it is not easy to observe these objects."
For a start they are relatively distant — the Lagrange points are as far away from us as the Sun is — so they are faint and can only be seen through large telescopes.
They also appear very close to the Sun in our sky, so astronomers only have a short window of time in the twilight before the Sun rises to see anything around L4.
"If you want to see this asteroid, you only have 15-20 minutes before the Sun starts bothering you," Dr Santana-Ros said.
It is also only possible to see objects orbiting the Lagrange points between late October and March.
The team had to convince large telescopes around the world to take a risk in order to investigate the possible asteroid.
"Astronomers like to point their telescope towards the night, we don't like to point towards the Sun," Dr Santana-Ros said.
In the end, they used three telescopes, including the 4-metre Lowell Discovery Telescope in Arizona and the 4.1m Southern Astrophysical Research Telescope in Chile, to confirm the asteroid.
Data from the observations indicated the object had a very tilted and elongated orbit around the Lagrange point, and was largely in shadow.
Dr Santana-Ros said these may have been reasons why it wasn't previously picked up by spacecraft fly-bys.
"Maybe they were not lucky enough when scanning the L4 point, the object was out of this area of the scan."
Where did it come from?
This is unclear, but it may have been jostled out of the main asteroid belt between Mars and Jupiter.
It is almost certainly a temporarily captured object, said Jonti Horner, an astronomer from the University of Southern Queensland not involved in the study.
"It's something that was moving through the inner Solar System minding its own business until gravitational nudges from all the other objects, Earth included, eased it into a Trojan orbit," Professor Horner said.
But this constant jostling in the inner Solar System affects the equilibrium of the Lagrange points, which means Trojans around Earth only hang around for a while before escaping.
"The Lagrange points of most of the planets are not as stable as those of Jupiter and Neptune," Professor Horner said.
"Jupiter and Uranus's Lagrange points are so stable that they can hold on to vast numbers of Trojans on timescales comparable to, or longer than, the age of the Solar System."
Dr Santana-Ros said modelling of this asteroid's orbit indicates that it will stick with us for roughly the next 4,000 years before either jumping to the L5 point or being booted back out into space by interactions with other asteroids or planets that destabilise the Lagrange points.
"From time to time there is something called Venus coming close to the Earth," Dr Santana-Ros said.
"And if it is coming close to the Earth, it is coming close to this object as well."
Could we visit it?
Visiting asteroids is "incredibly powerful" because they can tell us about the Solar System's past, Professor Horner said.
Samples of rock returned from an asteroid between Earth and Mars by Japanese Hayabusa2 mission revealedRyugu could be a source of very rare meteorites.
A NASA mission will return rocks from Bennu, another asteroid in the same area, next year.
NASA has also launched the Lucy mission to fly by the Trojan asteroids around Jupiter.
These primordial asteroids are made from the same dust and dirt that swirled together to form the planets billions of years ago, and can provide clues about how our Solar System formed.
Because the orbit of Earth Trojans is very tilted, it would be very costly to land on them to bring back rocks, but a fly-by could be possible, Dr Santana-Ros said.
And of the two Earth Trojans we now know of, the latest one would have a bigger launch window, according to calculations by the team.
While the discovery of these temporary Trojans was important, the ultimate goal would be to find a Trojan around Earth like the primordial ones around Jupiter, Dr Santana-Ros said.
