The asteroid is the first known rock to have made its way into our neighbourhood from elsewhere in the universe. It is the second alien rock to cause a stir in recent months – after 'Oumuamua arrived in our own solar system, but then flew out the other side, unlike the newly discovered asteroid.
As well as shining a light on the make-up of our solar system, the discovery could give clues about how life emerged on Earth.
The asteroid is named (514107) 2015 BZ509 and is currently in orbit around Jupiter. It was always known to be a bizarre object, and scientists have finally found why: because it didn't begin life here.
In our solar system, all of the planets and most of the rest of the objects move around the Sun in the same direction. But the alien rock moves the opposite way.
"How the asteroid came to move in this way while sharing Jupiter's orbit has until now been a mystery," explains Dr Fathi Namouni, lead author of the study.
"If 2015 BZ509 were a native of our system, it should have had the same original direction as all of the other planets and asteroids, inherited from the cloud of gas and dust that formed them."
Lost interstellar asteroid 'Oumuamua' enters solar system and baffles scientists
To find the origin of the asteroid, scientists ran simulations all the way back to the very beginning of the Solar System, 4.5 billion years ago. They show that the asteroid has always moved in this strange way, and so must have come from another solar system.
"Asteroid immigration from other star systems occurs because the Sun initially formed in a tightly-packed star cluster, where every star had its own system of planets and asteroids," comments Dr Helena Morais, the other member of the team.
"The close proximity of the stars, aided by the gravitational forces of the planets, help these systems attract, remove and capture asteroids from one another."
The newly discovered asteroid is not the first alien rock to have caused a stir in recent months. The mysterious rock known as 'Oumuamua became famous when it arrived in our own solar system in 2017.
But that was only a visitor. The new asteroid is a long-term resident and is now in orbit in our neighbourhood.
The discovery could shed light on a range of mysteries about how our solar system, planets and perhaps even life itself were formed. Exploring how the asteroid came into our own solar system could shed light on the beginnings of the Sun, and whether the ingredients of life were brought to Earth on another visitor from elsewhere in the universe.
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