An asteroid from another star system has been discovered sharing Jupiter's orbit - but travelling in the wrong direction.
It is the second time an interstellar space rock has been spotted in the solar system.
The first, a tumbling cigar-shaped object named 'Oumuamua, hit the headlines last year after flying into our galactic backyard and out again.
But the new 3km (1.8 mile) wide rock - code-named 2015 BZ509 - appears to be a long-term resident that has found itself a new home.
It stands out because it circles the sun in a "retrograde" orbit, opposite the direction of travel taken by all the planets and almost every other object in the solar system.
Dr Fathi Namouni, from the Cote d'Azur Observatory in France, a member of the team studying the object, said: "How the asteroid came to move in this way while sharing Jupiter's orbit has until now been a mystery.
"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."
Computer simulations suggest that the asteroid made the jump to our solar system 4.5 billion years ago when the sun was part of a tightly packed star cluster.
Co-author Dr Helena Morais, from the Estadual Paulista University in Brazil, said: "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 research is published in the journal Monthly Notices of the Royal Astronomical Society: Letters.
Since asteroids and comets carry organic chemicals and water, the discovery could shed light on the origins of life on Earth, say the scientists.