All of the planets in our solar system, and the vast majority of other objects as well, travel around the Sun in the same direction.
Scientists have discovered the first known permanent immigrant to the solar system - an asteroid, currently nestling in Jupiter's orbit, that migrated from another star system.
All of the planets in our solar system, and the vast majority of other objects as well, travel around the Sun in the same direction. However the immigrant asteroid named (514107) 2015 BZ509 is different - it moves in the opposite direction in what is known as a 'retrograde' orbit.
"How the asteroid came to move in this way while sharing Jupiter's orbit has until now been a mystery," said Fathi Namouni, from National Scientific Research Centre (CNRS) in France.
"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," said Namouni, lead author of the study published in the journal Monthly Notices of the Royal Astronomical Society: Letters.
An object known as 'Oumuamua was the last interstellar interloper to hit the headlines in last year. However it was just a tourist passing through, whereas this former exo-asteroid is a long-term resident.
The team ran simulations to trace the location of 2015 BZ509 right back to the birth of our solar system, 4.5 billion years ago when the era of planet formation ended.
These show that 2015 BZ509 has always moved in this way, and so could not have been there originally and must have been captured from another 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," said Helena Morais, from Universidade Estadual Paulista (UNESP) in Brazil.
"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," said Morais.
The discovery of the first permanent asteroid immigrant in the solar system has important implications for the open problems of planet formation, solar system evolution, and possibly the origin of life itself.
Understanding exactly when and how 2015 BZ509 settled in the solar system provides clues about the Sun's original star nursery, and about the potential enrichment of our early environment with components necessary for the appearance of life on Earth.