Astronomers have just confirmed the discovery of the solar system’s first interstellar immigrant. Unlike `Oumuamua, a fast-moving tourist that was spotted passing through our neighbourhood last year, this asteroid is here to stay.

The solar system is swirling with millions of asteroids. These hunks of rock mainly hang out between Mars and Jupiter, and are thought to be leftovers from the formation of the solar system.

Almost all of the asteroids — and all planets, for that matter — orbit in the same direction around the sun. But back in 2014, Hawaii’s Pan-STARRS telescope spotted 2015 BZ509, an asteroid that’s about three kilometres wide, nestled in the same orbit as Jupiter, but going in the wrong direction

“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,” explains Fathi Namouni, an astronomer from the Observatoire de la Côte d’Azur in France.

Now a study published in the journal Monthly Notices of the Royal Astronomical Society: Letters has shed light on the origins of Jupiter’s backwards little friend. Namouni and co-author Helena Morais from the Universidade Estadual Paulista in Brazil, both experts in retrograde asteroids, precisely simulated the asteroid’s motion all the way back to the birth of the solar system over four billion years ago.

“We did not expect that the asteroid would remain retrograde and bound to Jupiter’s resonance and that it would hang on in there for 4.5 billion years,” Namouni says. “But it did.”

This means that it can’t have formed along with the rest of our solar system. It must be an “exo-asteroid”, captured out of the interstellar medium by the sun’s gravity.{%recommended 4860%}

According to Morais, “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.

“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.”

So, asteroid immigration is a natural phenomenon, no visas required.

Further study could tell us whether interstellar asteroids helped enrich our solar system — even, for instance, contributing to the water content on Earth and thus indirectly helping along life.

Namouni says that many more interstellar asteroids could have settled down in the solar system. In fact, 2015 BZ509 could have been captured along with many similar objects. Simulations show these objects may have since moved into the region beyond Neptune, and the team are working on positively identifying a shortlist of candidates.