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An asteroid in Jupiter's orbit may have come from outside our Solar System, according to a new study.
Unlike 'Oumuamua, the interstellar object which briefly visited the Solar System earlier this year, 2015 BZ509 (affectionately known as BZ) seems to have been here for 4.5 billion years.
This makes it the first known interstellar asteroid to have taken up residence orbiting the Sun.
It is not yet known where the object came from.
"That's what we need to figure out next," laughs Dr Fathi Namouni from the Universite Cote d'Azur, one of the study's authors.
"Because 'Oumuamua was just passing by… it's not that difficult to go back and pinpoint where it came from," he told BBC News. "BZ reached the Solar System when it was forming, when the planets themselves were not exactly where they are now. So it's a little more tricky to figure out where it came from."
Prof Sara Russell from the Natural History Museum points out that the Solar System is actually littered with small bodies.
"There used to be this idea that the asteroids were stuck in a belt between Mars and Jupiter, and the Kuiper Belt was full of icy objects. Now we know that's completely untrue, everything's kind of jumbled up," she explains.
BZ itself was discovered in a wide-sky survey of such objects in 2015, but it took some time for its unusual behaviour to be understood.
Going against the flow
BZ is in a retrograde orbit, moving around the Sun in the opposite direction to the eight planets, and the majority of other objects in the Solar System.
This immediately makes it stick out, as almost everything which formed from the disk of gas and debris around the Sun follows the direction of the star's rotation.
While this backwards habit doesn't necessarily mean that a body has been captured into its current orbit, it is certainly a strong hint.
Thinking that BZ might have been drawn in from another group of small bodies in the Solar System, Namouni and colleague Helena Morais from the Universidade Estadual Paulista in Brazil set out to model the object's orbital history.
Long time neighbour
"We had this simulation which uses intense computing... to actually trace back in time to where this asteroid was when the planets finished forming," Dr Namouni explains.
The results suggested that BZ's only possible origin was outside our Solar System.
"Honestly we didn't have any idea what we were going to find. The last thing we expected... was that BZ has been in its current position most of the time."
Previously, it was thought that life among the giant planets was a turbulent one for small objects. The gravitational pull of the larger bodies would make asteroid behaviour unpredictable over long time spans, with their orbits changing after mere millions of years.
BZ's orbital longevity may be a result of its pattern - which matches, or is in "resonance" with Jupiter's.
One year in the asteroid's life lasts as long as a year on Jupiter, even though they travel in opposite directions around the Sun.
Prof Russell, who was not involved in the study, felt that its "implications were amazing."
"From a sense of our understanding of how the Solar System formed, it's quite plausible. There could have been some crossover of young stars' material," she told BBC News.
There is much left to learn about BZ's origin and composition, but it may also have something to tell us about other planets in the early history of the Solar System.
Working back through the asteroid's entry into Jupiter's orbit may help to determine what the giant planet's motion was during its early history - a key question for models of Solar System formation.
"We can work back and constrain the position of the different planets. Now that we have this object we can hope to discriminate between the various models... but it's going to be difficult," commented Dr Namouni.
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