Is This Backwards-Orbiting Asteroid an Interstellar Visitor?

The space rock could have been captured from another star system during the early days of our solar system

Images of 2015 BZ509 captured by the Large Binocular Telescope Observatory. C. Veillet / Large Binocular Telescope Observatory

Last year, astronomers were surprised to find a long, needle-shaped asteroid buzzing through our solar system. Dubbed ‘Oumuamua, the unusual space rock was the first interstellar object observed in our celestial neighborhood, but it didn’t stick around for long. Now, researchers argue they’ve found another interstellar visitor that’s actually a permanent resident of our solar system. But as Michael Greshko at National Geographic reports, their conclusions are controversial.

The space rock, known as 2015 BZ509, was discovered during sky surveys just a few years ago. There are about 779,000 known asteroids that orbit the sun counterclockwise, leftover chunks of the disk of rock, gas and ice that formed the planets in our solar system. But not all spin the same way. As Greshko reports, researchers have identified 95 that orbit the sun in the other direction. BZ509 is one of those objects, and it almost perfectly follows the orbit of Jupiter—but in the opposite direction.

According to the study, published in the journal Monthly Notices of the Royal Astronomical Society: Letters, the research team modeled the movements of BZ509, taking it all the way back to the birth of the solar system, some 4.5 billion years ago. They looked at a million clones of BZ509, each with a slight tweak to its orbit. Half of them didn’t even survive seven million years. Out of the rest of those space rocks, only 46 had orbits stable enough to keep them from crashing into the sun or being flung out of the solar system. In total, 27 of the simulated paths resembles BZ’s orbit.

The team concluded that their space rock has likely always orbited against the flow of traffic, which hints that it may not have been part of the swirling disc that created our solar system. Instead, it’s possible BZ is an asteroid from somewhere else that was captured by Jupiter's gravity.

“How the asteroid came to move in this way while sharing Jupiter’s orbit has until now been a mystery,” says Fathi Namouni of the Côte d’Azur Observatory and lead author of the study, in a press release. “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.”

Back when the solar system was forming, our sun was part of closely packed star cluster, according to Helena Morais, researcher at Universidade Estadual Paulista in Brazil and co-author of the new study. The gravitational tug of newly formed planets around all of these stars likely pulled asteroids and space debris between the various solar systems. “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,” she says.

The idea that early planetary systems were swapping asteroids is an exciting prospect, planetary scientist Licia Ray of Lancaster University, who was not involved in the study, tells Nicola Davis at The Guardian. “That means you can get a lot of cross-contamination, for lack of a better word, of stellar planetary systems during their formation,” she says. “It definitely could mean that you could get organic building blocks [of life] spread between different systems.”

But not everyone is comfortable with the interpretation that BZ comes from outside our solar system. “The median lifetime [of BZ509's clones] is so short, I’d be looking for short-term solutions,” Bill Bottke of the Southwest Research Institute tells Greshko. Instead, he thinks BZ could be an object bumped out of the Oort Cloud, a region of rock and ice chunks at the edge of the solar system and captured by Jupiter’s gravity a few million years ago.

The team hopes to look at more of the clockwise-rotating objects, and work through models to understand how BZ entered into Jupiter’s orbit. As the BBC reports, the project could also help tease apart the big planet’s motion in the early days of the solar system.

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