If you could float above the plane of the solar system, you'd notice that more than 99.9 percent of the objects whirling around the sun orbit counter-clockwise, set into motion by the spinning disk of dust and gas that birthed our planets, asteroids, and comets.

But bizarrely, of the more than 779,000 known asteroids, at least 95 drive against our solar system's flow of traffic. Now, two researchers are making an intriguing, if controversial, claim: One of these unusual asteroids—2015 BZ509—goes backward because it was adopted from another star system entirely.

“When we started working, we did not want to know whether it was interstellar,” says Fathi Namouni, an astronomer with the Côte d'Azur Observatory. Instead, Namouni and