An asteroid that flouts the norms of the solar system might not be from around here.

The renegade asteroid travels around the sun in reverse — in the opposite direction of the planets and most other asteroids (SN: 5/13/17, p. 5). Now two scientists suggest that’s because the space rock originated from outside the solar system, according to a paper published May 21 in Monthly Notices of the Royal Astronomical Society Letters.

Astronomers Fathi Namouni of the Côte d’Azur Observatory in Nice, France, and Helena Morais of Universidade Estadual Paulista in Rio Claro, Brazil, used computer simulations to show that the asteroid, which shares its orbit with Jupiter, could have been traveling in reverse ever since the solar system’s youth. Because asteroids in the infant solar system formed from one swirling cloud, they should have all been traveling in the same direction. So the best explanation, the duo suggests, is that the rock, known as 2015 BZ₅₀₉, migrated here from another star’s planetary system.

In 2017 astronomers spotted the first interstellar asteroid, dubbed ‘Oumuamua, which cruised through the solar system and back out again (SN Online: 12/1/17). Asteroid 2015 BZ₅₀₉, however, appears to be a long-term inhabitant.

“It’s certainly an interesting possibility,” says astronomer Martin Connors of Athabasca University in Canada. But, he says, the study doesn’t nail down whether the asteroid actually came from outside the solar system.

Such asteroids are faint and hard to get information from, Connors says. “There isn’t really a blazing sign saying, ‘Hey, I’m not from here.’ ”