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There’s an alien living among us. An asteroid out near Jupiter has been behaving strangely, and astronomers now think that may be because it didn’t come from our solar system.

Originally discovered in 2015, asteroid 2015 BZ509 orbits in the opposite direction of all the planets, including Jupiter, whose orbit the asteroid crosses periodically. A constant push-and-pull from those near passes with the gas giant has stabilized the asteroid’s orbit over billions of years.

It likely arrived here early in our solar system’s history from another planetary system, and Jupiter simply kept a tight grip on it, according to work by at the University of Côte d’Azur in France and at Universidade Estadual Paulista in Brazil.

Behaves so strangely

Namouni says the asteroid may have been ripped from its home system. “Stars form in closely-packed clusters. A large cluster density means that strong gravitational perturbations are exerted by star systems on one another,” he says. “This enables them to strip asteroids from one another and eventually capture them.”

He and Morais used models to figure out why this particular asteroid seemed to be so odd. Their simulations took into account what we know of the formation timelines of the eight planets and other objects in our solar system, as well as interactions with the interstellar areas around it.

They showed that BZ509 has been its present position going back to the near-beginning of the solar system – but if it was from here, it should have either gotten into line early on or been flung out, as hundreds of other such objects with similar configurations did in their simulations.

Not alone

There may be other interstellar objects still orbiting the sun as well, but they may not have been as lucky as this one. “Other asteroids were likely captured by Jupiter like 2015 BZ509, but left its vicinity because of perturbations by the other planets,” Namouni says.

at the University of Arizona says that the paper produces an interesting model and she didn’t expect the orbit to be so stable over long time periods. But she isn’t entirely sure about the interstellar origin. She says our understanding of the Kuiper Belt isn’t robust enough to rule out a more local origin. There’s a possibility an unknown large planet nicknamed “Planet Nine” has thrown a number of dwarf planets and small icy bodies into strange, oblong orbits.

One way we can figure out if it truly is from elsewhere is to learn more about its chemical abundances. Alessondra Springmann of the University of Arizona says that a close look at the spectra of light reflecting off it could reveal its origins. If it doesn’t look like anything else around it, Namouni’s hypothesis could hold more weight.

“It would be really great if there were (interstellar objects) in the solar system because then you can go to in situ exploration,” Volk says. “I’m just not convinced [BZ509] is one of them, but it certainly merits looking at in more detail.”

Read more: Our sun probably didn’t steal Planet Nine from outer space