2017 Western U., Athabasca U., Large Binocular Telescope Observatory.

There’s an asteroid in Jupiter’s lane that orbits the sun in the wrong direction – and it may have been doing so for more than a million years.

The asteroid 2015 BZ509 was discovered in 2015, orbiting near Jupiter but in the opposite direction. Like Jupiter and the other asteroids tied to its orbit, which are called Trojans, it takes 12 Earth years to orbit the sun.

It is the only asteroid we know of that shares a planet’s orbital space while moving in the opposite, or retrograde, direction. Paul Wiegert at the University of Western Ontario and his colleagues examined this strange orbit to figure out why BZ509 doesn’t crash head-on into Jupiter.


There are only 95 known asteroids that orbit in retrograde, most of them far from larger planets. “This makes sense: if a clown car is going to survive going the wrong way around the track, best to stay away from the big trucks,” Wiegert wrote on his website.

BZ509, on the other hand, comes within 176 million kilometers of Jupiter at its nearest, close enough for Jupiter to shift the asteroid’s orbit. Wiegert and his colleagues found that this shift actually keeps the asteroid safe.

The asteroid passes Jupiter twice per orbit: once when it slips between the planet and the sun, and once on the planet’s far side. Each pass provides a small gravitational tug, which keeps BZ509’s path just to one side of Jupiter’s so they don’t collide.

Stable and safe

Wiegert and his colleagues calculated that, despite the orbit’s apparent delicacy, it is actually fairly stable and safe for the asteroid. They showed that it has been stable for at least million years and ought to remain so for a million more.

“Finding such a long-lived object in this unusual configuration is certainly a surprise,” says Wiegert. What’s still not clear is how BZ509 got on its backward path.

“We have to understand which mechanisms put this asteroid in this kind of orbit,” says Mattia Galiazzo at the University of Vienna. “It’s very peculiar, so we need to know how it got there.”

It could be the captured core of an icy comet. Many more comets have retrograde orbits than asteroids, so it could be easier for the sun and Jupiter to pull one of them into this peculiar orbit than a rocky asteroid.

“This is a common process for comets, but not for asteroids,” says Rudolf Dvorak at the University of Vienna. It could have been captured on a single orbit, but more likely it was caught gradually after a series of encounters, Dvorak says.

The discovery of one object with such a strange orbit, whether it turns out to be an asteroid or a comet, suggests that we could find other bodies orbiting in ways we haven’t considered.

“It’s exciting that, of all the possible and unusual niches for asteroids to live in in our solar system, they all seem to be occupied,” says Wiegert. “They’re not just theoretically possible, but they’re real objects.”

Journal reference: Nature, DOI: 10.1038/nature22029