Jupiter is widely credited with providing Earth with a bit of protection. The immense gravity of the gas giant typically either sucks in asteroids and comets or flings them out into orbits where they pose our planet little danger. But astronomers have now identified an asteroid that's in a stable orbital interaction with Jupiter. That interaction sends the asteroid around our Solar System backward and causes it to shift between two radically different orbits without ever settling into either.

The planets and other bodies in the Solar System mostly orbit in a single direction, inherited from the spinning disk of material from which they formed. A few bodies orbit in the opposite direction—called retrograde—but these tend to have odd, highly elliptical orbits. They're also very rare; only 0.01 percent of the known asteroids have retrograde orbits. Orbiting in the wrong direction around the Solar System tends to bring an object into relative proximity to a planet twice an orbit, and the resulting gravitational interactions will eventually destabilize the orbit.

Or so we thought. Some mathematical work showed that it's possible to have a stable retrograde orbit that overlaps with the orbit of a planet. In this case, the gravitational interactions are the key to stability. Each of the two passes of a single orbit would provide a nudge that counteracted the results of the previous one. While the orbit would shift with each nudge, it would remain stable due to this cancellation.

Showing something's mathematically possible, however, is different from showing that it happens. Nevertheless, with the theoretical background in place, people were excited by an asteroid discovered two years ago, called 2015 BZ 509 . It seemed to be in the right place to interact with Jupiter, and a very preliminary check of 2015 BZ 509 's orbit suggested it might be moving in the wrong direction. So, a US-Canadian team of astronomers decided to get a careful look at its orbit.

While they did find that the asteroid was orbiting in a retrograde manner, they also found that Jupiter was giving it more than a nudge. Instead, the giant planet appears to act like a railway switch, getting 2015 BZ 509 to alternate between two very different orbits.

After one pass with Jupiter, 2015 BZ 509 gets shifted below the plane of the Solar System and sent on an orbit that brings it about half way between Jupiter's orbit and that of Mars on its closest approach to the Sun. But the orbit is off-center from the Sun, meaning it swings farther out again, where the timing is such that it has a second encounter with Jupiter. When that takes place, 2015 BZ 509 is flung above the plane of the Solar System and pushed into an orbit that has it loop out to about 1.5 times the Sun-Jupiter distance.

Amazingly, this is all stable. The authors calculate that the cycle can continue for at least a million years, and quite possibly longer.

What's less clear is how any body could have ended up falling into this sort of arrangement in the first place. The authors suspect an interaction with both Jupiter and Saturn could have pushed a body into the right location for Jupiter to take over. And, based on that location, it would probably be a comet. But the asteroid shows no sign of comet-like behavior during its closest approach to the Sun. So the identity of 2015 BZ 509 remains a mystery.

But its orbit is no longer a mystery, even if it is rather odd.

Nature, 2017. DOI: 10.1038/nature22029 (About DOIs).