Jupiter is accompanied in its journey around the Sun by about 6,000 asteroids called Trojans, one swarm preceding and another following the giant planet. They all move in the same direction. Except one.
2015 BZ509, BZ for short, is a retrograde asteroid, which means it travels in the opposite direction of all the planets and the vast majority of known objects in the Solar System. Being retrograde is clearly unusual enough, but BZ also shares its orbit with Jupiter, which makes it solid proof that these complex gravitational interactions are possible.
The results, published in Nature this week, build on several theoretical predictions describing the existence of objects in this state, known as retrograde co-orbital resonance. BZ is the first known example of this type of object.
The configuration is stabilized by how the tiny asteroid and the giant planet meet during the orbit. Over a revolution around the Sun, they encounter each other twice, once when BZ is farther out and the other time when it is closer in.
Animation of the path of Jupiter, the Trojans, and BZ. Western U. Athabasca U. Large Binocular Telescope Observatory,
This interaction is firm but gentle. It has helped keep BZ's orbit stable for millions of years, with the closest encounter between the two at 176 million kilometers (109 million miles) apart, farther than the distance between the Earth and the Sun.
BZ was discovered in 2015, but there was too much uncertainty about its orbit to clearly understand how it was moving around the Sun. For this study, the Canadian-American team used the Large Binocular Telescope Observatory to learn more about it.
It is assumed that it’s an asteroid, but it could also be an inactive comet. This is only based on the retrograde motion. There are have been no observations supporting the comet hypothesis, and BZ is a long way away from the Sun to sprout a tail. However, several known retrograde objects are comets, for example Halley’s Comet.
Co-orbital asteroids are a curious but fairly common phenomenon. Even our own planet has several of these objects, the most famous being 3753 Cruithne.