Asteroid 2015 BZ509 does two do-si-dos with Jupiter every 12 years as it travels backwards around the sun
An asteroid that orbits near Jupiter has been traveling in the wrong direction for at least a million years. All the planets, most asteroids and many comets orbit the same direction around the sun — counterclockwise when viewed from above the Earth’s north pole. 2015 BZ509 orbits clockwise, one of only about 95 asteroids to do so. Most of those circle the sun far from larger planets. Not this one. It’s the only asteroid we know of that shares space with Jupiter while moving in a contrary direction.
2015 BZ509 was discovered in 2015 and like Jupiter takes 12 years to orbit the sun. Why it hasn’t crashed into the giant planet yet is something of a miracle. It manages this little feat by passing inside Jupiter’s orbit on one side of the sun and outside the planet’s orbit on the opposite side of the sun. When nearest, BZ is 109 million miles from Jupiter, close enough to have its orbit stretched by Jupiter’s gravitational pull. But the inside-outside tugs cancel out, keeping the asteroid safe from collision with the planetary heavyweight.
“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,” writes Paul Wiegert (University of Western Ontario), lead author on a recent paper on 2015 BZ509, on his website.
So far so good. But how did this asteroid arrive on its backward path? It might just be a comet that dropped in a few millions years ago from the rich repository at the fringes of the solar system called the Oort Cloud after being nudged by the gravity of a close-passing star. Clockwise or retrograde orbits are more common among comets than asteroids. Pried loose by the star, the comet headed toward the sun but was captured by Jupiter. As far as having a tail or showing other signs of activity we usually associate with comets, either 2015 BZ509 has fizzled out or it’s far enough from the sun that it’s only comes to life now and again.
No disrespect to comets, but the object might be something more special. Its contrary orbit could mean it came from somewhere else entirely. Maybe even another solar system around a distant star. In which case it would be only the second interstellar asteroid known after ‘Oumuamua, a bizarre object discovered in October 2017. ‘Oumuamua zipped by the sun last September and is now on its way out of the solar system, while 2015 BZ509 appears to be here to stay.
Researchers Fathi Namouni and Helen Morais in their recent paper describe how they ran a simulation of the asteroid’s orbit back in time and discovered that it could have arrived at a stable orbit 4.5 billion years ago when the solar system first formed. But why backwards? They considered many alternative explanations but prefer an interstellar origin. Other astronomers are skeptical of their conclusions and say a local origin makes more sense. I tend to agree only because comets are such surprising creatures.
I suppose we won’t know for certain until we can figure out what it’s made of, a task that would require a space mission. Let’s get it on the list!