Astronomers have discovered a brand new type of object in our solar system: an active binary asteroid. That means the object, named Body 288P, is the first known hybrid of two rare types of asteroid: a binary that's made up of two rocks orbiting each other, and an active asteroid that acts more like a comet, leaving a trail of gas and dust in its wake.
Traditionally, the line between asteroids and comets was fairly clear: asteroids are chunks of rock and metal, while comets are icier, causing them to leave a vapor tail when the Sun heats them up. But the more we study these objects, the more that line blurs. Active asteroids kick up clouds of dust and gas, giving them the appearance of comets, but these are rare, with currently only about 20 known examples. When they orbit within the asteroid belt that lies between Mars and Jupiter, these objects are called main-belt comets.
Body 288P was discovered in 2011, and images captured by the Hubble Telescope back then indicated that it was active. But at that time, the asteroid was too far from Earth to get a good look. In September 2016 the asteroid was on its closest approach to the Sun, and as it passed within 200 million km of Earth, astronomers from the Max Planck Institute used Hubble to peer closer. And that's when it became clear that it was made up of not one, but two separate pieces. That makes 288P the first binary main-belt comet.
These observations revealed some of 288P's many quirks. Most binary asteroids are made up of one larger body with a smaller "moon" orbiting it, but 288P's pieces are roughly the same size, each measuring about 1 km (0.62 mi) wide. And they're much further apart than usual, orbiting each other at a distance of about 100 km (62 mi).
According to the Max Planck astronomers, rapid rotation may have caused 288P to break up in the first place, but the fact that the object is still active suggests that this would be a relatively recent development, occurring around 5,000 years ago. Then, the gases it spews likely drove the two pieces apart to their current distant orbit.
"288P's activity probably played a decisive role in its further evolution," says Jessica Agarwal, main author of the study. "The most probable formation scenario of 288P is a breakup due to fast rotation. After that, the two fragments may have been moved further apart by sublimation torques."
Among the many thousands of pieces of debris floating around out there, Body 288P is a completely unique object. Further study is required to figure out how common binary main-belt comets might be, and what clues they can reveal about the origins of the Solar System.
The research was published in the journal Nature. Check out 288P in action in the video below.