Astronomers have discovered rings around an asteroid-like body whose orbit is between those of Saturn and Uranus. At just 250 kilometres across, Chariklo is the smallest body so far found to have rings. Previously, only the giant planets — Jupiter, Saturn, Uranus and Neptune — have been seen sporting them. Published online today in Nature, the finding indicates that rings may be a more common feature than previously thought. The discovery was an accident and a surprise, says lead author Felipe Braga-Ribas, an astronomer at the National Observatory in Rio de Janeiro. Rings are interesting celestial features, as they are often a first step in the formation of planets and moons. But matter would typically struggle to stay stable around a body with only a tiny gravitational pull. “We thought that maybe having rings was linked with the mass of the object.
So finding them on a small object was very unexpected,” he says. Chariklo belongs to a class of objects called Centaurs, which traverse the outer Solar System in unstable orbits and can share characteristics with both asteroids and comets. Because they are small, dark and far away, studying them is a challenge, Braga-Ribas says. His team discovered the rings while observing the way in that the asteroid blocked out light from a distant star as it passed between the star and Earth on 3 June 2013. The researchers found a dip in brightness, as expected, when the asteroid traversed the star. But they also detected two much smaller dips, both before Chariklo passed in front of the star, and after. Piecing together results from telescopes at seven sites across South America, the team deduced that the blips were caused by two distinct, narrow rings that were respectively 7 and 3 kilometres wide. The same technique was used to discover the rings around Uranus in 1977. More