But until now it seemed that only giant planets had the gravity to hold on to the billions of bits of orbiting ice and dust that make up a ring.
In a paper published today in Nature, astronomers report the discovery of two icy rings around a small object named Chariklo that orbits between Saturn and Uranus. Chariklo is only 154 miles (248 kilometers) across. (Related: "Asteroids and Comets.")
"When I first heard of it, it was unbelievable," says Joseph Burns, an astronomer at Cornell University in Ithaca, New York, who wasn't involved with the ring discovery.
"It's a mind-blowing kind of observation," says Marc Buie of the Southwest Research Institute in Boulder, Colorado, who also wasn't involved with the study.
"This probably will be the biggest discovery of my career," says Felipe Braga-Ribas of the National Observatory in Brazil, who led the team that found the rings, and who received his Ph.D. just last year.
A Comet? An Asteroid? A Minor Planet?
Chariklo, which was discovered in 1997, is the largest of a strange class of objects known as centaurs. There are a few hundred of them. Like the mythological hybrid of horse and human, they straddle two realms: that of the rocky asteroids, which orbit closer to Earth than Jupiter, and that of the icy comets, which mostly lie beyond Neptune.
Centaurs may hail originally from the comet belt, but they now circulate between Jupiter and Neptune on unstable orbits that cross the path of one of the giant planets. That means they're probably doomed, within ten million years or so, to be flung into the inner solar system or out into interstellar space.
There's a lot that's not known about centaurs, including how many have enough gravity to pull themselves into a round shape, which would earn them the title of "dwarf planet."
On June 3, 2013, Braga-Ribas and his colleagues observed Chariklo as it passed in front of a distant star—an event called an occultation. By using seven different telescopes around South America to determine exactly when and for how long the star's light was dimmed by the centaur, they hoped to measure Chariklo's size and shape precisely.
The event lasted only a few seconds. At first, the team thought the brief, unexpected dips in starlight they recorded were evidence that Chariklo was giving off jets of gas, like a comet. The same phenomenon had been seen previously on Chiron, another centaur.
But from the pattern of the light dips, which came before and after the dimming caused by Chariklo itself, it soon became clear to the researchers that they had observed two closely spaced but distinct rings passing in front of the star.
"It was a complete surprise," says Braga-Ribas.
The rings are about four and two miles (seven and three kilometers) wide, and are respectively 243 miles (391 kilometers) and 251 miles (405 kilometers) from the center of Chariklo. They are chock-full of water ice, Braga-Ribas says, and are bright and dense, resembling a miniature version of Saturn's majestic bands.
In retrospect, the discovery of the rings may clear up a mystery about Chariklo's behavior. From 1997 to 2008 it got progressively dimmer, and signs of water ice seemed to disappear from its spectrum. Braga-Ribas says the most likely explanation is that Chariklo's bright rings were gradually assuming an edge-on orientation and becoming invisible to observers on Earth. As the rings tilted back to face Earth, Chariklo brightened and signs of water—most likely locked up in the rings—became apparent again.
The Power of the Occult
Astronomers aren't sure how Chariklo got its rings. But they provide a great opportunity to learn how rings form in general, Burns says.
One possibility is that a collision with a smaller object created a disk of debris around Chariklo, and that the smaller pieces were then corralled into rings by the gravity of some of the bigger pieces. Such "shepherd satellites" help give Saturn's rings their sharp edges. The shepherd satellites that may orbit Chariklo, however, could be less than a mile across, making them difficult to detect with a telescope.
Since making their discovery, Braga-Ribas and his colleagues have watched Chariklo pass in front of another star, and they're planning to observe more such events this year. They're also keeping their eye out for rings around 50 other centaurs that are predicted to occult.
"If this sort of thing can happen and is commonplace, what is that telling us?" says Buie. "This is really new stuff and it's going to take a while for the full ramifications to sink in."
"Discovering a new object is a pretty good experience," Braga-Ribas says. "We are making history here."