"It was an extremely surprising discovery," said James Bauer, a planetary astronomer at NASA's Jet Propulsion Laboratory in La Cañada Flintridge who was not involved in the finding. "No one has ever seen rings around a comet or an asteroid before. This is a brand-new area."
The two concentric rings around Chariklo are dense, thin and bright, according to a report published online Wednesday by the journal Nature. Observations made with seven different telescopes in June 2013 indicate that the outer ring is about 2 miles wide and the inner ring measures roughly 4 miles wide. The elliptical rings are separated by a gap of about 5.5 miles, and they are probably made of water ice, scientists reported.
Chariklo, a 155-mile-wide rock that orbits the sun between Saturn and Uranus, is part of a class of objects known as centaurs. These space rocks are thought to be part comets, part asteroids.
Scientists believe Chariklo's surface is made of carbon silicates and organics, with icy materials trapped below. It reflects just 3% of the light that hits it, making it darker than asphalt — and thus difficult to study from Earth.
The rings were discovered when an international team of scientists observed Chariklo passing in front of a distant star. Using a technique known as occultation, they took careful measurements of how long the asteroid blocked the star's light.
The scientists, led by Felipe Braga-Ribas of Brazil's National Observatory in Rio de Janeiro, were hoping to get a more precise idea of the centaur's size. But something unexpected happened. The star's light went out briefly twice, then for a long time, and then briefly twice again. Something around Chariklo was clearly blocking the star as well.
"We had puzzling data that we did not know how to reconcile," said Jose Ortiz of the Institute of Astrophysics of Andalusia in Spain, a member of the research team and coauthor of the paper. "But when I realized that an elliptical ring could explain all the secondary occultation events, I almost jumped out of my seat!"
Scientists used a similar method to discover the rings of Uranus and of Neptune. "This is a great technique for finding faint rings, and the evidence is very credible," Bauer said.
Amy Mainzer, an astronomer at the Jet Propulsion Laboratory who was not involved in the study, said that "until now, we had no idea that anything but the giant planets could have rings." She added that the team's discovery "is amazingly cool."
Nobody knows exactly how Chariklo's rings formed, but the Nature authors suggest their origin is probably tied to some sort of collision. One scenario is that another asteroid slammed into Chariklo, knocking ice off its surface that reformed as rings. Another possibility is that something hit a moon orbiting the centaur and the debris from that collision made the rings. Or perhaps two small moons of the asteroid smashed together, forming the two circles.
"I particularly like the scenario of the total or partial disruption of a moon through a relatively small collision, but so far we have little information to decide which mechanism is best to explain what we saw," Ortiz said.
The scientific team is hoping to get some time on the Hubble Space Telescope to collect actual images of Chariklo and its rings. They are also planning to observe it with a network of telescopes during three more occultation events expected to occur this year. The centaur is currently moving through a crowded star field, so such opportunities are relatively frequent.
Although Chariklo is the first small planetary body to be discovered with a ring system, it may not be the last.
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