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When astronomers got a rare look at an asteroid passing in front of a distant star last year, they discovered something rarer still: the first asteroid with rings.

Telltale variations in light show that the space rock, known as Chariklo, has two dense, narrow rings. That makes it only the fifth ringed world known to exist in the solar system, after Saturn, Jupiter, Uranus and Neptune.

"We weren't looking for a ring and didn't think small bodies like Chariklo had them at all, so the discovery — and the amazing amount of detail we saw in the system — came as a complete surprise!" Felipe Braga-Ribas of Brazil's Observatorio Nacional/MCTI said in a news release announcing the find.

An artist's conception shows the icy asteroid Chariklo and its rings, which are 4 miles and 2 miles (7 kilometers and 3 kilometers) wide and separated by a 6-mile-wide (9-kilometer-wide) gap. The ring system extends about 500 miles (810 kilometers) in diameter.Lucie Maquet

Braga-Ribas planned the campaign to observe Chariklo, and he's also the lead author of a paper on the asteroid's rings published in Thursday's issue of the journal Nature.

Chariklo, a 160-mile-wide (258-kilometer-wide) world named after a nymph in Greek mythology, orbits between Saturn and Uranus and is the largest of a class of objects known as Centaurs. Astronomers calculated that Chariklo would eclipse the star UCAC4 248-108672 on June 3, 2013, as seen from South America. So they flocked to several southern locations to catch the show.

Braga-Ribas and his colleagues recorded the dip in starlight as the asteroid passed over — as well as two additional dips, a few seconds before and again a few seconds after the main event. That's what tipped them off about the rings.

Astronomers suggest that the rings (nicknamed Oiapoque and Chu', after rivers in Brazil) are made up of debris left over from a cosmic collision (like the one thought to have given rise to Earth's moon). The fact that the rings are so sharply confined suggests that "shepherd moons" may be keeping them in line.

"It's likely that Chariklo has at least one small moon still waiting to be discovered," Braga-Ribas said.

Braga-Ribas is among 64 authors of the Nature paper, titled "A Ring System Detected Around the Centaur (10199) Chariklo."

Correction for 5:55 p.m. ET March 26: An earlier version of this report misstated the metric conversion for Chariklo's diameter.