Icy Chariklo asteroid has ring system

Artist's impression of of Chariko and rings An artist's impression of how the rings might look from the surface of Chariklo

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The asteroid Chariklo has been confirmed as the smallest object in the Solar System to display a ring system.

Encircling bands of material are more usually associated with the giant planets, such as Saturn and Uranus.

Chariklo may be just 250km wide but observations made when it passed in front of a distant star reveal the presence of two distinct rings.

Astronomers tell the journal Nature that the rings are 7km and 3km wide, separated by a clear gap of 9km.

They probably comprise ice particles and perhaps small fragments of rock.

"Our best guess for the origin of the rings is that there was a collision on Chariklo and that this collision injected a disc of material around the body," Bruno Sicardy, from the Paris Observatory, France, told BBC News.

Even the Earth may have had such bands early in its history if our theory for the Moon's formation is correct.

This describes an impact that threw material into the sky that then encircled the globe before eventually coalescing into the planet's familiar satellite.

Icy Chariklo, too, may have experienced such a bombardment, but the rings in this case are unlikely to form a moon.

"The rings are very nearby and there are forces called tides, and every time the particles try to accrete and form a satellite, the tidal forces will disrupt them," explained Prof Sicardy. "The fact that they are so close to Chariklo will maintain them as rings."

Chariklo is technically known as a centaur. Like the mythical creature of the same name, it exhibits a half-way character, having the traits of an asteroid but also looking very comet-like.

Today, Chariklo moves beyond the orbit of Saturn. However, it is very probable that it formed much further out in the Solar System and was then perturbed inwards.

The observations that led to the rings' detection were carried out on 3 June 2013 using seven telescopes sited in South America.

Scientists had worked out that Chariklo would pass in front of the star called UCAC4 248-108672 and were therefore able to coordinate the various facilities.

The occultation, as it is known, lasted just five seconds.

Nonetheless, the researchers were able to use the event to better describe the size and shape of Chariklo, as well as pick out the surprising rings.

Artist's impression of of Chariklo and rings This visualisation gives a sense of the size of Chariklo's rings compared to the asteroid itself
Saturn Chariklo moves beyond the orbit of the planet with the best known ring system - Saturn

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos

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