An asteroid with rings: astronomers discover rare oddity in our solar system

Astronomers have known for years now that all the planets of the outer solar system — Jupiter, Saturn, Uranus and Neptune — have rings, but now, for the first time ever, they've found an asteroid out there that has them as well.

10199 Chariklo is the largest centaur — a 'cross' between an asteroid and a comet — that astronomers have detected so far, roughly 250 kilometres across, with an orbit that takes it around the sun between the orbits of Saturn and Uranus. Astronomers first found Chariklo over 17 years ago, but recent observations turned up something really unexpected.

"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!" said Felipe Braga-Ribas an astronomer at the National Observatory in Rio de Janeiro, Brazil, who led the campaign to observe Chariklo and write up this amazing discovery.

This video shows an artist's impression of the centaur and its rings, revealing how there is one thicker inner ring and a very thin outer ring beyond it.

According to the team that discovered them, the inner ring is roughly seven kilometres across, while the outer ring is just three kilometres across, and they're separated by a nine-kilometre-wide gap.

They found these rings on June 3, 2013, by observing Chariklo directly as it passed in front of (occulted) a star known as UCAC4 248-108672. Chariklo completely blocked the light from the star, as expected, but they also spotted a very brief dip in the star's light both before the occultation and after, as seen in this video:

Since numerous telescopes across South America were trained on Chariklo at the time, the team was able to capture the event from many different angles. Even though the differences in angle were very small (since South America is quite small compared to the distance between Earth and Chariklo), it was still enough to allow them to figure out exactly how big the rings are.

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Although the description of Chariklo flips back and forth between asteroid and centaur, both here and in the European Southern Observatory news release, it is valid to use both titles for this object. If a centaur was flung into the inner solar system by venturing too close to one of the outer planets, it would likely act like a comet as it approached the sun. However, at that distance out, it's so cold that Chariklo is completely inert and it behaves like an asteroid.

Also, you may have noticed something from the first video above ... it shows a small moon orbiting Chariklo. That's no mistake, and it's not an artist's embellishment. The discovery of these rings points to a collision with another object at some point in Chariklo's past, but for the debris from that impact to form these two distinct rings is an indication that there's possibly some other object there to 'shepherd' it all into that formation, as we've seen with some of the moons of Saturn. Further observations will be needed to confirm if there's a moon there or not, but for the time being, the astronomers have nicknamed the rings Oiapoque and Chuí, after two rivers in Brazil.

(Image courtesy: ESO/L. Calçada/M. Kornmesser/Nick Risinger)

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