Surprise! Astronomers Discover Rings Around An Asteroid

Artist’s impression shows how the rings might look from close to the surface of asteroid 10199 Chariklo which orbits between Saturn and Uranus. The two dense, narrow rings were discovered by surprise last June. Click to enlarge. Credit: ESO/L. Calcada/Nick Risinger

Ever heard of the asteroid Chariklo (KAR-ik-lo)? I didn’t either until today, but this obscure object will now enter the textbooks as the first asteroid ever discovered with rings. That’s right – rings. Like Saturn, Jupiter, Uranus and Neptune. That makes this new ringbearer only the 5th body known to possess this feature and the smallest by far at only 155 miles wide.

Another artist’s impression of Chariklo and its pair of skinny rings as it might look from an orbiting space probe. The rings are just 240 miles above its surface. The asteroid orbits between Saturn and Uranus and might be made of mostly ice like a comet. Credit: ESO/L. Calcada/Nick Risinger

Two dense, sharply confined rings just 4.3 and 1.8 miles wide separated by a gap of 5.6 miles orbit this remote Centaur asteroid. How they got there is anyone’s guess, but they’re likely debris from a long-ago collision with another asteroid or comet. Scientists believe their distinctive shapes and crisp edges are shaped and herded by the gravitational influence of a tiny, yet to be discovered “sheparding” moon embedded within the ring plane. Sheparding moons are responsible for many of the narrow gaps and crisp record-groove-like ringlets around Saturn.

Saturn’s moons Pandora (left) and Prometheus confine the planet’s F-ring and create curious warps and waves within it. Something similiar may help define Chariklo’s two rings. Credit: NASA/JPL/Space Science Institute

The discovery was serendipitous. Astronomers using seven telescopes across southern South America watched Chariklo pass in front of or occult a star. To their surprise, the star’s light dipped in brightness both a few seconds before and few seconds after the main occultation. Something around the asteroid was blocking the light. By comparing observations from all seven sites, they learned not only the shape and size of the asteroid, but also the shape, widths and orientation of the rings.

“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 of the Observatório Nacional/MCTI in Brazil, who planned the observation campaign and is lead author on the new paper.

This artist’s impression shows the view from inside the ring system, with Chariklo behind and a putative shepherding satellites also visible. The leaders of this project are provisionally calling the rings by the nicknames Oiapoque and Chuí, two rivers near the northern and southern extremes of Brazil. Credit: ESO/L. Calcada/Nick Risinger

Chariklo belongs to a class of asteroids called Centaurs that orbit between Jupiter and Neptune and cross the orbit of one or more giant planets as they circle the sun. They’re believed to have originated in the outer asteroid belt beyond Neptune called the Kuiper Belt and perturbed by Neptune’s gravity to their present location.

Because Centaurs’ orbits cross those of the giant planets their days are numbered. If they don’t eventually collide with a giant planet, they’ll be flung by its gravity straight out of the solar system. Centaurs have lifetimes measured only in millions of years – that’s why it’s thought they’re continually replenished by Neptune’s massaging of the the inner Kuiper Belt.

Size comparison of Chariklo, Pluto and the moon. Credit: Wikipedia

Some Centaurs mimic comets, developing hazy comas of gas and dust, but all we so far of Chariklo’s composition is that it shows signs of amorphous carbon and possibly water ice.

Astronomers made the ring system discovery in June 2013 when Chariklo – and its rings – passed in front of a faint star, briefly blocking its light. Credit: ESO

It was discovered in 1997 and is the largest known Centaur, orbiting the sun every 63 years. The rings are dense and orbit just 240 miles above the asteroid’s surface. What a sight they’d be slicing across the starry sky!

4 Responses

    1. astrobob

      Hi Carol,
      Thanks. The labeling in the illustration is correct, but for some reason I put Earth in the caption. Call me geocentric. I appreciate you pointing out the error. It’s fixed now.

    1. astrobob

      I agree. That fact that no one was looking for rings around this one makes it all the more satisfying a discovery.

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