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.
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.
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.
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.
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!