Surprising rings circle comet-asteroid hybrid

View the video

Rings of orbiting ice crystals encircle all giant planets in our solar system: Jupiter, Saturn, Uranus and Neptune. Now a much smaller planetoid — an object too small to qualify for planet status — can join them on the list of ringed objects. It is the first time rings have been found around such a small object.

Astronomers have discovered a pair of thin rings encircling 10199 Chariklo. It’s a rock-and-ice asteroid-comet hybrid, also known as a centaur. Chariklo’s diameter is only about 250 kilometers. That’s a span about equal to the distance separating Chicago, Ill., and Davenport, Iowa. Chariklo orbits the sun between Saturn and Uranus. Its rings probably result from something colliding with it, astronomers now suspect.

The rings revealed themselves during an occultation. That’s a chance passage of some object (here Chariklo) in front of a distant star. Because the planetoid is so much closer to Earth than the star, it appears far bigger (at least when viewed from Earth). So Chariklo can briefly eclipse the star’s light as it passes in front. By knowing how far the planetoid is from Earth and how long the star behind it disappears, astronomers can calculate the planetoid’s size and shape.

Felipe Braga-Ribas coordinated observations of this occultation by more than a dozen telescopes scattered across South America on June 3, 2013. An astronomer, Braga-Ribas works at Observatório Nacional in Rio de Janeiro, Brazil. Scientists at each telescope timed how long the star appeared to vanish.

By combining all the observations, his team concludes a double ring, composed mostly of water ice, must encircle the planetoid. The astronomers reported their data March 26 in Nature.

“There’s no doubt that there’s a ring,” says David Jewitt, who was not involved in the study. Indeed, “nobody knows what it means,” says this astronomer at the University of California, Los Angeles. The source of even planetary rings remains a puzzle, he notes. But scientists have their suspicions. Another celestial body could have smashed into Chariklo, excavating ice from the centaur’s interior, authors of the new study note. Alternatively, Chariklo’s gravity may have ripped apart an icy moon orbiting the centaur. It’s even possible that two of the centaur’s moons might have collided.

Indeed, Braga-Ribas and Jewitt say, Chariklo likely still has unseen moons. They would seem necessary to keep the rings in shape. Chariklo’s gravity, collisions and radiation pressure from the sun should all have ripped those rings apart — unless gravity from such “shepherd moons” held them in place.

Jewitt says that the rings mesh well with what astronomers already know about the outer solar system. Before settling into its current orbit, Chariklo most likely lived in the Kuiper belt. That’s an icy debris field beyond Neptune. And many Kuiper belt objects are known to have at least one moon.

Chariklo has recently shown some odd behavior. The rings may now help explain that.

From 1997 to 2008, the centaur grew fainter and the amount of ice it hosted seemed to drop. After 2008, however, the centaur’s ice levels gradually appeared to return to normal. Researchers now know that the planetoid’s ice is mostly in the rings — not on Chariklo’s surface. The new observations suggest that in 2008, the centaur’s rings would have appeared edge-on (as seen from Earth). Because those rings blocked some of its light, Chariklo appeared fainter. And its ice appeared to vanish only when those icy rings were being viewed edge-on.

Astronomers have not yet observed many centaur occultations. That they have already found a ring, Jewitt says, suggests that such icy rings rings may be relatively common.  Braga-Ribas agrees. “We don’t think it’s the only one,” he says. “We may have others.”

This animation show how light from a distant star (top right) was briefly eclipsed as Chariklo passed in front. The graph superimposed over the video shows the big drop in starlight during that occultation — and smaller dips before and after. Those tinier dips point to Chariklo’s two rings. Credit: Lucie Maquet, F. Braga-Ribas et al/Nature 2014, adapted by Ashley Yeager

Power Words

asteroid  A rocky object in orbit around the sun. Most orbit in a region that falls between the orbits of Mars and Jupiter. Astronomers refer to this region as the asteroid belt.

astronomy  The area of science that deals with celestial objects, space and the physical universe as a whole. People who work in this field are called astronomers.

celestial object   Any naturally formed objects of substantial size in space. Examples include comets, asteroids, planets, moons, stars and galaxies.

centaur     (in astronomy) A celestial object that is a hybrid between an asteroid and comet.

comet  A celestial object consisting of a nucleus of ice and dust. When a comet passes near the sun, gas and dust vaporize off the comet’s surface, creating its trailing “tail.”

gravity  The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.

Kuiper belt  An area of the solar system beyond the orbit of Neptune. It is a vast area containing leftovers from the formation of the solar system that continue to orbit the sun. Many objects in the Kuiper belt are made of ice, rock, frozen methane and ammonia.

moon  The natural satellite of any planet.

Neptune  The furthest planet from the sun in our solar system. It is the fourth largest planet in the solar system.

occultation   Any event where an object is briefly hidden from view when another object passes in front of it. In astronomy, this usually refers to objects such as asteroids passing in front of a star.

planetoid    Also known as asteroids, these celestrial objects are sometimes referred to as minor planets. They orbit the sun. Some are spherical, others have an irregular shape.

solar system  The eight major planets and their moons in orbit around the sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.

telescope  A light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.