Even before astronomers pointed their telescopes at a dim star over Chile last June, they knew it would darken for a few seconds as an asteroid passed in front of it. What they didn’t expect were two brief flickers a few seconds beforehand and afterward, suggesting that the asteroid was encircled by Saturn-like rings. The find is the first evidence for such rings around anything in our solar system other than a giant planet.
Like other teams positioned in a 1500-kilometer-wide swath across South America, the astronomers had started out the night with one mission: They intended to measure the size of Chariklo, an icy body that circles the sun between the orbits of Saturn and Uranus. Knowing Chariklo’s speed across the sky, they could use the duration of the mini-eclipse to estimate the distant asteroid’s size, says Felipe Braga-Ribas, an astronomer at the National Observatory in Rio de Janeiro, Brazil.
“But then we saw the flickers,” he says. Because the team members weren't looking for a ring, the discovery came as a complete surprise, he notes.
Braga-Ribas and his colleagues were measuring the brightness of the target star—one in the constellation Scorpio that's too dim to see with the naked eye—every 0.1 seconds. That sampling rate was fast enough to discern that there were two flickers before and after the main occultation, or blockage of light—evidence of two dense, narrow rings with a small but distinct gap between them. (Other groups also recorded dips in light caused by the rings, but couldn’t discern the gap between them because their sensors weren’t taking data samples often enough.)
Using data gathered at six other sites along the mini-eclipse’s path, the researchers estimate that Chariklo has a radius of about 124 kilometers. The same data sets suggest that material in the rings orbit the asteroid at radii of about 391 and 405 km, the researchers report online today in Nature. If all the material in each ring were gathered together, the inner ring would measure about 2 km across and the outer ring would be about half that diameter, Braga-Ribas says.
The newly discovered ring system answers a couple of mysteries, says Joseph Burns, a planetary scientist at Cornell University. After Chariklo was discovered in 1997, he notes, its brightness gradually dropped by about 40%, reaching a low in 2008. Over the same period, signs of water ice—certain wavelengths missing from the light reflected back toward Earth—faded. Since 2008, though, Chariklo has regained its brightness and its watery signature.
Burns says those trends can be explained by the shifting view of the rings as seen from our planet: Six years ago, when Chariklo was in a different part of its orbit, the rings would have been seen edgewise from Earth, an invisible sliver that reflected little or no light in our direction. Data suggest that although the ice-rich rings have only 15% of Chariklo’s surface area, they reflect almost three times as much light on an area-to-area comparison. Particles in the rings are on average much brighter than Chariklo’s surface because they often collide, exposing fresh, bright ice; meanwhile, Chariklo itself continues to accumulate dust, he suggests.
At present, it’s not clear how Chariklo’s ring system formed. One possibility is that a slow-motion collision with a smaller asteroid or comet once blasted debris into orbit, and the gravitational pull of the largest bits of debris have shepherded the smaller bits into a sharply defined ring. (The presence of such “shepherd moons” could help explain the distinct, 9-kilometer-wide gap between Chariklo’s two rings as well, Braga-Ribas notes.)
In another scenario, a loosely consolidated object (possibly a comet) passing near Chariklo was captured to become a satellite but subsequently was ripped apart by gravitational interactions with its new parent.
Regardless of its origin, analyses of the new ring system—which is much smaller and simpler than those circling Jupiter, Saturn, Uranus, and Neptune—may help scientists develop better computer models of how rings behave and evolve, Burns says. The particles in Chariklo’s rings take about 10 hours to orbit the asteroid, he estimates, about the same time the small bits of material near the middle of Saturn’s broad ring system take to circle that planet.