The journal Nature, in its new issue published today, reported the discovery of rings unexpectedly orbiting one of the Solar System's small bodies. Until then, rings had been confirmed only around planets such as Saturn, Jupiter, Uranus, and Neptune, all of them gas giants. The article therefore describes Chariklo as the fifth Solar System body—and by far the smallest non-gaseous body then known—to have confirmed rings.
Chariklo, about 250 kilometres in diameter, orbits the Sun between the orbits of Uranus and Neptune. Astronomers observing simultaneously from seven locations in South America were waiting, according to their calculations, for Chariklo to occult the star designated UCAC4 248-108672 when they unexpectedly detected the rings.
Just as Earth or the Moon can block sunlight to produce lunar and solar eclipses, other celestial bodies can pass in front of one another. Examples include planets being occulted by the Moon and stars being occulted by planets, moons, or asteroids. A planetary occultation by the Moon can be an interesting subject for astrophotographers, but observing a stellar occultation by a planet or asteroid is a specialized scientific measurement requiring precise instruments.
At Chariklo's great distance, the source explains, available instruments could not directly resolve an object like it in sufficient detail, so researchers used techniques such as occultation to obtain the information they needed. During an occultation, astronomical instruments measure changes in the light received from the background object, providing valuable information about both the object being occulted and the object passing in front of it.
On June 3, 2013, several teams of European astronomers at seven observatories in South America—including ESO's La Silla Observatory in Chile—were waiting for the predicted stellar occultation by Chariklo. A few seconds before the main occultation, however, the star's light unexpectedly dimmed slightly. It then disappeared as predicted when Chariklo itself passed in front of it. After the body moved past the star, the light did not return immediately to its previous level; instead, it recovered in steps.
The explanation was the presence of rings around Chariklo. Because the event was observed simultaneously from several locations and all teams recorded the same behaviour, the article notes that instrumental error or a similar mistake could be ruled out. More importantly, observing the occultation from several sites allowed researchers to derive detailed information not only about Chariklo itself but also about the rings' structure, shape, and orientation.
The data indicated two narrow rings orbiting close together. The inner ring was reported as about 7 kilometres wide and the outer ring about 3 kilometres wide, separated by a gap roughly 9 kilometres across.
The original article links to the Nature paper, images of participating observatories, and an ESO vodcast about the discovery; those links are preserved below. [source] [source] [source] [source]