Rings were observed by chance around the icy Chariklo asteroid, which orbits between Saturn and Uranus. A discovery that makes it a unique object, and opens a new chapter in the study of small solar system bodies. The researchers working on the TRAPPIST project – a University of Liège telescope based in Chile - took part in this observation. Chariklo is a Centaur, a small body (barely 250 km in diameter) made of rock and ice, halfway between an asteroid and a comet. It’s study can shed more light on questions as varied as the chemical composition of the early solar system, the formation of the moon, the stability of planetary rings, and the increasingly teneous line between comets and asteroids.

It was during the night of 3 June 2013 that Emmanuël Jehin, astrophysicist at the University of Liège and F.R.S-FNRS research associate, contributed to an unprecedented observation - with the PhD student Cyrielle Opitom - while at the controls of TRAPPIST, a robotic telescope 60 cm in diameter installed in the Atacama desert in Chile (see Liège astrophysicists in seventh heaven). Rings revolving around an asteroid were discovered for the first time. They are the subject of a publication in Nature (1). “We knew that there were rings around giant planets”, the researcher explains. “We’ve known about Saturn’s since the 17th century and they are the finest example. The smaller ones of Jupiter, Uranus and Neptune were discovered much more recently, thanks in particular to the images from the Voyager probe at the end of the 1970s. But we would never have imagined that a ring could form around an object as small as an asteroid and remain stable. And in this case, we discovered a double ring.” The surprise was considerable since the observation of the Chariklo asteroid was orchestrated for quite another reason. “With a diameter of 250 kilometres, Chariklo is the biggest asteroid in the family of Centaurs, and therefore ideal for observation. And we wanted to acquire more precise information about its size.” 

Chariklo orbits between Saturn and Uranus at about 2.5 billion kilometres from Earth. At this distance, it is just another spot of light among others. Even with the biggest telescopes, the asteroid is too small and too far away to measure its size or see its surface by direct observation. To learn more about its characteristics, indirect methods must be used, such as the stellar occultation technique.

“We knew that on 3 June 2013, Chariklo was going to occult a star that was quite bright to observers, situated in a band passing over South America”, the astrophysicist remembers. “It really was a godsend because this star is visible even by telescopes of a modest size. Usually, occultations, which require a perfect alignment between the asteroid and the star, take place in front of relatively faint stars, because there are more of them. It's impossible for telescopes that aren’t very sensitive to observe them. In this case, we were able to mobilize a whole set of telescopes in Brazil, Argentina and Chile, and match up all the data to ensure we wouldn’t miss the event.” And the occultation happened as planned.  But instead of the star simply disappearing for a few seconds after the asteroid had passed in front of it, several brief eclipses were also observed.

“This allowed us to detect the presence of two rings. The  duration of the occultation even allowed us to determine their sizes. The one furthest away is 3 kilometres wide, and the other 7, with a diameter of 800 kilometres. Their optical width is also different. It doesn't block the light in the same way. One is more opaque than the other, which means that its density is greater, and perhaps has particles of a different kind. But to establish this, we will need more precise measurements, using spectroscopy among other things.”

(1) “A ring system detected around the Centaur (10199) Chariklo”, by F. Braga-Ribas et al.,  online in the journal Nature on 26 March 2014.