This ESOcast shows how observations at many sites in South America, including ESO’s La Silla Observatory, have made the surprise discovery that the remote asteroid Chariklo is surrounded by two dense and narrow rings. This is the smallest object by far found to have rings and only the fifth body in the Solar System — after the much larger planets Jupiter, Saturn, Uranus and Neptune — to have this feature. The origin of these rings remains a mystery, but they may be the result of a collision that created a disc of debris. Credit ESO.
In 1997 SSERVI Director Yvonne Pendleton and husband Dale Cruikshank observed spectra for the Centaur 1997 CU26 with their team at the Keck Observatory. The data showed strong absorptions at 1.52 and 2.03 micrometers attributable to water ice on the asteroid’s surface. The reflectance spectrum matched a mixture of low-temperature, particulate water ice and spectrally featureless but otherwise red-colored material. Water ice dominated the spectrum of 1997 CU26, whereas methane or methane-like hydrocarbons apparently dominated the spectrum of the Kuiper belt object 1993 SC, perhaps indicating different origins, thermal histories, or both for these two objects. Asteroid 1997 CU26 is the largest known centaur, orbiting the Sun between Saturn and Uranus. It was the first Centaur object discovered to have water ice on its surface (ref Science Magazine). Later the asteroid 1997 CU26 was renamed Chariklo.
When astronomers observed Chariklo passing in front of a distant star last year, they discovered that it was also the first asteroid with rings.
Felipe Braga-Ribas of Brazil’s Observatorio Nacional/MCTI, in a paper published in Thursday’s issue of the journal Nature, reports that Chariklo has two dense, narrow rings. That makes it only the fifth ringed world known to exist in the solar system, after Saturn, Jupiter, Uranus and Neptune.
Astronomers suggest that the rings are made up of debris left over from a cosmic collision (like the one thought to have given rise to Earth’s Moon). The fact that the rings are so sharply confined suggests that “shepherd moons” may be keeping them in line.
“It’s likely that Chariklo has at least one small moon still waiting to be discovered,” Braga-Ribas said.