In an observational feat of high precision, scientists used a new technique with NASA's James Webb Space Telescope to capture the shadows of starlight cast by the thin rings of Chariklo. Chariklo is an icy, small body, but the largest of the known Centaur population, located more than 2 billion miles away beyond the orbit of Saturn. Chariklo is only 160 miles (250 kilometers) or ~51 times smaller than Earth in diameter, and its rings orbit at a distance of about 250 miles (400 kilometers) from the center of the body.
We asked members of the science team observing Chariklo to tell us more about this unique system, the occultation technique, and what they learned from their Webb observations.
In 2013, Felipe Braga-Ribas and collaborators, using ground-based telescopes, discovered that Chariklo hosts a system of two thin rings. Such rings had been expected only around large planets such as Jupiter and Neptune. The astronomers had been watching a star as Chariklo passed in front of it, blocking the starlight as they had predicted. Astronomers call this phenomenon an occultation. To their surprise, the star blinked off and on again twice before disappearing behind Chariklo, and double-blinked again after the star reemerged. The blinking was caused by two thin rings – the first rings ever detected around a small solar system object.
Pablo Santos-Sanz, from Instituto de Astrofísica de Andalucía in Granada, Spain, has an approved "Target of Opportunity" program (program 1271) to attempt an occultation observation as part of Webb's solar system Guaranteed Time Observations (GTO) led by Heidi Hammel from the Association of Universities for Research in Astronomy. By remarkable good luck, we discovered that Chariklo was on track for just such an occultation event in October 2022. This was the first stellar occultation attempted with Webb. A lot of hard work went into identifying and refining the predictions for this unusual event.
On Oct. 18, we used Webb's Near-Infrared Camera (NIRCam) instrument to closely monitor the star Gaia DR3 6873519665992128512, and watch for the tell-tale dips in brightness indicating an occultation had taken place. The shadows produced by Chariklo's rings were clearly detected, demonstrating a new way of using Webb to explore solar system objects. The star shadow due to Chariklo itself tracked just out of Webb's view. This appulse (the technical name for a close pass with no occultation) was exactly as had been predicted after the last Webb course trajectory maneuver.
The Webb occultation light curve, a graph of an object's brightness over time, revealed that the observations were successful! The rings were captured exactly as predicted. The occultation light curves will yield interesting new science for Chariklo's rings. Santos-Sanz explained: "As we delve deeper into the data, we will explore whether we cleanly resolve the two rings. From the shapes of rings' occultation light curves, we also will explore the rings' thickness, the sizes and colors of the ring particles, and more. We hope gain insight into why this small body even has rings at all, and perhaps detect new fainter rings."
The rings are probably composed of small particles of water ice mixed with dark material, debris from an icy body that collided with Chariklo in the past. Chariklo is too small and too far away for even Webb to directly image the rings separated from the main body, so occultations are the only tool to characterize the rings by themselves.
