January 25, 2022
The thin rings of Chariklo cast shadows of starlight, which were captured by researchers using a new technique with NASA's James Webb Space Telescope. The largest known population of Centaurs is represented by Chariklo, an icy, tiny body that lies more than 2 billion kilometers outside of Saturn's orbit. Chariklo's diameter is only 160 miles (250 kilometers), which is 51 times smaller than that of Earth, and its rings are located roughly 250 miles (400 kilometers) from the body's core.
Using ground-based telescopes, Felipe Braga-Ribas and associates found that Chariklo is home to a system of two thin rings in 2013. Only around massive planets like Jupiter and Neptune had such rings been anticipated. As expected, Chariklo passed in front of a star, blocking its light as the astronomers watched it. This occurrence is known as an occultation by astronomers. To their surprise, the star double-blinked once more after emerging from behind Chariklo then twice more before blinking off and on again twice more. The first rings ever discovered surrounding a minor solar system object were the two thin rings that were responsible for the blinking.
In order to attempt an occultation observation as part of the Webb solar system Guaranteed Time Observations (GTO) led by Heidi Hammel from the Association of Universities for Research in Astronomy, Pablo Santos-Sanz from the Instituto de Astrofsica de Andaluca in Granada, Spain, has been granted a "Target of Opportunity" program (program 1271). By an amazing stroke of fate, we found that Chariklo was on schedule for a similar occultation event in October 2022. This was the first attempt at a star occultation with Webb. Finding and improving the predictions for this uncommon event required a lot of effort.
On October 18, we closely monitored the star Gaia DR3 6873519665992128512 using Webb's Near-Infrared Camera (NIRCam) sensor to look for the telltale dips in brightness indicating an occultation had occurred. Clear evidence of the shadows cast by Chariklo's rings was found, illuminating a fresh method for using Webb to investigate solar system objects. The star shadow caused by Chariklo moved out of Webb's line of sight. This appulse, or near pass without an occultation, occurred exactly as had been foreseen following the previous Webb course trajectory maneuver.
The observations were successful, as shown by the Webb occultation light curve, a graph of an object's brightness over time! As expected, the rings were successfully captured. For Chariklo's rings, the occultation light curves will produce intriguing new scientific findings. "As we go deeper into the data, we will study whether we cleanly resolve the two rings," Santos-Sanz said. We will investigate the thickness of the rings, the sizes and colors of the ring particles, and more from the shapes of the occultation light curves of rings. We hope to learn more about why this little body has rings at all and potentially find some new, fainter rings.