James Webb telescope observes the rings of Chariklo with a high-precision occultation technique

In 2013, an international science team watched as Chariklo, a minor planet between Saturn and Uranus, passed in front of a background star, a technique known as occultation. Remarkably, the star blinked twice before and after disappearing behind Chariklo, revealing the presence of two thin, dense rings, the first ever detected around a Solar System object other than a giant planet. The rings have now been observed by the James Webb Space Telescope (JWST), in its first observed stellar occultation.

"In a high-precision observational milestone, on 18 October 2022 we were able to observe the brightness dips that the passage of Chariklo produced on the star Gaia DR3 6873519665992128512. The shadows of the rings were clearly detected, showing a new way to study the Solar System with James Webb," says Pablo Santos-Sanz, a researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC) who led the observing campaign.

Chariklo's rings are probably composed of small particles of water ice mixed with dark material, remnants of an icy body with which it collided in the past. Chariklo is too small and too far away for even the JWST to directly image the rings, so occultations are the only tool for characterizing the rings.

"As we dig deeper into the data, we will explore whether we cleanly resolve the two rings. From the occultation light curves we will explore the thickness of the rings, as well as the sizes and colours of the particles that form them. We hope to better understand why this small body has rings, and perhaps detect fainter ones", adds Pablo Santos-Sanz.

Shortly after the occultation, JWST targeted Chariklo again, this time to observe the sunlight reflected by Chariklo and its rings, which provided a spectrum with a clear presence of water ice in the system. "Spectra from ground-based telescopes had hinted at the presence of this ice, but the exquisite quality of the JWST revealed for the first time the clear signature of crystalline ice", says Noemí Pinilla-Alonso, who is responsible for JWST's spectroscopic observations of Chariklo. "Since high-energy particles transform ice from crystalline to amorphous, the detection of crystalline ice indicates that the Chariklo system undergoes continuous micro-collisions that expose pristine material or trigger crystallisation processes", says Dean Hines, an STSI researcher involved in the work.

Most of the light reflected in the spectrum comes from Chariklo, and models suggest that the area of the ring observed with JWST is about five times smaller than the area of the body itself. The high sensitivity of JWST, in combination with detailed models, will make it possible to distinguish the signatures due to the material of the rings and Chariklo. "By observing Chariklo with JWST for several years, we will be able to isolate the contribution of the rings as our viewing angle to them changes", says Pinilla-Alonso (NASA).

The high sensitivity and infrared capability of JWST is coupled with the unique scientific performance of the occultations and the detail provided by the spectra. These successful spectroscopic and occultation observations of Chariklo with JWST open the door to a new way of characterising small objects in the outer Solar System.