An international team observed in 2013 how Chariclo, a minor planet or planetoid located between Saturn and Uranus, passed in front of a background star, a technique known as occultation. The star blinked twice before and after it disappeared behind Chariclo, revealing the presence of two thin, dense rings. These are the first detected around an object in the Solar System that wasn’t a giant planet. The rings have already been captured by the James Webb Space Telescope (JWST), in its first stellar occultation.
“In a high-precision observation frame. On October 18, 2022, we could see the decrease in brightness that Cariclo’s passage produced on the star Gaia DR3 6873519665992128512. The shadows of the rings were clearly detected, showing a new way of studying the Solar System with James Webb” , he indicates Pablo Santos-Sanz, a researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC) who led the observation campaign.
The planet is too small and too far away for direct images of its rings.
These rings are likely composed of small particles of water ice mixed with dark material, remnants of an icy body that collided with it in the past. Chariclo is too small and too far away even for JWST to image the rings directly, so occultations are the only tool to characterize them.
The concealment light curve obtained by Webb’s NIRCam infrared camera. / NASA/ESA/CSA
“As we dig deeper into the data, we’ll explore whether we resolved the two rings cleanly. From the hidden light curves, we’ll explore their thickness, as well as the sizes and colors of the particles that make them up. We hope to better understand why this small body has rings and, who knows, detecting the weakest ones”, adds the scientist.
JWST refocused on Chariclo to observe how the planet and its rings reflect sunlight. The telescope detected a clear presence of water ice in the system. “The spectra from ground-based telescopes suggested 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, responsible for the observations.
The exquisite quality of JWST revealed the clear signature of crystalline ice for the first time
Noemi Pinilla-Alonso
“As the high-energy particles transform ice from a crystalline to an amorphous state, the detection of crystalline ice indicates that the Cariclo system experiences continuous micro-collisions that expose pristine material or trigger crystallization processes,” said Dean Hines, a researcher at STSI, who participates in the work.
The high sensitivity of the telescope makes it possible to distinguish the shapes of material in the rings and the planet
Most of the reflected light in the spectrum comes from the minor planet, and models suggest that the area of the ring seen with the JWST is about five times smaller than the area of the body itself. The telescope’s high sensitivity, in combination with detailed models, makes it possible to distinguish ring shapes and planet material.
“By observing Cariclo with the JWST over several years, we will be able to isolate the contribution of the rings as our angle of view changes,” says Pinilla-Alonso.
In addition to the JWST’s infrared sensitivity and capability, add the unique scientific performance offered by occultations and the detail provided by spectra. These successful spectroscopic and occultation observations by Cariclo open the door to a new way of characterizing small objects in the outer solar system.