Centaur Charliko 10199 Light Curve

In an observational feat of high precision, scientists used a new technique with the NASA/ESA/CSA 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 3.2 billion kilometres away beyond the orbit of Saturn. Chariklo is only 250 kilometers or ~51 times smaller than Earth in diameter, and its rings orbit at a distance of about 400 kilometers from the center of the body.

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 18 October 2022, the team 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. The rings are likely 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. Shortly after the occultation, Webb targeted Chariklo again, this time to collect observations of the sunlight reflected by Chariklo and its rings (program 1272). The spectrum of the system (shown in the graphic above) shows three absorption bands of water ice in the Chariklo system.

This graphic shows the reflectance spectrum of the double-ringed centaur 10199 Chariklo, captured by Webb’s Near-Infrared Spectrograph (NIRSpec) on 31 October 2022. This spectrum shows clear evidence for crystalline water ice on Chariklo’s surface. A reflectance spectrum shows variations in the brightness of different wavelengths (colors) of reflected sunlight. The dips in the spectrum are related to the presence of water ice, which absorbs these wavelengths, decreasing the amount of light that is reflected back to the telescope.  The background illustration of Chariklo and its rings is based on current understanding of the planet. Webb has not captured a direct image of Chariklo and its rings. 

[Image Description: The graphic shows a reflectance spectrum in the form of a graph of the Brightness of Light (relative reflectance) on the vertical y-axis versus Wavelength of Light in microns on the horizontal x-axis. The spectrum is plotted as a continuous jagged white line. The overall shape of the line is curvy, with broad peaks and valleys. Three prominent valleys are highlighted in blue and labeled “Water Ice, H2O.” In the background is a grayscale illustration of Chariklo and its rings, as seen from an oblique angle.]