JWST occultation reveals unforeseen complexity in Chariklo's ring system

TL;DR

This study reports near-infrared JWST occultation observations of Chariklo's double-ring system, revealing an unexpected rise in the inner ring's opacity and a disappearance or strong weakening of the outer ring in the JWST bands. By predicting and observing the event from space using two near-infrared channels, the authors derive ring parameters and compare them across decades and wavelengths, uncovering temporal and potentially wavelength-dependent changes. They explore three scenarios—azimuthal variability, material or cross-section changes, and grain-property effects—and find that the inner ring has become substantially more opaque while the outer ring may be transient or evolving rapidly, suggesting a hitherto unseen level of complexity in small-body ring systems. The results imply rapid ring evolution and possible confinement/replenishment mechanisms, challenging current models and offering new insights into transient ring architectures in the outer Solar System.

Abstract

Ring systems have been discovered around several small bodies in the outer Solar System through stellar occultations. While such measurements provide key information about ring geometry and dynamical interactions, little is known about their origins, lifetimes, evolutionary pathways, or compositions. Here we report near-infrared observations with the James Webb Space Telescope (JWST) of a stellar occultation by (10199) Chariklo, a Centaur known to host a double-ring system. Our JWST measurements show that Chariklo's inner dense ring has become significantly more opaque than in previous observations, pointing to ongoing replenishment processes or dynamical restructuring. In contrast, the outer ring exhibits a much weaker near-infrared occultation signature than seen in earlier visible-light detections. This discrepancy may reflect material loss, suggesting that the outer ring could be transient, or may arise from wavelength-dependent opacity. These scenarios, which are not mutually exclusive, point to an unprecedented level of complexity in small-body ring systems, distinct from those observed around any other minor bodies in the Solar System.

Figures (10)

• Figure 1: Light curves obtained with the James Webb Space Telescope (JWST) Near Infrared Camera (NIRCam), using the F150W2 (1.5 $\mu$m) and F322W2 (3.2 $\mu$m) filters, reveal the characteristic drop in stellar brightness as Chariklo’s rings occulted a background star on 18 October 2022 (UT). Although Chariklo’s body did not occult the star from JWST’s perspective, its rings did. In the upper panels, the locations of the C1R and C2R ring detections in both filters are highlighted in orange. The zoomed-in lower panels further illustrate these events, with clear diffraction spikes marking the sharp ingress and egress edges of the C1R ring. The light green shaded region indicates where C2R is detected or expected. Black dots represent the observed flux, while the red curve shows the best-fit model to the ring occultations.
• Figure 2: Temporal evolution of Chariklo’s rings. (a) Temporal evolution of the equivalent widths ($E_P$) of the C1R ring, derived from stellar occultation events between January 2013 and October 2022. Each marker corresponds to an individual $E_P$ measurement (in km), with asymmetric error bars reflecting observational uncertainties (see \ref{['tab:c1r_equivalent_widths']}). Marker shapes indicate the observing sites and instruments, while black and grey colours represent the 1$^{\text{st}}$ and 2$^{\text{nd}}$ ring contacts, respectively. A quadratic polynomial fit, obtained via the RANSAC (Random Sample Consensus) algorithm, is overplotted to highlight potential long-term variations or structural evolution within C1R. (b) Time series of C2R equivalent width ($E_P$) measurements derived from multiple stellar occultation events spanning 2013–2022, illustrating a possible decline in opacity over the past decade. Each data point corresponds to a single $E_P$ estimate (in km), with asymmetric error bars indicating uncertainties (see \ref{['tab:c2r_equivalent_widths']}). Marker shapes and colours identify the observing sites and instruments, while ‘X’ and ‘o’ symbols represent the 1$^{\text{st}}$ and 2$^{\text{nd}}$ ring contacts, respectively. A quadratic polynomial fit, obtained via the RANSAC algorithm, is overplotted to emphasise the long-term trend of the ring.
• Figure 3: Comparison of observed normal fractional transmission ($1 - p_N$) with radiative-transfer models using different compositions and grain sizes for C1R. Black symbols represent the observed normal fractional transmission values (filled circles: pre-JWST observations, squares: JWST data). The solid blue curve shows the best-fit model (lowest $\chi^2$) using only pre-JWST measurements, while the solid red curve represents the best-fit model including the JWST data. The shaded blue (filled) and orange (striped) regions indicate the approximate parameter space where the models provide the best fits.
• Figure : Extended Data Figure 1: Occultation map derived from the reconstructed JWST ephemeris. The map displays the relative positions of Chariklo’s main body and ring system at the time of occultation, based on the reconstructed JWST trajectory. The actual chord observed by JWST is also shown, revealing that the star crossed the rings without being occulted by the body itself, enabling a clear detection of the ring features.
• Figure : Extended Data Figure 2: Positional discrepancy between successive JWST ephemeris solutions. The plot shows the difference in Chariklo’s predicted position using two consecutive JWST ephemeris releases, both issued on 27 September 2022. The offset highlights the sensitivity of occultation predictions to small updates in spacecraft trajectory data, underscoring the challenge of planning such events from a moving observatory in L2 orbit.