JWST occultation reveals unforeseen complexity in Chariklo's ring system

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.

💡 Research Summary

The authors present the first near‑infrared stellar occultation of the Centaur (10199) Chariklo obtained with the James Webb Space Telescope (JWST). On 18 October 2022 JWST’s NIRCam observed a background star as Chariklo’s double‑ring system passed in front of it, using the F150W2 (1.5 µm) and F322W2 (3.2 µm) filters. The spacecraft chord passed only ~7.4 km from Chariklo’s centre, sampling essentially the central cross‑section of both rings. No direct occultation of the body itself was seen, but both rings produced clear dips in the stellar flux.

The inner dense ring (C1R) is detected in both filters with pronounced Fresnel diffraction spikes at ingress and egress, indicating sharp edges and strong confinement. Quantitatively, the normal opacity (p_N) derived from the JWST data is 0.431 ± 0.012, a ∼40 % increase over the pre‑JWST average of 0.303 ± 0.028. The statistical significance is high (z = 4.2, p ≈ 3 × 10⁻⁵). In contrast, the outer ring (C2R) is only marginally detected at 1 σ in the 1.5 µm band and not at all at 3.2 µm, implying a much lower opacity than previously measured in the visible. This wavelength‑dependent discrepancy suggests either a loss of material, a dominance of sub‑micron grains whose scattering efficiency drops in the infrared, or that JWST sampled a sparser azimuthal segment of the ring.

To place the JWST results in context, the authors compiled all published stellar‑occultation measurements of Chariklo’s rings from 2013 to 2022. They derived the equivalent width (E_P), a proxy for the integrated cross‑section of ring particles, for each event. Over the 9‑year interval, C1R’s E_P increased by ~0.90 km (≈ 50 %), while C2R’s E_P decreased by ~0.094 km (≈ 60 %) between 2017 and 2022. The magnitude of the change in C1R is an order of magnitude larger than that in C2R, indicating that the material lost from the outer ring cannot simply be transferred inward; instead, the system appears to be gaining material overall or undergoing a restructuring of particle size distribution.

Three explanatory scenarios are explored: (1) azimuthal density variations—high‑density clumps could have been coincidentally sampled by JWST; statistical analysis yields low probabilities (p ≈ 0.004 for C1R, p ≲ 0.002 for C2R), making this unlikely as the sole cause; (2) genuine changes in the amount or effective cross‑section of ring material without wavelength dependence; (3) wavelength‑dependent optical properties arising from grain composition and size. Radiative‑transfer modeling with mixtures of water ice, silicates, and carbonaceous material shows that the observed increase in C1R opacity cannot be reproduced by any plausible combination of grain sizes and compositions that also fits the pre‑JWST visible‑infrared data. The best‑fit models for the JWST data require predominantly silicate grains of 1–10 µm, but the fits remain poor, reinforcing the conclusion that C1R’s opacity truly increased. For C2R, the lack of well‑resolved profiles prevents a detailed compositional analysis, yet the rapid decline in E_P and the weak infrared signal point toward a population dominated by sub‑micron particles that are efficiently removed by radiation pressure and collisional grinding.

The authors draw parallels with other low‑mass dusty rings in the Solar System—Saturn’s D, E, and F rings, Neptune’s Adams arcs, and Uranus’s λ ring—all of which exhibit decadal‑scale variations in opacity, width, or confinement driven by micrometeoroid bombardment, electromagnetic forces, or satellite interactions. By analogy, Chariklo’s outer ring may be a transient structure undergoing active dispersal, while the inner ring could be being replenished by collisional cascades or external sources.

In summary, JWST’s high‑precision, multi‑wavelength occultation demonstrates that Chariklo’s rings are dynamically evolving on timescales of a few years, with the inner ring becoming markedly more opaque and the outer ring fading. The findings highlight the importance of space‑based occultations for probing the physical and compositional properties of small‑body ring systems, and they motivate future coordinated observations across the visible to mid‑infrared to map the temporal evolution and material exchange processes within such enigmatic structures.