The two rings around the distant Centaur Chariklo no longer look the way they did in earlier observations. In a 2022 observation with the James Webb Space Telescope, the dense inner ring appeared substantially more opaque than before, while the outer ring produced a surprisingly weak signal. The contrasting changes suggest that Chariklo’s ring system is not as static as earlier observations had indicated.
Chariklo is a small body in the outer Solar System surrounded by two sharply confined rings. Known as C1R and C2R, the rings orbit roughly 390 and 405 kilometers from Chariklo’s center, respectively. Their origin, composition and particle-size distribution have remained poorly constrained despite more than a decade of observations.
The new observation came on Oct. 18, 2022, when JWST watched Chariklo pass in front of a background star. This technique, called a stellar occultation, uses the brief changes in starlight as an object or its surrounding material crosses the star’s line of sight. The pattern of fading can reveal the structure of otherwise difficult-to-observe rings.
From JWST’s viewpoint, the stellar path came within about 7.4 kilometers of Chariklo’s surface while still crossing the ring system. The reconstructed closest approach to Chariklo’s center was 132.2 kilometers, slightly outside the body’s volume-equivalent radius of 124.8 kilometers, so Chariklo itself did not block the star. No additional brightness dips above the 2σ photometric noise level were detected along the path, providing no evidence for additional ring material or satellites along that chord.
JWST observed the event simultaneously at two near-infrared wavelengths, centered at 1.5 and 3.2 micrometers. The observation was notable because stellar occultations of small-body rings at wavelengths beyond 3 micrometers had not previously been obtained.
The inner ring is substantially more opaque
The inner ring, C1R, produced a strong and clearly defined signal. Its light curve contained pronounced diffraction spikes at ingress and egress, features associated with its sharply defined edges and strong confinement.
But its opacity was different from what earlier occultations had measured.
Before the JWST observation, measurements gave C1R an average normal opacity of 0.303 ± 0.028. JWST measured 0.431 ± 0.012. The difference corresponds to a z score of 4.2 and a probability of about 3 × 10⁻⁵ under the comparison used by the authors.
The JWST measurements also gave normal opacities of 0.425 ± 0.008 and 0.417 ± 0.013 for C1R in the first and second contacts at 1.5 micrometers, and 0.438 ± 0.017 and 0.444 ± 0.014 at 3.2 micrometers. Its equivalent widths were about 3 kilometers in the 1.5-micrometer observations and just over 3 kilometers at 3.2 micrometers.
Equivalent width is an effective measure that combines a ring’s opacity and radial width. In an occultation, it provides a way to characterize how much ring material is intercepted along the observed path. The authors emphasize that these values describe the effective average properties along the sampled chord rather than the detailed local structure of the ring.
The outer ring moves in the opposite direction
C2R behaved very differently.
The outer ring was only marginally detected at 1.5 micrometers and was not detected at 3.2 micrometers, even though the two JWST observations were simultaneous and sampled the same segment of the ring. Compared with earlier visible-light occultations, the result points to particularly low opacity in C2R.
The difference is also visible when the new measurements are placed alongside observations made between 2013 and 2017. The 2022 JWST equivalent-width measurements for C2R were about 60% below the 2017 measurements. C1R, meanwhile, showed the opposite behavior, with an increase of about 50%.
The change was especially striking because the two rings did not simply appear to exchange the same amount of material. The change in C1R’s equivalent width was about 10 times larger than the change measured for C2R. The authors therefore conclude that material missing from C2R cannot by itself explain the increase seen in C1R.
That leaves several possible explanations.
Several explanations remain possible
One possibility is that the rings are not uniform around Chariklo. If different parts of a ring contain different amounts of material, JWST could simply have sampled an unusually dense section of C1R and an unusually sparse section of C2R.
The researchers tested that possibility using models in which ring opacity or equivalent width varies periodically with longitude. For C2R, only about 4 × 10⁻⁴ to 7 × 10⁻⁴ of the simulated cases reproduced a sequence of low equivalent-width measurements like the observed one. For C1R, the probability of obtaining opacities as high as those measured by JWST was about 10⁻³ in the 1.5-micrometer band and about 4 × 10⁻⁵ in the 3.2-micrometer band, even for a single measurement. Requiring both contacts from the same JWST occultation made the probabilities still smaller.
The results therefore indicate that differences in the sampled longitude alone are unlikely to explain the changes.
Another possibility is that the amount of material, or the effective cross section of the particles, has changed. The authors note that an increase in C1R’s effective cross section could, for example, result from collisions among larger particles that produce a cascade of smaller fragments.
For C2R, the fading signal could indicate depletion. But the available observations cannot yet establish whether the ring is steadily losing material or underwent a relatively sudden change between 2017 and 2022. The sparse earlier measurements also cannot rule out the possibility that depletion began before 2017.
The infrared wavelengths add another complication
The contrast between old visible-light measurements and the new JWST observations also raises the possibility that the ring particles behave differently at different wavelengths.
The researchers modeled how mixtures of materials, including water ice, olivine and carbon, and different particle sizes would affect the observed extinction. For C1R, some of the older measurements could be reproduced with mixtures containing micrometer-sized water ice and submicrometer silicate grains. Other combinations, including pure silicate models with different grain sizes, could produce comparable fits, so the solution was not unique.
But adding the JWST measurements changed the picture. No combination of materials and grain sizes tested could reproduce all of the measurements simultaneously, particularly those from JWST’s F150W2 and F322W2 filters together with the earlier VLT/Ks observations. The best-fitting models were mainly composed of silicates with grain sizes between 1 and 10 micrometers, but the overall fits remained poor.
That result argues against wavelength-dependent scattering alone as the explanation for C1R’s change after 2017. The authors instead conclude that its opacity appears to have genuinely increased.
C2R is harder to assess this way because there are few well-resolved occultation profiles. If the outer ring really is undergoing depletion, however, the authors say that would be consistent with a population dominated by small grains. Submicrometer particles are more vulnerable to processes that can disperse them.
The rings have stayed in place despite the changes
The changing opacity presents another problem. If the rings contain sufficiently small particles, radiation pressure could remove micrometer-sized grains on relatively short timescales, potentially distorting or dispersing the ring structure.
Yet occultations covering nearly a decade show no substantial changes in the radial positions of either C1R or C2R. That stability suggests that some mechanism must be helping to stabilize or replenish the rings.
For C1R, one proposed source of confinement is a strong 1:3 spin-orbit resonance with Chariklo. In this configuration, a ring particle completes one orbit for every three rotations of Chariklo. The same resonance can also drive outward migration, however, potentially leading eventually to dispersal.
A small external shepherd satellite has therefore been proposed as a way to prevent that drift. If such a moonlet exists, the authors suggest it could share an orbit with C2R and potentially supply material to the rings.
The JWST data also provide an estimate for how quickly material released locally in C2R would spread around the ring through Keplerian shear. The median azimuthal spreading timescale is 0.34 years, with a 1σ range of 0.21 to 0.96 years. The authors suggest that a co-orbiting shepherd satellite could potentially both help maintain sharp ring edges and continuously supply fresh material.
These mechanisms remain proposed explanations, not established causes of the changes observed by JWST.
C2R may be a short-lived structure
The researchers also consider the possibility that Chariklo’s outer ring is transient.
Their analysis notes that solar radiation pressure can affect the stability of particles in highly tilted rings such as Chariklo’s. Depending on particle size and composition, some size ranges may remain stable while others are more vulnerable to loss. When Chariklo’s triaxial shape is included, the stability limits change further.
If C2R’s apparent decline represents an ongoing depletion process taking place over only a few years, the authors say it could indicate a transient structure. They also note that episodic activity and temporarily bound structures have been suggested around the Centaur Chiron.
But the JWST observation is only one epoch. It cannot by itself distinguish between continuing depletion and a change that occurred relatively abruptly sometime between earlier observations and 2022.
A future stellar occultation observed at visible wavelengths could provide a direct test of the trends. Such an observation could help separate genuine changes over time from differences caused by wavelength-dependent scattering.
For now, the two rings present a contrasting picture: C1R is significantly more opaque than it was in previous measurements, while C2R has become much harder to detect in the near infrared. The observations establish that the properties measured along Chariklo’s rings have changed, but whether those changes primarily reflect evolving ring material, wavelength-dependent opacity, or a combination of both remains unresolved.
The study was published in Science Advances.
