TL;DR: James Webb Space Telescope observations from Oct. 18, 2022, published Sept. 9, 2026, show Chariklo's inner ring grew much more opaque while its outer ring faded.

Webb catches a tiny world’s rings changing course

A team led by the Institute of Astrophysics of Andalusia (IAA-CSIC) used the James Webb Space Telescope to record a stellar occultation by the Centaur object Chariklo on Oct. 18, 2022, according to the EurekAlert! release titled “James Webb Space Telescope discovers that Chariklo’s invisible rings are changing.”

The study, published in Science Advances on Sept. 9, 2026, compared the Webb near-infrared light curves with stellar-occultation measurements collected from the ground over the previous decade, that release reported.

The comparison revealed opposite behavior in the two narrow rings, designated C1R and C2R. The dense inner ring, C1R, displayed significantly higher opacity than during earlier occultations, while the outer ring, C2R, displayed a much weaker near-infrared occultation signature than previously observed, according to the Science Advances paper titled “JWST stellar occultation reveals unexpected changes in Chariklo’s ring system.”

Lead researcher Pablo Santos-Sanz, an IAA-CSIC researcher, said the team found opposite changes after comparing Webb data with other occultations from the last decade, with higher opacity in the inner ring and lower opacity in the outer ring, according to that EurekAlert! release.

The result is the first demonstration that the ring system around this small outer Solar System body has changed on timescales of only a few years, that release stated.

A 250-kilometer Centaur between Saturn and Uranus

Chariklo is barely 250 kilometers in diameter and orbits between Saturn and Uranus at nearly 17 times the Earth-Sun distance, according to the EurekAlert! release.

Chariklo belongs to the Centaurs, asteroidal bodies whose orbits lie between those of the giant planets, according to the Observatoire de Paris report titled “The ‘pocket rings’ of the small body Chariklo are evolving.”

That report placed Chariklo about two billion kilometers from Earth and gave its radius as about 125 kilometers, describing it as the largest known Centaur.

Chariklo was discovered in 1997 and was the first of this family to be discovered, while its rings were discovered in 2013, according to the Sky & Telescope article titled “Webb Telescope Discovers Surprising Changes in an Asteroid’s Rings.”

Until just over a decade ago, ring systems were thought to be exclusive to the giant planets Jupiter, Saturn, Uranus and Neptune, according to the EurekAlert! release. In 2013, Chariklo joined that group when astronomers discovered two dense rings around it during a ground-based stellar occultation.

The rings, C1R and C2R, orbit at 390 and 405 kilometers from Chariklo’s center, respectively, according to that Science Advances paper. The Observatoire de Paris report described both rings as only a few kilometers wide.

Chariklo was the first small Solar System body found to host a ring system composed of two sharply confined components, according to that Science Advances paper. Rings have since been found around four other minor planets, according to the Sky & Telescope article.

How Webb watched a star blink behind the rings

The team studied the rings through a stellar occultation, a technique that measures the decrease in a star’s light when an object passes in front of it, according to the EurekAlert! release.

The rings are so narrow, and Chariklo is so far away, that they cannot be photographed directly, even with Webb or the largest ground-based telescopes, that release stated. Stellar occultations allow astronomers to study them indirectly by measuring brief dips in a star’s brightness as each ring passes in front of it.

The Oct. 18, 2022 event was the first stellar occultation specifically predicted and planned for observation with Webb and successfully observed from the space telescope, that release reported.

Achieving the observation required knowing with extraordinary precision the orbit of Chariklo, the position of the star from ESA’s Gaia mission, and the trajectory of Webb itself around the L2 Lagrange point, according to that EurekAlert! release.

Webb follows an orbit around L2, a region about 1.5 million kilometers beyond Earth away from the Sun, that requires periodic corrections through station-keeping maneuvers, according to co-author Yücel Kilic, a postdoctoral researcher at IAA-CSIC, as quoted in that release.

At the time of the occultation, Chariklo was moving relative to Webb at just 2.5 kilometers per second, a low relative speed that provided unprecedented spatial resolution for studying the rings’ structure, that release stated.

The team had originally hoped to catch an occultation of both Chariklo and its rings, but a last-minute course correction moved Webb barely out of the shadow cast by the asteroid, according to the Sky & Telescope article. The rings would still occult the star, so the team proceeded, that article reported.

A near miss that still crossed both rings

The Webb chord passed just about 7.4 kilometers from Chariklo’s surface, an exceptionally close approach that still intersected the ring system, according to that Science Advances paper.

The reconstructed geometry corresponded to a closest approach of 132.2 kilometers, slightly larger than Chariklo’s volume-equivalent radius of 124.8 kilometers, confirming that the main body did not occult the star, that paper reported.

No additional dips in brightness were detected above the 2-sigma photometric noise level in the full Webb time series, indicating the absence of additional ring material or satellites along the Webb chord, that paper reported.

The observation was conducted simultaneously in two near-infrared bands, centered at 1.5 and 3.2 microns, designated F150W2 and F322W2, according to that Science Advances paper.

That observation included the first stellar-occultation detection of a minor-body ring system at wavelengths beyond 3 microns, a regime inaccessible from the ground, that paper reported. Webb’s ability to observe in the near-infrared up to about 5 microns made it possible to study the rings at wavelengths previously inaccessible from the ground, according to the Observatoire de Paris report.

Webb was able to measure the dimming in two different wavelength bands at once, including infrared wavelengths blocked for ground-based observers by Earth’s infrared-observing atmosphere, according to the Sky & Telescope article.

Principal investigator Pablo Santos-Sanz of the Institute of Astrophysics of Andalucía, Spain, said predicting the occultation accurately enough was one challenge, adding that once Webb was in the right place at the right time its capabilities were exceptional for this kind of observation, according to that Sky & Telescope article.

Inner ring denser, outer ring fainter

A striking aspect of the results is that the positions of the two rings are essentially unchanged, but their properties appear to have been modified, according to the Observatoire de Paris report.

The inner ring, C1R, now appears about 50% more opaque than before, that report stated. Conversely, the outer ring, C2R, exhibits a much weaker signal, with opacity down by about 60% compared with 2017, that report stated.

The Science Advances paper described the same pattern in physical terms: the dense inner ring had become significantly more opaque than in previous occultations, suggesting ongoing replenishment or dynamical restructuring, while the outer ring’s weaker signature may reflect material loss, a transient nature, or wavelength-dependent opacity.

The EurekAlert! release said this unexpected behavior indicates that Chariklo’s ring system is dynamic and may involve more complex physical processes than previously thought.

Until now, scientists considered the rings around small bodies relatively stable, that release stated. The changes detected in Chariklo challenge that view and suggest these systems may be much more dynamic than previously thought.

Santos-Sanz said the results force a rethink of how the rings form, evolve and remain stable, adding that detecting these changes opens a new window for understanding the evolution of these systems and possibly other Solar System ring systems, according to that release.

Team member Heidi Hammel of AURA, who contributed discretionary time on Webb, said the idea that small bodies would have rings at all was surprising, questioning why such tiny objects would have ring systems, let alone multiple systems, according to the Sky & Telescope article. The team launched the project to learn how such rings form and evolve, that article reported.

Time evolution, lumps, or wavelength effects

What Webb measured robustly is a change in the occultation properties of the rings, particularly the opposite behavior of the two components, Santos-Sanz said, while moving from that result to a specific formation or evolutionary scenario is much more difficult, according to the Sky & Telescope article.

The physical origin of the detected changes remains an open question: they could reflect temporal evolution of the rings, differences related to the use of different filters, or a combination of both effects, according to the EurekAlert! release.

For C2R, the decrease in opacity could reflect a gradual loss of material, while the increase in opacity of C1R could result from an influx of material or from collisions that broke larger particles into many finer grains, according to the Observatoire de Paris report.

Those variations do not appear attributable simply to Webb observing different portions of the rings, but instead indicate actual evolution of the system, that report stated.

Another possibility is that the rings themselves are lumpy and uneven, rather than changing uniformly over time, according to the Sky & Telescope article. Hammel pointed to Neptune’s non-uniform rings and the early controversy over Neptune occultations, in which some observers saw them and others did not, before Voyager close-up images revealed density variations, that article reported.

Alternatively, the rings might appear differently at the infrared wavelengths Webb observes compared with ground-based visible-light observations, that article reported. Comparing infrared measurements with near-infrared and visible measurements could provide clues about composition and origins, that article added.

If the changes represent dynamic changes on short timescales, they could suggest ongoing mass transfer between the rings or an unseen shepherding moon that shapes the inner ring and contributes material to it, according to that Sky & Telescope article.

Future occultations, particularly in the visible spectrum, will make it possible to determine whether the observed changes are continuing and to establish whether they represent temporal evolution or wavelength-related effects, according to the Observatoire de Paris report.

The work was led by IAA-CSIC through scientific design, occultation prediction, data analysis and physical interpretation, with ring modeling and statistical analysis, in collaboration with researchers from Spain, Brazil, France, Hungary and the United States, according to the EurekAlert! release. Part of the French contribution was developed within the ANR Roche project and the ERC Lucky Star project on stellar occultations, according to the Observatoire de Paris report.

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