JAMES WEBB REVEALS CHARIKLO’S TWO RINGS ARE CHANGING IN OPPOSITE DIRECTIONS

The inner ring surrounding the distant centaur has grown approximately 50 percent more opaque while the outer ring’s signal has weakened by approximately 60 percent, exposing a dynamic system that scientists still cannot fully explain.

The James Webb Space Telescope has detected unexpected differences within the two narrow rings surrounding Chariklo, a small icy body orbiting the Sun between Saturn and Uranus.

Chariklo’s dense inner ring appears approximately 50 percent more opaque than it did during earlier stellar occultations. Its outer ring produced a far weaker signal, indicating a decrease of approximately 60 percent between observations conducted in 2017 and 2022.

The positions of both rings remained essentially unchanged.

That combination presents astronomers with a significant mystery. The rings continue orbiting at their established distances from Chariklo, yet the amount of starlight they block has changed in opposite directions.

The findings were reported by a research team led by Pablo Santos-Sanz of the Institute of Astrophysics of Andalusia in Spain.

The study does not establish that scientists watched ring particles physically moving from one location to another. Webb recorded the rings during a single occultation in October 2022, and researchers compared those measurements with ground-based observations collected during the preceding decade.

The comparison provides strong evidence that Chariklo’s ring system has changed or contains optical properties that earlier visible-light observations could not reveal.

The inner ring’s increased opacity cannot be explained solely by the different infrared wavelengths used by Webb, according to the team’s modeling. The outer ring remains less certain because its signal was extremely weak.

The results expose a ring system far more active and complicated than its small size would suggest.

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CHARIKLO IS ONLY ABOUT 250 KILOMETERS ACROSS

Chariklo is approximately 250 kilometers, or 160 miles, across. Its volume-equivalent radius is about 124.8 kilometers, placing its diameter near 250 kilometers.

Chariklo remains the largest known member of the centaur population despite its comparatively small dimensions. Centaurs are icy minor bodies whose orbits generally exist between Jupiter and Neptune and cross or approach the orbital regions of the giant planets.

Their name reflects their mixed characteristics. Centaurs can display properties associated with asteroids while containing volatile material and sometimes developing cometary activity.

Many are thought to have originated farther from the Sun before gravitational encounters with the outer planets redirected them into dynamically unstable orbits. Their paths can change over astronomical time as they interact with the gravity of Jupiter, Saturn, Uranus, and Neptune.

Chariklo follows an orbit between Saturn and Uranus at distances extending billions of miles from Earth. It completes one journey around the Sun in approximately 63 years.

Its remote location and small size make direct study extremely difficult.

The rings are far too narrow and faint for Webb to photograph as two visibly separated bands around the body. Astronomers instead measure their shadows.

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THE FIRST RINGS DISCOVERED AROUND A SMALL SOLAR SYSTEM BODY

Chariklo’s rings were discovered during a stellar occultation on June 3, 2013.

Astronomers had predicted that Chariklo would pass in front of a distant star. Telescopes positioned across South America monitored the star as the centaur moved across the line of sight.

The researchers expected one primary decrease in brightness when Chariklo blocked the star. They recorded additional dips before and after the central occultation.

Those smaller, symmetrical interruptions revealed two narrow rings surrounding Chariklo. The discovery made Chariklo the first small Solar System body known to possess a ring system.

Before that event, confirmed rings had been associated with the four giant planets: Jupiter, Saturn, Uranus, and Neptune.

The inner ring was designated C1R. The outer ring became C2R.

C1R orbits approximately 390 kilometers, or 242 miles, from Chariklo’s center. C2R circles the body at approximately 405 kilometers, or 252 miles, from the center.

The inner ring is approximately 6 to 7 kilometers wide. The outer ring is approximately 2 to 4 kilometers wide, with a gap of roughly 9 kilometers separating the two structures.

Those dimensions are extraordinary when placed beside Chariklo itself. The rings occupy an orbital region extending far beyond the surface of a body with an average radius near 125 kilometers.

Their narrowness also creates a long-standing dynamical problem.

Particles in a ring do not automatically remain assembled within sharply defined boundaries. Collisions, gravitational disturbances, solar radiation pressure, and differences in orbital velocity can cause ring material to spread, migrate, or escape.

Chariklo’s rings have remained detectable since 2013, indicating that some process must preserve, replenish, or repeatedly reconstruct them.

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WEBB MEASURED THE RINGS THROUGH A DISTANT STAR

The Webb observation occurred on October 18, 2022.

Researchers used the telescope’s Near-Infrared Camera to monitor Gaia DR3 6873519665992128512 as Chariklo and its rings passed across the star from Webb’s perspective.

The main body of Chariklo narrowly missed blocking the star. Its rings crossed the line of sight and produced measurable decreases in brightness.

That near miss proved scientifically valuable. It allowed the team to study the rings without Chariklo’s solid body completely removing the star’s light during the critical portion of the event.

Webb observed the occultation simultaneously through two near-infrared filters centered near wavelengths of 1.5 and 3.2 microns.

The 3.2-micron measurement represented the first stellar-occultation detection conducted beyond 3 microns, a wavelength region inaccessible to comparable observations from the ground because Earth’s atmosphere absorbs much of the incoming infrared radiation.

Chariklo was moving across Webb’s line of sight at approximately 2.5 kilometers per second, or 1.5 miles per second. That is relatively slow for an occultation involving objects moving through the Solar System, giving the telescope time to collect a highly detailed brightness profile.

The observation produced a light curve recording how much of the star’s radiation reached Webb from moment to moment.

Each change in brightness contained information about the rings’ widths, opacity, boundaries, and particle distribution.

C1R produced a strong signal with distinct diffraction features at its entrance and exit. Those features indicate that the inner ring possesses sharp edges and remains tightly confined, bearing a structural resemblance to some of the narrow rings surrounding Uranus.

C2R nearly disappeared from the data.

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THE INNER RING HAS BECOME MORE OPAQUE

Researchers calculated that observations recorded before Webb gave C1R an average normal opacity of approximately 0.303. Webb measured an average near 0.431.

The difference was statistically significant and represented an increase approaching 50 percent. C1R blocked a greater proportion of the background star’s light than it had during the earlier occultations.

Opacity does not provide a simple particle count.

A ring can become more opaque if it gains material. It can also block more light if collisions break large particles into numerous smaller grains, expanding the combined cross-sectional area facing the observer without requiring an equal increase in total mass.

Imagine one large block of material breaking into thousands of grains. The total mass may remain almost identical, yet the collection can intercept more light because a greater amount of surface area has been distributed across the ring.

Collisional fragmentation inside C1R could therefore explain part of the measured increase.

An outside source could also be replenishing the ring. Material may be entering C1R through impacts, the breakup of a larger object, interactions with an undiscovered moonlet, or a continuing process that has not yet been identified.

The team tested whether Webb’s infrared wavelengths alone could account for the increase.

Its radiative-transfer models examined mixtures involving water ice, silicates, carbon, and particles of different sizes. Earlier observations could be approximated by certain mixtures of small silicate particles and water-ice grains.

No tested combination successfully reproduced the complete set of earlier and Webb measurements.

The researchers concluded that wavelength dependence alone cannot explain C1R’s stronger opacity. The inner ring appears to have undergone a real change in structure, particle cross-section, or material content.

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THE OUTER RING IS FADING OR BECOMING TRANSPARENT IN INFRARED LIGHT

C2R presented the opposite result.

The outer ring was detected only marginally in Webb’s 1.5-micron data and was not detected in the simultaneous 3.2-micron channel. Comparison with the 2017 ground-based occultation indicated that its equivalent width had fallen by approximately 60 percent by 2022.

Equivalent width combines a ring’s opacity and radial width into a measurement related to the effective cross-section of the material blocking the star.

A lower value can indicate that the ring contains less material, has become narrower, contains particles that interact differently with the observed wavelength, or has developed a different distribution along its orbit.

One possibility is that C2R is losing material and may be a temporary structure rather than a permanent ring. Particle collisions can grind material into finer grains. Solar radiation pressure can then alter the orbits of those particles, raising their eccentricity and moving them away from the narrow ring.

Some of the displaced material could eventually cross the orbit of C1R.

That does not mean C2R supplied enough matter to produce C1R’s full increase. The researchers determined that the change measured in the inner ring was approximately ten times greater than the loss measured in the outer ring.

A simple transfer of the missing outer-ring material into the inner ring cannot balance the observations.

C2R may also contain submicron particles whose ability to scatter or absorb light decreases at Webb’s longer infrared wavelengths. Under that explanation, at least part of the apparent fading would be an optical effect rather than a physical loss of material.

The current measurements cannot fully separate those possibilities.

A future visible-light occultation could establish whether C2R has weakened across wavelengths or appears faint only in the infrared.

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WEBB PROBABLY DID NOT SAMPLE TWO RANDOM ABNORMAL SECTIONS

A ring does not need to possess the same density along its full circumference. It can contain arcs, gaps, clumps, dense segments, and regions with fewer particles. Webb could theoretically have observed an unusually dense portion of C1R and an unusually sparse portion of C2R.

The research team tested that possibility through numerical modeling. Its calculations indicated that the probability of Webb randomly encountering the measured high-density portions of C1R during both ring crossings was approximately 0.004.

For C2R, the probability of randomly sampling two unusually low-density regions was estimated at no greater than 0.002.

Those probabilities do not make localized variations impossible. They show that ring fragmentation alone is unlikely to explain the entire result.

The measurements instead favor some combination of real temporal evolution, material redistribution, particle fragmentation, replenishment, depletion, and wavelength-dependent behavior.

The three explanations are not mutually exclusive. Chariklo’s rings could be evolving physically while also containing different particle populations that interact differently with visible and infrared radiation.

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SOMETHING MUST BE KEEPING THE RINGS TOGETHER

The survival of C2R creates another puzzle.

Based upon its narrow dimensions and current measurements, the researchers calculated a median dynamical spreading timescale of only 0.34 years. The estimated range extended from approximately 0.21 to 0.96 years.

That calculation does not mean the ring must disappear on a precise date. It demonstrates that an unconfined ring with those properties should spread rapidly on astronomical timescales.

C2R has been observed across nearly a decade without a significant shift in its radial location.

Some stabilizing or replenishing process must therefore be operating, or current models are missing part of the system’s dynamics.

One proposed explanation involves a small shepherd moon.

Shepherd moons use gravity to influence nearby ring particles and maintain narrow boundaries. Small satellites perform that function within the ring systems of the giant planets.

A moonlet near Chariklo’s rings could confine the material while also supplying fresh particles through impacts or surface shedding.

No such satellite was detected during the Webb occultation. The absence of a detection does not rule out an object too small, dark, or poorly positioned to register in the available data.

The Webb observation found no additional brightness dips indicating another ring, cloud, or orbiting satellite along the specific path sampled by the star.

The ring particles themselves may also occupy size ranges that remain more stable against solar radiation pressure due to Chariklo’s highly tilted ring geometry.

Scientists do not yet possess enough information to determine whether a moonlet, resonant interaction, particle-size effect, repeated replenishment, or several mechanisms are maintaining the system.

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AN OBSERVATION REQUIRING EXTREME PRECISION

The Chariklo campaign was a major technical achievement in addition to its scientific value.

A stellar occultation may last only seconds. The target, star, telescope, and observer must align with extreme precision at the correct moment. Webb does not orbit Earth.

It operates around the Sun-Earth L2 region approximately 1.5 million kilometers, or 930,000 miles, beyond Earth on the side opposite the Sun. The observatory follows a halo orbit around that region and performs periodic station-keeping maneuvers.

Researchers had to calculate Chariklo’s path, the position and motion of the background star, and Webb’s changing location in space.

The Gaia mission supplied the high-precision stellar measurements required to identify the star’s position and predict the alignment.

Updates to Webb’s trajectory could shift the projected occultation path. The team refined its calculations as newer spacecraft-position information became available, including updates shortly before the event.

This was the first stellar occultation specifically predicted and successfully observed through a planned Webb campaign. The star passed behind the rings but missed Chariklo’s body almost exactly as the final calculations had projected.

The success demonstrated that Webb can be used for precision occultation science involving extremely small and distant Solar System structures that the telescope cannot resolve through direct imaging.

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CHARIKLO’S RINGS MAY NOT BE PERMANENT

Ring systems are often imagined as stable structures surviving unchanged for immense periods.

Chariklo challenges that picture.

If C2R is truly losing material on a timescale measured in years, the outer ring may be transient. Astronomers could be observing it during a limited period between its formation and dispersal. Its existence would then raise another question: why did humanity discover it during this particular stage?

Temporary rings could be more common around small outer Solar System bodies than present observations indicate. They would remain difficult to detect because successful occultations require rare geometric alignments, precise predictions, and instruments positioned within a narrow shadow path.

Other small bodies have already complicated the traditional understanding of rings.

Evidence of ring structures or surrounding material has been reported around the centaur Chiron. Rings have also been discovered around the dwarf planet Haumea and the trans-Neptunian object Quaoar.

Quaoar’s rings orbit beyond the classical Roche limit, where standard expectations suggested that material should assemble into a moon rather than remain distributed as a ring.

Chariklo presents a different problem: two narrow rings occupying stable radial locations while their measured opacities appear to evolve in opposite directions.

These systems show that small-body rings cannot be understood solely by reducing the models created for Saturn, Uranus, Jupiter, and Neptune.

Their low gravity, small particle populations, exposure to solar radiation, potential moonlets, collision histories, and unstable outer Solar System environments create a separate class of dynamical questions.

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THE NEXT OCCULTATION COULD SETTLE THE CENTRAL QUESTION

The most valuable follow-up would be another well-resolved stellar occultation observed in visible light.

If a future visible-light measurement confirms that C1R remains more opaque and C2R remains weaker, the case for physical evolution will become substantially stronger.

If C2R appears much stronger in visible light than it did in Webb’s infrared data, particle size and composition may account for a greater portion of the discrepancy.

Repeated observations could also determine whether the measured changes are continuing, reversing, or fluctuating.

Scientists could then test whether C1R is being continually replenished, whether collisions are producing new fine particles, whether C2R is actively dispersing, and whether an unseen moonlet is influencing both structures.

Webb’s observation provides one decisive point in time. Establishing the rings’ behavior requires a continuing record.

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TRJ VERDICT

The James Webb Space Telescope did not directly photograph Chariklo’s rings transforming.

It accomplished something more precise.

Webb measured how the rings blocked a distant star in two infrared wavelength bands and allowed researchers to compare their structure with occultations recorded between 2013 and 2017.

The dense inner ring, C1R, showed an opacity increase approaching 50 percent. The outer ring, C2R, produced a signal approximately 60 percent weaker than the 2017 measurement. Both rings remained at essentially the same orbital locations.

The increased opacity of C1R cannot be explained solely by wavelength differences. Its material content, particle distribution, or effective cross-section appears to have changed.

C2R may be losing material, dispersing, or containing grains that become far less effective at blocking longer-wavelength infrared light. The present evidence cannot fully distinguish those explanations.

The missing material from C2R is also insufficient to explain the far greater change measured in C1R. One ring cannot be treated as simply feeding the other.

Chariklo’s two rings are responding in opposite directions while retaining their narrow orbital positions. That requires confinement, replenishment, particle restructuring, or physical processes not yet fully identified.

The mystery is no longer whether a body only 250 kilometers across can possess rings. That question was answered in 2013.

The new question is how those rings continue to survive while changing before astronomers’ instruments.

Webb has shown that Chariklo is not carrying a frozen relic through the outer Solar System.

It is carrying an active and evolving ring system.

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Pablo Santos-Sanz et al., “JWST Occultation Reveals Unforeseen Complexity in Chariklo’s Ring System,” arXiv:2510.06366v2, December 17, 2025. (Free Download)

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Supplementary Materials Credit: Pablo Santos-Sanz et al., “JWST Stellar Occultation Reveals Unexpected Changes in Chariklo’s Ring System,” Science Advances, Vol. 12, eaeh4794 (2026). (Free Download)

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