JWST finds Chariklo’s inner ring growing denser while its outer ring fades

Chariklo’s inner ring is getting denser. Its outer ring is fading. Both changes are happening at the same time — and, for the moment, nobody knows why.

A team of astronomers led by Spain’s Andalusian Institute of Astrophysics has compared the first-ever planned James Webb Space Telescope observation of a small solar system body with a decade of ground-based data, and found that Chariklo’s two narrow rings are not the static structures that models had assumed. The inner ring, known as C1R, showed a significantly stronger signal in 2022 than in any previous measurement. The outer ring, C2R, was measurably weaker. The opposing changes appear simultaneous, which no existing theory of small-body ring dynamics had predicted.

The finding matters beyond Chariklo itself. Ring systems around planets had already shown that rings can evolve — Saturn’s F ring reshapes visibly on timescales of months — but rings around small bodies were considered too weakly perturbed, too gravitationally quiet, to change on human timescales. Chariklo’s rings have now shown otherwise.

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How astronomers measured rings the width of a city block

Chariklo is a centaur: a small solar system body about 125 kilometres across whose orbit threads between Saturn and Uranus, at a distance from Earth that ranges between roughly 13 and 18 astronomical units. Its rings span a width of no more than a few kilometres each — far narrower than any structure a telescope could image directly.

The only way to measure them is through stellar occultation: watching Chariklo pass in front of a background star and timing the exact moments the star’s light dips. Each dip, one on each side of Chariklo, marks a ring. The depth of the dip encodes opacity; its duration encodes ring width. The technique demands pinpoint knowledge of both Chariklo’s position and the star’s, which the European Space Agency’s Gaia mission now provides at milliarcsecond precision.

On October 18, 2022, the JWST pointed its NIRSpec instrument at a star in Chariklo’s predicted path. This was the first stellar occultation deliberately planned and executed using the telescope — an operation that required tracking Chariklo’s orbit with the accuracy of ESA’s Gaia catalogue and commanding JWST to maintain sub-arcsecond pointing as both the telescope and the target moved. The observation achieved spatial resolution on Chariklo’s ring plane of approximately one kilometre: finer than any previous measurement of a small-body ring from Earth.

The team then compared this 2022 JWST profile against the best available ground-based occultation data spanning roughly a decade, taken from observatories across South America, Europe, and Africa.

One ring grows denser. The other fades. At the same time.

Chariklo’s rings were first detected in 2013, also through stellar occultation, by a ground-based network in South America. At the time of discovery they appeared as two narrow, distinct features: C1R, the inner and denser ring, and C2R, the outer and more tenuous one. Their mere existence was a surprise — no object of Chariklo’s size was known to host rings, and their origin remained debated.

Over the following decade, repeated stellar occultations tracked the system but found nothing obviously amiss. The rings appeared stable.

The JWST data changed that picture. Compared to the ground-based baseline, C1R had grown more opaque: starlight passing through it dimmed more than before. C2R, meanwhile, had dimmed less — the ring had become more transparent, or perhaps thinner, or both. The changes are opposite in direction and appear to have occurred over the same period of roughly ten years.

For scale: the change in C1R’s optical depth is detectable against the noise floor of JWST’s exquisite photometry, but the absolute difference in ring mass remains unconstrained. No telescope can yet determine whether C1R is accumulating particles, compressing its existing material, or changing its grain properties. The difference in signal is real; its physical cause is an open question.

What the models expected — and why it does not settle

Three explanations are currently in play, and the research team is careful to present all three without endorsing any.

The first is genuine temporal evolution: material redistributed between the rings, perhaps driven by a collision event, a resonance with one of Chariklo’s moons, or the slow gravitational sculpting of the centaur’s irregular shape. If this is the answer, Chariklo’s rings are demonstrably short-lived on cosmic timescales, which raises a harder question: if rings around small bodies come and go, why did we happen to find them?

The second is an observational effect: the two JWST and ground-based datasets used slightly different wavelength filters, and the rings’ optical properties may vary with wavelength. If the dust grains have a particular size distribution, they could scatter differently in JWST’s near-infrared band than in the visible-light bands used from the ground, producing an apparent change that reflects grain properties rather than ring mass.

The third is a combination of both: real ring evolution overlaid on a wavelength-dependent signal, in proportions the current data cannot resolve.

The paper’s lead author, Pablo Santos-Sanz of the Andalusian Institute of Astrophysics, notes that the physical origin of the changes remains undetermined. The team describes this not as a failure of the observation — the JWST data are the highest-resolution measurement of Chariklo’s rings ever obtained — but as the limit of what a single stellar occultation can determine. Ring width, opacity, and compositional properties would need to be disentangled by a dedicated observational campaign, ideally with multiple JWST occultations at different wavelengths over several years.

The limitation matters: the three explanations have very different implications. If rings around small bodies are genuinely transient, the number of centaurs that currently host undetected rings should be substantially higher than the one or two confirmed so far — and surveys are looking in the wrong timescale. If it is a wavelength effect, the JWST dataset is not the final word on whether Chariklo’s rings actually changed at all.

What Chariklo’s rings reveal about small-body ring systems

Before 2013, no rings had been found around any object smaller than a giant planet. Chariklo’s discovery changed the picture: centaurs, trans-Neptunian objects, even some asteroids might host ring systems too faint or too narrow to detect except through the right stellar occultation. Subsequent searches have found candidate ring features around a handful of other small bodies, though none as clearly resolved as Chariklo’s.

The theoretical question is formation. Ring systems around giant planets are maintained by the gravity of nearby moons that confine particles into narrow bands and replenish material eroded by collisions and radiation pressure. Chariklo has no confirmed large moon capable of performing that function. Two scenarios dominate: the rings formed from a collision that left debris in orbit, or they are the remnants of a tidally disrupted object captured at some point in Chariklo’s history. Neither scenario can yet be confirmed or excluded.

If Chariklo’s rings are genuinely evolving — not just appearing to because of wavelength differences — they may be on their way out. The timescale would depend on the mass involved. A ring system of modest mass would dissipate on timescales of thousands to millions of years: long by human standards, but essentially instantaneous in the five-billion-year history of the solar system. That would make their discovery a fortunate accident.

Common questions about Chariklo’s rings

What is Chariklo, and where is it in the solar system? Chariklo is a centaur — a class of small solar system bodies that orbit between the giant planets, in this case between Saturn and Uranus. With a radius of about 125 kilometres, it is the largest known centaur. At its current distance, it takes roughly 63 years to complete one orbit of the Sun.

How were Chariklo’s rings discovered in the first place? They were found in June 2013 when ground-based observatories across South America watched Chariklo pass in front of a background star. Instead of a single dip in the star’s brightness, telescopes recorded five dips — one main one as Chariklo itself blocked the star, and two smaller ones on either side, one for each ring. The discovery was announced in Nature in 2014.

Is the change in Chariklo’s rings definitely real? The JWST data shows a clear difference in ring opacity compared to earlier measurements. Whether this reflects actual changes in ring mass and structure, or partly reflects the different wavelengths at which JWST and the ground-based telescopes observed, has not been determined. The paper presents both possibilities explicitly and recommends further JWST observations to separate them.

Could Chariklo’s rings disappear? If the changes reflect genuine mass redistribution or loss, and if the process continues at a comparable rate, the rings could weaken significantly on timescales of centuries to millennia. No prediction of ring lifetime is currently possible from the available data. What the current data does confirm is that Chariklo’s ring system is not locked in a stable equilibrium.

Are there other ring systems around small solar system bodies? A small number of candidate ring features have been detected around other trans-Neptunian objects and centaurs through stellar occultation, but Chariklo’s remain the most clearly characterised. Quaoar, a trans-Neptunian object, was confirmed to carry a ring in 2023, and Chiron — another centaur — has shown features consistent with ring material in multiple occultation datasets.

Next confirmed step: the research team has identified future stellar occultations by Chariklo for which JWST pointings can be planned, with at least one candidate event in 2027. Observations at multiple near-infrared wavelengths would test whether the opacity difference is grain-size-dependent, and a multi-epoch campaign could determine whether C1R’s density continues to increase or has stabilised. The answers will determine whether Chariklo’s rings are a snapshot of ring formation in progress or a system in the early stages of dissolution.

Reference: Santos-Sanz et al., “JWST stellar occultation reveals unexpected changes in Chariklo’s ring system,” Science Advances, 2026. DOI: 10.1126/sciadv.aeh4794

Tags: astronomy, JWST, Science Advances, centaur, Chariklo, planetary rings

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