In 2013, astronomers discovered two rings around 250-km-wide Chariklo; nine years later, James Webb found the inner ring more opaque and the outer ring weaker

Credit: L. Maquet, Observatoire de Paris

In 2013, astronomers discovered two narrow rings around Chariklo, a 250-kilometre-wide minor planet orbiting between Saturn and Uranus, marking the first time rings had been found around such a small body. Nine years later, the James Webb Space Telescope observed the same system during a rare stellar occultation and found that the inner ring had become noticeably more opaque while the outer ring appeared weaker and more transparent. This opposite evolution suggests that Chariklo’s rings are far more dynamic than previously thought, changing on timescales of just a few years rather than remaining stable over centuries, according to a study led by the Institute of Astrophysics of Andalusia and reported by EurekAlert.

A tiny world with giant-planet features

Chariklo, formally known as 10199 Chariklo, is classified as a centaur, a type of icy minor planet that orbits in the outer Solar System between the giant planets. With a diameter of only about 250 kilometres, it is tiny compared to Saturn or Uranus, yet it hosts a ring system that rivals those worlds in complexity. The two rings, designated C1R and C2R, are extremely narrow and lie close to the body, at distances of roughly 390 and 405 kilometres from Chariklo’s centre.Before 2013, ring systems were considered a feature reserved for the four giant planets: Jupiter, Saturn, Uranus and Neptune. The discovery that a small, distant object like Chariklo could sustain dense rings upended that assumption and raised new questions about how such delicate structures form and survive. Were they the result of a recent collision, the breakup of a small moon, or material lifted from Chariklo’s surface by some unknown process? And how could they remain stable in an environment where gravitational tugs from nearby giants and passing objects might be expected to tear them apart?

How astronomers study invisible rings

Chariklo’s rings are too narrow and too far away to be photographed directly, even by the James Webb Space Telescope or the largest ground-based observatories. Instead, they rely on a technique called stellar occultation. When Chariklo passes in front of a background star, the starlight is briefly blocked by the body and its rings as they cross in front of the star. Using fast and precise instruments to measure these tiny drops in brightness, scientists can deduce the width, spacing and opacity of the rings.The 2022 observation marked a major milestone. It was the first stellar occultation specifically predicted and planned for the James Webb Space Telescope. Achieving this required extraordinarily precise knowledge of Chariklo’s orbit, the position of the target star, thanks to data from the European Space Agency’s Gaia mission, and the exact trajectory of JWST as it orbits the L2 Lagrange point about 1.5 million kilometres beyond Earth. At the time of the occultation, Chariklo was moving relative to JWST at just 2.5 kilometres per second, giving the team unprecedented spatial resolution for probing the ring structure.

Opposite changes in the two rings

When the team compared the 2022 JWST data with earlier occultation measurements from the past decade, they found that the two rings were evolving in opposite directions. The inner ring, C1R, was much more opaque, blocking more starlight and probably containing more material or more tightly packed particles than previously. The outer ring, C2R, displayed lower opacity, suggesting it had become more tenuous or less dense over the same period.These changes occurred over less than a decade, a very short time in planetary terms. Until now, scientists had generally assumed that rings around small bodies were relatively stable, persisting with little change over long periods. Chariklo’s behaviour challenges that view and implies that these systems can be surprisingly active, responding to internal dynamics or external influences on human-observable timescales.

What might be driving the changes

The physical origin of the detected changes remains an open question. One possibility is that the rings are undergoing genuine temporal evolution. Collisions between ring particles, gravitational interactions with invisible shepherd moons, or perturbations from Chariklo ’s irregular shape could redistribute material between the two rings or within each ring. Over time, such processes could make one ring denser and another more diffuse.Another possible reason may be the different filters and observation set-ups used in the various occultation campaigns . The JWST observations were taken in specific infrared filters, while the previous ground-based campaigns were in visible or near-infrared wavelengths. If the particles that make up the rings are of different sizes, then the rings can appear to have different opacities when viewed with different wavelengths of light. The study authors note that the observed changes could reflect a combination of real evolution and observational differences, and they call for further monitoring to disentangle these effects.

A new window on ring dynamics

The ability to detect changes in Chariklo’s rings over just a few years opens a new window for understanding how ring systems evolve. If small bodies can host dynamic rings that shift on short timescales, similar behaviour might be found in other minor-planet systems yet to be discovered. This could reshape models of how rings form, how long they last and what mechanisms keep them confined and stable in the face of disruptive forces.For planetary scientists, Chariklo has become a natural laboratory. Its rings are close enough to the central body and narrow enough that subtle changes can be measured with current technology, yet distant enough that they exist in a different gravitational and collisional environment than the rings of Saturn or Uranus. By watching how C1R and C2R change over time, researchers can test theories about particle collisions, wave propagation, confinement by small moons and the role of radiation and micrometeoroid impacts in shaping ring architecture.

A technical and collaborative achievement

The studytitled ‘JWST stellar occultation reveals unexpected changes in Chariklo’s ring system’, published in Science Advances, was led by researchers at the Institute of Astrophysics of Andalusia, part of the Spanish National Research Council. The team designed the project, predicted the occultation, coordinated the JWST observation, analysed the data and interpreted the results. Collaborators from Spain, Brazil, France, Hungary and the United States contributed to ring modelling, statistical analysis and the broader scientific context.Achieving the observation required more than just telescope time. It depended on precise orbital mechanics, accurate star positions from Gaia, and careful planning of JWST’s movements around L2, including station-keeping maneuvers that keep the telescope in its intended orbit. The success of the campaign shows how space-based observatories, in combination with ground-based networks and high-precision astrometry, can probe fine details of distant, faint systems that would otherwise be invisible.

Why Chariklo matters beyond rings

Chariklo’s story is part of a larger shift in how astronomers view the outer Solar System. Once seen as a relatively quiet region populated by static, unchanging bodies, the trans-Saturnian zone is now recognised as dynamic and complex. Centaurs like Chariklo migrate over time, sometimes evolving into short-period comets, sometimes being ejected from the Solar System entirely. Their surfaces can be altered by impacts, outgassing and radiation, and now their ring systems appear capable of changing on observable timescales.As telescopes and techniques improve, more ringed minor planets may be discovered, and existing ones may be monitored more closely. Each new system will add data points to a growing picture of how common rings are, how long they survive and what conditions allow them to persist. Chariklo, once a curiosity, has become a benchmark object for this emerging field.

Looking ahead

The authors emphasise that their results force a rethink of how small-body ring systems form, evolve and maintain stability. Future occultations, ideally with JWST and large ground-based facilities working together, could track further changes in Chariklo’s rings and search for similar behaviour in other objects. Over time, these observations may reveal whether the opposite trends seen in C1R and C2R are part of a cyclical process, a one-time redistribution of material or something more complex.For now, Chariklo stands as a reminder that even in a Solar System that has been studied for centuries, surprises remain. A 250-kilometre-wide world, billions of kilometres from Earth, is hosting a pair of rings that are changing before our eyes, inviting astronomers to watch, measure and refine their understanding of how such delicate structures can exist and evolve in the cold darkness beyond the giant planets.