James Webb Space Telescope reveals changes in Chariklo’s elusive rings

When the James Webb Space Telescope turned its mirrors toward a faint, icy body drifting in the outer Solar System on October 18, 2022, astronomers were not expecting a surprise. They were expecting a triumph of prediction and precision: the first stellar occultation ever planned specifically for Webb, a fleeting alignment in which a small, distant object would pass in front of a background star and briefly block its light. What they got, in addition to the technical milestone, was a genuine cosmic puzzle. A new study published in Science Advances, led by researchers at the Institute of Astrophysics of Andalusia (IAA-CSIC), reveals that the two narrow rings encircling the centaur Chariklo are changing—and changing in opposite directions—on timescales of just a few years.

Chariklo is an object barely 250 kilometers across, orbiting the Sun at a distance of nearly 17 times the Earth-Sun separation, in the unsettled zone between Saturn and Uranus. Until 2013, ring systems were believed to be the exclusive domain of the giant planets—Jupiter, Saturn, Uranus, and Neptune—each surrounded by vast disks of orbiting debris shaped by immense gravitational fields. The discovery that a tiny, irregularly shaped world could hold onto not one but two dense, well-defined rings upended that assumption. Chariklo became the smallest known body in the Solar System with rings, and astronomers have been puzzling over how such a modest object, with only a whisper of gravity, can keep a ring system stable at all.

The rings themselves are invisible in any conventional sense. Chariklo is so small and so remote that neither Webb nor the largest telescopes on Earth can photograph the rings directly. Instead, astronomers rely on stellar occultations, an elegant technique in which the light from a distant star is monitored with millisecond precision as the target object drifts across the line of sight. When Chariklo’s solid body passes in front of the star, the starlight vanishes for a moment. But when its rings pass in front of the star, they too dim the light—briefly, in two separate dips separated by the width of Chariklo itself. Each dip is a silhouette of the ring, encoded in photons, and by measuring the depth and duration of those dips scientists can reconstruct the ring’s width, opacity, and structure with a resolution that direct imaging could never achieve.

Planning such an observation for a space telescope operating a million and a half kilometers from Earth is an extraordinary feat of celestial bookkeeping. As Yücel Kilic, a postdoctoral researcher at IAA-CSIC and co-author of the study, explains, success required knowing Chariklo’s orbit with extraordinary precision, the exact position of the background star—provided by ESA’s Gaia mission—and the trajectory of Webb itself as it loops around the L2 Lagrange point, a gravitationally quiet region beyond Earth from which the telescope observes the universe. Webb’s orbit around L2 requires periodic station-keeping maneuvers, meaning its position at any future moment must be calculated with care. At the time of the occultation, Chariklo was moving relative to Webb at just 2.5 kilometers per second—an exceptionally slow relative velocity that translated into unprecedented spatial resolution along the occultation path, allowing the telescope to sample the fine structure of the rings in remarkable detail.

The results, when compared against a decade of ground-based occultation measurements of Chariklo, delivered something unexpected. The two rings, previously assumed to be relatively steady features, have diverged. The inner ring has grown significantly more opaque—it blocks more starlight than it did in earlier observations—while the outer ring has become less opaque, letting more light through. The two rings, circling the same tiny world, are evolving in opposite directions. “By comparing JWST observations with those obtained during other stellar occultations over the last decade, we discovered opposite changes in the two rings: while the inner ring shows significantly higher opacity, the outer ring shows lower opacity,” says Pablo Santos-Sanz, the IAA-CSIC researcher who leads the study.

This finding strikes at a foundational assumption in the study of small-body ring systems. Until now, scientists considered the rings around minor planets and centaurs to be comparatively stable—delicate but enduring structures, held in place by a balance of gravity, collisions among ring particles, and possibly the gravitational shepherding of unseen small moons. The rapid, divergent evolution detected at Chariklo suggests instead that these systems are intrinsically dynamic, churning and adjusting on timescales short enough to be witnessed within a single human decade. “Our results force us to rethink how they form, how they evolve, and what mechanisms maintain their stability,” Santos-Sanz notes. “The ability to detect these changes opens a new window for understanding the evolution of these systems and, possibly, that of other ring systems in the Solar System.”

What physical processes could drive such behavior? That question remains open, and the authors are careful to acknowledge the uncertainty. The detected changes could reflect genuine temporal evolution within the rings—particles colliding, spreading, clumping, or being perturbed by gravitational resonances with Chariklo or with hypothetical shepherd satellites. Alternatively, they could arise from observational factors: different occultations were recorded through different filters, and the way ring particles interact with light depends on wavelength. The truth may be a combination of both effects, and disentangling them will require future observations, ideally across a range of wavelengths and viewing geometries.

The stakes of this question extend well beyond Chariklo. Ring systems are laboratories for the physics of disks—systems of countless particles orbiting under gravity, colliding, dissipating energy, and organizing themselves into structures. The same physics operates, on vastly different scales, in planetary rings, in the debris disks around young stars, and perhaps in the disks of material from which planets themselves formed. If the rings of a 250-kilometer body can evolve measurably in a few years, then ring dynamics may be far more active everywhere in the Solar System than previously believed, and the delicate structures around Uranus and Neptune—long assumed to be fossilized remnants—might be more alive than anyone suspected.

The study itself was conceived and executed end to end by the IAA-CSIC team, which led the scientific design of the project, the prediction of the occultation for Webb, the data analysis, and the physical interpretation of the results. The group also spearheaded the ring modeling and the statistical analysis demonstrating that the observed changes are real and not artifacts of measurement. The work was carried out in collaboration with researchers from Spain, Brazil, France, Hungary, and the United States, reflecting the international, distributed character of occultation science, which often depends on networks of observers spread across the globe to catch a shadow that sweeps across the Earth in minutes.

For the occultation community, the Webb observation marks a turning point. Stellar occultations have historically been a ground-based technique, carried out by teams of astronomers positioning portable telescopes along the predicted shadow path. Extending the method to a space telescope—whose orbit must be predicted through an entirely different orbital regime, and whose observing schedule must be locked in years in advance—demonstrates that the technique can now be applied from beyond Earth’s atmosphere, free of clouds, weather, and atmospheric scintillation. Given Webb’s sensitivity and the growing catalog of precisely predicted occultations enabled by Gaia, the outer Solar System is suddenly open to a new mode of high-resolution exploration.

Chariklo itself remains an enigmatic protagonist. A centaur in the astronomical sense—a body transitioning between the realm of the Kuiper Belt and that of the short-period comets—its orbit is inherently unstable over millions of years, nudged by the giant planets. Its rings, sandwiched between Saturn and Uranus in a region of strong gravitational perturbation, may owe their existence, or their eventual destruction, to this dynamic environment. The new measurements provide the first direct evidence that the system is not frozen in time but is actively evolving, offering a rare opportunity: the chance to watch, in near real time, the processes that shape and reshape rings around small worlds.

The next occultations of Chariklo are already being predicted, and each will add another snapshot to the growing record of the ring system’s behavior. If the divergent trend in opacity continues, or reverses, or reveals a periodic component, the data will begin to discriminate between competing explanations—particle evolution, shepherding moons, resonant forcing, or the subtle effects of viewing geometry. For now, the message of the Webb observations is clear and, for planetary scientists, exhilarating: even the smallest ringed bodies in the Solar System are telling stories that change from year to year, and we are finally equipped to listen.