Webb Telescope Spots Changes in Chariklo's Rings
Chariklo's rings have changed within just a few years, a new study in Science Advances shows: the inner ring is denser, the outer ring significantly fainter.
The James Webb Space Telescope (JWST) recorded a stellar occultation by the centaur Chariklo, a small Solar System body with its own two-ring system, on October 18, 2022. A paper published September 9, 2026, in the journal Science Advances shows that those rings have changed within just a few years: the inner ring has grown denser, while the outer ring is significantly fainter. It was also the first stellar occultation ever planned in advance and successfully observed with this particular telescope. The research was led by the Institute of Astrophysics of Andalusia (IAA-CSIC).
Chariklo is the largest known centaur, a body orbiting between Saturn and Uranus with a radius of about 125 kilometers, meaning a diameter of roughly 250 kilometers. Sources differ on exactly how far away it is: the Paris Observatory puts its distance from Earth at about two billion kilometers, while the SINC agency states it lies about 17 astronomical units from the Sun. Chariklo’s two narrow rings were discovered in 2013, when a ground-based observation of a stellar occultation showed for the first time that rings are not exclusive to large planets like Jupiter and Saturn. The rings, designated C1R and C2R, sit at about 390 and 405 kilometers from the body’s center and are only a few kilometers wide.
The 2022 observation was carried out in the near-infrared, out to about five micrometers, while the relative speed between Chariklo and the telescope was around 2.5 kilometers per second. Predicting the occultation at all required knowing, with extreme precision, Chariklo’s orbit, the star’s position, and JWST’s own trajectory. Judzel Kilic, a postdoctoral researcher at IAA-CSIC and co-author of the study, explained:
Achieving this required knowing, with extreme precision, Chariklo’s orbit, the star’s position, thanks to ESA’s Gaia mission, and the trajectory of JWST itself around the L2 Lagrange point, a region of space about 1.5 million kilometers from Earth, in the direction opposite the Sun. JWST follows an orbit around that region that requires periodic corrections through station-keeping maneuvers.
According to the measurements, the opacity of the inner ring, C1R, increased by about 50 percent compared to earlier measurements, while the opacity of the outer ring, C2R, dropped by about 60 percent compared to 2017. The positions of both rings stayed essentially unchanged; what changed was only the amount of material blocking the star’s light.
Pablo Santos Sanz, an IAA-CSIC researcher and the paper’s lead author, said in a statement from the institute:
Our results are forcing us to reconsider how they form, how they evolve, and what mechanisms keep them stable.
The physical cause of the changes remains an open question. For the inner ring, researchers point to a possible replenishment of material or a dynamical rearrangement of particles, while for the outer ring they cite possible loss of material, the chance that it is a transient event, or the possibility that the measured opacity simply depends on the wavelength at which it was observed. Besides IAA-CSIC, teams from France worked on the paper, including the Paris Observatory, CNRS, and Sorbonne University, along with researchers from Brazil, Hungary, and the United States.
The study does not establish with certainty whether the changes in Chariklo’s rings are a lasting evolution of the system or simply the result of JWST observing at a different wavelength than earlier, ground-based measurements did. The authors say only future occultations, especially ones observed in the visible part of the spectrum, will show whether the changes are continuing and whether they reflect a genuine evolution of the rings or a wavelength-dependent effect.
