James Webb has revealed unexpected changes in Chariklo’s two rings, which appear to be evolving in opposite ways over only a few years.
For much of modern astronomy, rings were believed to belong only to the giant planets of the Solar System, including Jupiter, Saturn, Uranus, and Neptune. That assumption changed in 2013, when astronomers discovered two dense rings around Chariklo, a small body only about 250 kilometers across that travels between Saturn and Uranus at nearly 17 times the Earth-Sun distance.
On October 18, 2022, researchers led by the Institute of Astrophysics of Andalusia (IAA-CSIC) used the James Webb Space Telescope (JWST) to examine Chariklo’s rings during a stellar occultation. This method tracks how a star’s light dims as an object passes in front of it. A new study published in Science Advances, also led by the IAA-CSIC, now shows for the first time that Chariklo’s rings can change measurably over the course of only a few years.
“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, ” explains Pablo Santos-Sanz, an IAA-CSIC researcher who leads the study.
The contrasting changes suggest that Chariklo’s rings are not static structures. Instead, the system appears to be active and may be influenced by physical processes that are more complicated than astronomers previously realized.
James Webb Achieves a First
The observations also marked an important technical milestone. Chariklo’s passage in front of the star was the first stellar occultation that had been specifically predicted and planned for JWST and then successfully observed by the space telescope.
“Achieving this required knowing with extraordinary precision the orbit of Chariklo, the position of the star—thanks to ESA’s Gaia mission—and the trajectory of JWST itself around the L2 Lagrange point, a region of space located about 1.5 million kilometers beyond Earth, away from the Sun. JWST follows an orbit around this region that requires periodic corrections through station-keeping maneuvers,” notes Yücel Kilic, postdoctoral researcher at the IAA-CSIC and co-author of the study.
During the occultation, Chariklo was moving relative to JWST at only 2.5 kilometers per second. That unusually slow relative motion gave astronomers exceptional spatial resolution, allowing them to probe the fine structure of the rings in greater detail.
Chariklo’s rings are extremely narrow, and the object itself is so distant that the rings cannot be directly photographed, even by JWST or the largest telescopes on Earth. Stellar occultations provide another way to study them. As Chariklo and its rings pass in front of a background star, each ring produces a brief dip in the star’s brightness, revealing information about the ring’s structure and opacity.
Statement by Pablo Santos-Sanz (IAA-CSIC).
Chariklo Challenges Ideas About Ring Stability
Scientists had generally assumed that rings around small Solar System bodies remain relatively stable over time. The newly detected changes in Chariklo call that assumption into question and suggest that such ring systems may evolve much more quickly than expected.
“Our results force us to rethink how they form, how they evolve, and what mechanisms maintain their stability. 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,” says Santos-Sanz.
Exactly what caused the changes is still uncertain. They may reflect genuine evolution within the rings over time, differences caused by the filters used during separate observations, or some combination of the two.
An International Effort to Understand Chariklo
The Institute of Astrophysics of Andalusia (IAA-CSIC) directed every major stage of the project, beginning with the scientific planning and prediction of Chariklo’s occultation for JWST and continuing through data analysis and interpretation.
Researchers at the IAA-CSIC also played a central role in modeling the rings and carrying out the statistical analysis that confirmed the observed changes are real. The work involved collaborators from Spain, Brazil, France, Hungary, and the United States.
Reference: “JWST stellar occultation reveals unexpected changes in Chariklo’s ring system” by Pablo Santos-Sanz, Altair R. Gomes-Júnior, Bruno E. Morgado, Yucel Kilic, Csilla Kalup, Csaba Kiss, Chrystian L. Pereira, Bryan J. Holler, Nicolás Morales, José Luis Ortiz, Bruno Sicardy, Juan Luis Rizos, John Stansberry, Richard G. French, Heidi B. Hammel, Zhong-Yi Lin, Damya Souami, Josselin Desmars, Stefanie N. Milam, Felipe Braga-Ribas, Marcelo Assafin, Gustavo Benedetti-Rossi, Julio I. B. Camargo, Rene Duffard, Flavia L. Rommel, Estela Fernández-Valenzuela, Noemí Pinilla-Alonso and Mónica Vara-Lubiano, 9 September 2026, Science Advances.
DOI: 10.1126/sciadv.aeh4794
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