Astronomers using the James Webb Space Telescope have found that the two thin rings surrounding Chariklo, a small icy body orbiting between Saturn and Uranus, appear to have changed over roughly a decade. The discovery, published in Science Advances, challenges the long-held assumption that ring systems around minor planets are essentially stable over short timescales.
Chariklo is a centaur about 250 kilometers in diameter, located on average nearly 17 times farther from the Sun than Earth. Its rings were discovered in 2013 during a stellar occultation, making it the first small body known to host a ring system. Until then, rings had been confirmed only around Jupiter, Saturn, Uranus and Neptune. The rings are too narrow and Chariklo too distant to be photographed directly, even by Webb, so astronomers study them by watching the brief dimming of starlight when Chariklo passes in front of a star.
The new analysis relies on a stellar occultation observed with JWST on October 18, 2022, combined with earlier occultation data spanning about ten years. The 2022 event was the first stellar occultation ever predicted and planned specifically for Webb and then successfully observed. It required precise knowledge of Chariklo's orbit, the star's position from Gaia mission data, and Webb's trajectory around the L2 Lagrange point about 1.5 million kilometers from Earth. During the event, Chariklo moved relative to Webb at only 2.5 kilometers per second, a very low relative speed that allowed the ring structure to be resolved with unprecedented spatial detail.
Comparing that observation with previous occultations revealed that the two rings have not changed in the same way. The inner ring, known as C1R, now appears more opaque than in the past, while the outer ring, C2R, appears less opaque. The result, led by Pablo Santos-Sanz of the Instituto de Astrofísica de Andalucía, suggests that the rings may evolve on timescales of just a few years, far faster than previously thought.
The researchers caution that the differences could partly stem from the different filters used in the compared observations, or from a combination of real evolution and instrumental effects. Further occultations will be needed to confirm whether the ring system is genuinely changing and to identify the physical processes driving any evolution.
Chariklo remains invisible in direct photographs, but each time a star passes behind it at the right moment, astronomers may gain another clue about how its rings are transforming. The study appears in Science Advances under the title «JWST stellar occultation reveals unexpected changes in Chariklo's ring system».