For most of modern astronomy, rings were associated only with the Solar System's giant planets: Jupiter, Saturn, Uranus, and Neptune. That picture changed in 2013, when astronomers discovered two dense rings around Chariklo, a small object only about 250 kilometers across that travels between Saturn and Uranus, nearly 17 times farther from the Sun than Earth. Now, new observations from the James Webb Space Telescope (JWST) show that those rings are not as stable as scientists once assumed.
A study published in Science Advances, led by the Institute of Astrophysics of Andalusia (IAA-CSIC), provides the first evidence that Chariklo's ring system has changed over a period of only a few years. James Webb Detects Changes in Chariklo's Rings The key observation took place on October 18, 2022, when researchers used JWST to watch Chariklo pass in front of a distant star. This event, known as a stellar occultation, allows astronomers to study objects that are too small or distant to image directly.
As Chariklo and its rings crossed in front of the star, they briefly blocked some of its light, producing measurable dips in brightness. Researchers compared the JWST data with earlier stellar occultation observations collected over the previous decade. The comparison revealed that Chariklo's two rings have changed in different ways.
"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. In other words, the inner ring now blocks more light than before, while the outer ring blocks less. That unexpected contrast suggests that Chariklo's rings are active, evolving structures influenced by physical processes that may be more complicated than scientists previously realized.
A First for the James Webb Space Telescope The observation was also a technological milestone. It marked the first time a stellar occultation had been specifically predicted and planned for JWST and then successfully observed with the space telescope. Pulling off the observation required extraordinary precision.
Scientists needed highly accurate information about Chariklo's orbit, the location of the background star, and JWST's own movement through space. "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.
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