Chariklo, a small asteroid orbiting between Saturn and Uranus, surprised astronomers in 2013 when it became the first object beyond the giant planets found to have rings. Now, new observations from the James Webb Space Telescope show those rings are changing in unexpected ways. The inner ring appears significantly more opaque than before, while the outer ring has become so translucent it is barely detectable.
A team led by Pablo Santos-Sanz used JWST to catch Chariklo passing in front of a distant star, an event called a stellar occultation. Predicting the alignment was difficult because JWST's orbit is hard to forecast more than a few months ahead, but the gamble paid off. The telescope clearly detected both rings, yet the inner ring's opacity had jumped 42% compared to earlier ground-based observations, while the outer ring had faded dramatically.
The researchers have no definitive explanation for the opposing changes. One possibility is that collisions within the inner ring are altering how particles reflect light, while outer ring particles are drifting away. Another is that an unseen moon is reshaping the rings, feeding material inward. Heikki Salo, an astronomer not involved in the study, notes that Chariklo's irregular shape and the rings' orbital resonance may make them naturally time-variable, similar to Saturn's F ring. With the next occultation possibly five years away, the mystery of Chariklo's evolving rings is likely to persist for now.