Webb Finds Chariklo's Two Rings Are Changing in Opposite Ways

The James Webb Space Telescope revealed that Chariklo's inner ring is growing more opaque while its outer ring fades, upending ideas that small-body rings are stable.

Using the James Webb Space Telescope, astronomers have discovered that the two rings of Chariklo — a roughly 155-mile (250-kilometer) wide Centaur orbiting the Sun between Saturn and Uranus — are changing in opposite directions. According to space.com, a team led by Pablo Santos-Sanz of the Institute of Astrophysics of Andalusia (IAA-CSIC) began observing Chariklo with JWST in October 2022 and compared the results with stellar occultation measurements taken over the past decade. The inner ring now shows significantly higher opacity, while the outer ring shows lower opacity — a divergence nobody expected.

Small bodies were previously believed to have relatively stable rings, so seeing measurable, and opposing, changes in the two rings of an object only 250 kilometers across is a genuine surprise. "Our results force us to rethink how they form, how they evolve, and what mechanisms maintain their stability," Santos-Sanz said, as quoted by space.com. Chariklo's rings were only discovered in 2013, and the new data suggest the system experiences more complex physics than scientists realized — hinting that even the most diminutive bodies can host dynamic ring systems, not just giants like Saturn, Jupiter, Uranus and Neptune.

Even the JWST cannot directly image Chariklo, so astronomers rely on stellar occultation — measuring the dip in a star's light when the object passes in front of it. SSBCrack News reports that during the occultation, Chariklo moved at roughly 5,600 miles per hour (2.5 km/s) relative to JWST, which is slow by solar-system standards and allowed the rings to be resolved in unprecedented detail. The findings were published in the journal Science Advances on September 9, according to space.com.

Pulling off the observation required extraordinary coordination. "Achieving this required knowing with extraordinary precision the orbit of Chariklo, the position of the star, thanks to the European Space Agency's Gaia mission, and the trajectory of JWST itself around the L2 Lagrange point," team member Yücel Kilic of the IAA-CSIC said, per space.com. L2 lies about 1 million miles (1.5 million km) beyond Earth, and JWST's orbit there requires periodic station-keeping maneuvers. ua.news notes the observation similarly depended on Gaia's precise star positions.

The mystery cause of the changes remains unresolved, but the detection itself matters. As space.com reports, Santos-Sanz said the ability to detect such changes "opens a new window for understanding the evolution of these systems and, possibly, that of other ring systems in the solar system." Because Chariklo's rings were thought to be stable since their 2013 discovery, the finding challenges assumptions that could extend to ring physics broadly — including how Saturn's and Uranus's rings evolve over time.

Astronomers will need continued occultation campaigns to track whether the opacity trends persist or reverse, since direct imaging remains out of reach even for JWST. ua.news confirms the cause of the changes has not been determined, making future observations — paired with Gaia's star catalogs — the main path forward. Beyond Chariklo, the broader small-body census is advancing: Mashable reports that Webb and Hubble together identified 27 new Trans-Neptunian Objects, some only about six miles wide, in the most sensitive study of such worlds to date, hinting at more surprises from the solar system's small inhabitants.