Webb's First Planned Occultation Caught Chariklo's Rings Shifting

Webb's first-ever planned stellar occultation revealed Chariklo's inner ring growing denser while its outer ring thins, forcing a rethink of small-body ring systems.

Chariklo, a centaur only about 250 kilometers across that orbits between Saturn and Uranus, holds the distinction of being the first small solar system body found with rings, discovered in 2013. On October 18, 2022, the James Webb Space Telescope watched Chariklo pass in front of a distant star, measuring dips in starlight as the rings crossed the background star. Comparing that data with a decade of earlier occultation observations, a study led by the Institute of Astrophysics of Andalusia (IAA-CSIC) and published in Science Advances found opposite changes: the inner ring now blocks significantly more light than before, while the outer ring blocks less. "We discovered opposite changes in the two rings," said study lead Pablo Santos-Sanz.

The observation marked the first time a stellar occultation was specifically predicted and planned for JWST and then successfully captured by the space telescope. According to ScienceDaily, co-author Yücel Kilic of IAA-CSIC explained that the feat required knowing the orbit of Chariklo, the position of the background star — thanks to ESA's Gaia mission — and the trajectory of Webb itself around the L2 Lagrange point, roughly 1.5 million kilometers beyond Earth, where the telescope needs periodic station-keeping maneuvers. Live Science reports that Chariklo was moving relative to Webb at just 1.5 miles (2.5 kilometers) per second, an unusually slow relative speed that gave researchers exceptionally detailed spatial information about the rings' structure.

Scientists had generally regarded rings around small solar system bodies as relatively stable structures, so evidence that Chariklo's ring system shifted measurably within just a few years upends that assumption. The rings are too faint and narrow to be imaged directly even by Webb, which is why every observation to date — from the ground in 2013, 2014 and 2017, and now from space — has relied on stellar occultations, using a distant star as a backlight. As Live Science notes, Webb's finer resolution may itself explain part of the discrepancy, since it could resolve denser and sparser zones that earlier ground-based views smoothed over.

According to Live Science, the study authors offered three possible explanations: Webb's higher resolution revealing structure earlier observations missed, genuine changes in the rings' material or grains, or grains with wavelength-dependent optical properties that only Webb's infrared instruments could detect. ScienceDaily adds that the changes may represent real evolution, differences from the filters used across observations, or a combination of both. The researchers say a future occultation observation — the next time Chariklo passes in front of a star — would help disentangle temporal evolution from wavelength-dependent scattering effects. "Our results force us to rethink how they form, how they evolve, and what mechanisms maintain their stability," Santos-Sanz said, opening a new window on ring systems across the solar system.