James Webb Telescope Reveals Puzzling, Changing Rings on Centaur Object Chariklo

The James Webb Space Telescope (JWST) has discovered unexpected changes in the rings of Chariklo, a tiny centaur object orbiting between Saturn and Uranus. Published in Science Advances, observations reveal that Chariklo’s inner and outer rings have shifted in opposite opacity over the past decade, challenging long-held assumptions about outer solar system dynamics.

Tracking the Shift Between Saturn and Uranus

Chariklo is an unusual body, classified as a centaur because it exhibits characteristics of both asteroids and comets. Measuring only about 250 kilometers (155 miles), across, it holds the distinction of being the smallest known solar system object confirmed to possess a ring system. First discovered in 2013, Chariklo’s twin rings were subsequently studied using ground-based telescopes in 2014 and 2017.

When the James Webb Space Telescope turned its infrared instruments toward the distant object in October 2022, a decade of comparison revealed stark physical differences. “By comparing JWST observations with those obtained… 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,” Santos-Sanz explained.

According to data highlighted by Science Advances, the inner ring, designated C1R, is now roughly 50% more opaque. Conversely, the outer ring, C2R, is approximately 60% less opaque when evaluated against occultation data gathered between 2013 and 2017. Despite these shifting opacity levels, the actual radii of the rings have remained static, indicating that large-scale physical movement of material is not driving the transformation.

High-Precision Astrometry and Stellar Occultations

Because Chariklo and its rings are far too faint for direct imaging, astronomers rely on stellar occultations. This technique measures the precise dips and increases in starlight as the system passes in front of a distant background star. For the October 2022 campaign, JWST observed Chariklo as it occulted the star Gaia DR3 6873519665992128512.

Photo: science.report

This project marked the first time the James Webb Space Telescope was intentionally scheduled around a pre-planned stellar occultation of a minor solar system body. The execution required extreme operational precision. Researchers utilized data from the European Space Agency’s Gaia mission, which charts the precise positions and movements of over two billion stars. By factoring in Gaia’s star catalog and calculating the exact trajectory of JWST—orbiting roughly 930,000 miles, or 1.5 million kilometers, from Earth in the opposite direction of the sun—the team successfully captured the event.

Chariklo’s relative speed during the maneuver was a sluggish 1.5 miles per second, or 2.5 kilometers per second. This low relative velocity allowed for fine spatial sampling of the ring structure, a feat previously hampered by atmospheric distortion when using ground-based observatories.

Hypotheses on Ring Instability and Evolution

The research team has proposed three primary theories to account for the puzzling shifts recorded by JWST. First, the telescope’s advanced spatial resolution may simply be resolving denser and sparser zones within the rings that older ground-based instruments blurred together. Second, actual physical changes might be occurring within the ring material or individual icy grains. Third, JWST’s specialized infrared instruments could be detecting grains with wavelength-dependent optical properties, meaning the variability stems from composition and grain-size visibility rather than physical evolution.

Photo: livescience.com

Santos-Sanz emphasized that these findings compel astronomers to reconsider the mechanics of small ringed bodies. “Our results force us to rethink how they form, how they evolve, and what mechanisms maintain their stability,” he stated. Even objects orbiting far outside the immediate gravitational domains of the gas giants experience active environmental shifts.

To untangle temporal evolution from wavelength-dependent scattering effects, the study authors recommend executing a theoretical follow-up observation during the next available stellar occultation. Such data, ideally gathered across multiple instruments and wavelengths, will help clarify the complex physical processes actively shaping Chariklo’s mysterious rings.