Chariklo is challenging our understanding of ring systems
Until 2013, ring systems were thought to be exclusive to the giant planets of the Solar System, such as Jupiter, Saturn, Uranus, and Neptune. However, in 2013, a small body barely 250 kilometers in diameter, located at nearly 17 times the Earth-Sun distance, joined this small group. The object is Chariklo, a small body orbiting between Saturn and Uranus, around which astronomers discovered two dense rings.
On October 18, 2022, the Institute of Astrophysics of Andalusia (IAA-CSIC) led an observation with the James Webb Space Telescope (JWST) to study Chariklo’s rings through a stellar occultation, a technique that measures the decrease in a star’s light when an object passes in front of it (cf. Figure 1). Now, a new study published in Science Advances and also led by the IAA-CSIC demonstrates, for the first time, that Chariklo’s ring system has undergone changes on timescales of just a few years.
“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.
This unexpected behavior indicates that Chariklo’s ring system is dynamic and may be subject to more complex physical processes than previously thought.
The James Webb Space Telescope achieves a first-of-its-kind stellar occultation
The study also represents a significant technological advance : the occultation by Chariklo was the first stellar occultation specifically predicted and planned for observation with JWST and successfully observed from the space telescope (cf. Figure 2).
“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.
At the time of the occultation, Chariklo was moving relative to JWST at just 2.5 kilometers per second. This exceptionally low relative speed provided unprecedented spatial resolution for studying the structure of its rings. These rings are so narrow, and Chariklo is so far away, that they cannot be photographed directly, even with the James Webb Space Telescope or the largest ground-based telescopes. Stellar occultations allow astronomers to study them indirectly by measuring brief dips in a star’s brightness as each ring passes in front of it.
Until now, scientists considered the rings around small bodies in the Solar System relatively stable. The changes detected in Chariklo challenge this view and suggest that these systems may be much more dynamic than previously thought.
“Our results force us to rethink how they form, how they evolve, and what mechanisms maintain their stability. The ability to detect these changes opens a new window for understanding the evolution of these systems and, possibly, that of other ring systems in the Solar System,” says Santos-Sanz.
The physical origin of the detected changes, however, remains an open question : they could reflect temporal evolution of the rings, differences related to the use of different filters, or a combination of both effects.
This work forms part of the ANR-funded ‘Roche’ research project (ANR-23-CE49-0012), led by Nicolas Rambaux at the LTE, in which Bruno Sicardy (co-author of the article) is participating. The aim of the project is to gain a better understanding of the environment and interactions of small bodies in the Solar System. The authors also include Josselin Desmars (LTE) and Damya Souami (LIRA), who are working on stellar occultations as part of the “Lucky Star” project, which brings together teams from France, Spain and Brazil.
