In October 2022, a team of Spanish astronomers leveraged the powerful eye of NASA’s James Webb Space Telescope to examine one of the most unexpected discoveries in the cosmos: the rings encircling a curious asteroid in the outer Solar System.
The unusual object, formally known as 10199 Chariklo, is a class of small celestial objects astronomers call centaurs, whose orbit around the Sun falls between Jupiter and Neptune, and which cross the orbits of at least one of our Solar System’s giant planets. Because of their unique placement, these objects have unstable orbits that are expected to last just a few million years.
Originally discovered in 1997, additional observations of Chariklo in 2013 during a stellar occultation event revealed a pair of dense rings encircling the minor planet—an unexpected discovery that identified it as the second smallest object known to possess such a feature; another centaur, Chiron, remains the smallest similar ringed object.
As recently as a decade ago, only giant planets were thought to possess ring systems, and the discovery of ringed centaurs fundamentally upended past thinking about such features, prompting additional observations of the unusual minor planets. On October 18, 2022, one of these observations commenced as astronomers with the Institute of Astrophysics of Andalusia (IAA-CSIC) peered at Chariklo’s rings with the Webb telescope during another stellar occultation.
The results of those observations, now published in Science Advances, have revealed for the first time that several changes have occurred in Chariklo’s rings over the last few years, revealing an unexpectedly dynamic ring system encircling the minor planet.
Changes in an Unusual Ring System
Pablo Santos-Sanz, an IAA-CSIC researcher and lead researcher behind the new study, said the team’s observations four years ago revealed changes in each of Chariklo’s rings that were opposite from the other.
“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,” Santos-Sanz said.
This unusual finding appears to suggest that Chariklo’s ring system may be influenced by a broader range of processes than astronomers anticipated, and also marks a new milestone for Webb as the first stellar occultation predicted, planned, and successfully observed using NASA’s premier space observatory.
“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,” said study co-author Yücel Kilic, a postdoctoral researcher at the IAA-CSIC, noting the telescope’s position approximately 1.5 million kilometers from Earth at the L2 Lagrange point, close to where NASA’s newly-launched Nancy Grace Roman Space Telescope is not also positioned in orbit.
“JWST follows an orbit around this region that requires periodic corrections through station-keeping maneuvers,” Kilic added, emphasizing the careful timing required for the team’s observations.
During the 2022 stellar occultation, Chariklo’s orbit carried it along at around 2.5 kilometers per second relative to Webb’s position, providing a very low relative speed that was optimal for observations of the centaur’s rings.
Due to the extremely narrow width of the rings and their great distance, photographing them is virtually impossible without a stellar occultation, which provides astronomers a chance for indirect study of these features through observations of short dips in the brightness of a distant star as each of the rings passes in front of it, briefly obscuring its light.
While many questions remain, such as what specifically drives the changes observed in the Chariklo’s rings, the team’s “force us to rethink how [rings] form, how they evolve, and what mechanisms maintain their stability,” Santos-Sanz said of the team’s observations.
“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,” he added.
The team’s study, “JWST stellar occultation reveals unexpected changes in Chariklo’s ring system,” appeared in Science Advances.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.