Strange 'centaur' object between Saturn and Uranus has mysterious, changing rings, James Webb telescope study hints

The rings around a strange small body orbiting between Saturn and Uranus look mysteriously different from previous views, new observations from the James Webb Space Telescope (JWST) suggest.

The rings encircle Chariklo, which is a centaur, a strange type of object that shows characteristics of both asteroids and comets. Chariklo is only about 400 miles (250 kilometers) in diameter and is part of a large group of tiny objects in the distant solar system.

Scientists first discovered Chariklo's rings in 2013 — making it the first small solar system body found to have rings — and studied them with ground-based telescopes again in 2014 and 2017. When JWST took another look at Chariklo in 2022, a decade had brought a lot of difference, according to a study published Sept. 9 in the journal Science Advances.

"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, " Pablo Santos-Sanz, a researcher at the Institute of Astrophysics of Andalusia and first author of the study, said in a statement.

Researchers don't know exactly how the changes took place, but the study authors offered three suggestions. The first is that JWST, which has a fine resolution, was able to see both denser and sparser zones of the rings than previous observations showed. The second is that there were changes in the rings' material or in the grains themselves. Finally, a third possibility is that JWST could see "grains with wavelength-dependent optical properties," meaning the telescope saw effects in composition and grain size that explain the variability.

Santos-Sanz said the research suggests that small bodies with rings — even those orbiting far away from the gravitational influences of planets — may have more changes in their systems than scientists previously thought. "Our results force us to rethink how they form, how they evolve, and what mechanisms maintain their stability," he said.

A theoretical follow-up observation, taken the next time Chariklo passes in front of a star, would help to confirm the new JWST results, the researchers added — especially the apparent decreased opacity of one of the rings and the increased opacity of the other.

"These observations would help disentangle temporal evolution from wavelength-dependent scattering effects, and clarify the physical processes shaping Chariklo’s rings," the researchers stated in the study.

A JWST milestone

The new observations of Chariklo also mark the first time that scientists have planned a JWST viewing campaign specifically around a stellar occultation — that is, the researchers timed the viewing precisely to when Chariklo passed in front of a distant star from the telescope's point of view.

All observations of Chariklo's rings, on the ground and in space, have used stellar occultations. That's because the rings are too faint to be imaged directly, so astronomers must instead learn more about the rings by measuring the dips and increases in brightness as Chariklo passes in front of the background star. Scientists use a similar method to study the atmospheres of alien planets with JWST's infrared instruments.

Chariklo's speed relative to JWST was low, at 1.5 miles per second (2.5 kilometers per second), allowing for fine imaging of the rings. To time the occultation, the researchers used info from the European Space Agency's Gaia mission, which precisely charts the locations and movements of more than 2 billion stars, as well as calculated the orbit of JWST at a location roughly 930,000 miles (1.5 million km) from Earth in the opposite direction of the sun. According to the researchers, the planning and execution of the mission required "extreme precision."