Webb Watched a 250-Kilometer Rock Pass in Front of a Star. Its Rings Had Changed in Just a Few Years, and Nobody Expected That.

For most of the space age, rings were a giant-planet phenomenon. Jupiter, Saturn, Uranus and Neptune had them; nothing else did. That changed in 2013, when astronomers found two dense rings around Chariklo, a body barely 250 kilometers across that orbits between Saturn and Uranus at nearly 17 times the Earth-sun distance. Now a study led by Spain's Institute of Astrophysics of Andalusia (IAA-CSIC) reports that those rings are not sitting still. They changed measurably within a few years, and in opposite directions.

The result comes from an observation on Oct. 18, 2022, when the James Webb Space Telescope watched Chariklo pass in front of a background star. The rings are far too narrow and too distant to photograph directly, even with Webb. Instead, astronomers measure the brief dips in starlight as each ring crosses the line of sight, a technique called stellar occultation. By comparing the Webb data with occultations recorded over the previous decade, the team found the inner ring had become significantly more opaque while the outer ring had become less so.

"By comparing JWST observations with those obtained during other stellar occultations over the last decade, we discovered opposite changes in the two rings," said Pablo Santos-Sanz, the IAA-CSIC researcher who led the study, published in Science Advances. "While the inner ring shows significantly higher opacity, the outer ring shows lower opacity."

Until now, ring systems around small bodies were assumed to be relatively stable. Chariklo says otherwise. "Our results force us to rethink how they form, how they evolve and what mechanisms maintain their stability," Santos-Sanz said. The team is candid that the cause is still open: the shifts could reflect real evolution in the rings, differences introduced by observing through different filters, or some mix of the two. A statistical analysis led by the IAA-CSIC team concluded the changes are real.

The observation was a technical first as well. It was the first stellar occultation specifically predicted, planned and successfully observed with Webb. Pulling it off required knowing Chariklo's orbit, the star's position from ESA's Gaia mission, and Webb's own trajectory around the L2 Lagrange point, about 1.5 million kilometers from Earth, where the telescope makes periodic station-keeping burns, said Yücel Kilic, an IAA-CSIC postdoctoral researcher and co-author.

Geometry helped. At the moment of the occultation, Chariklo was moving relative to Webb at just 2.5 kilometers per second. That unusually slow relative motion stretched the event out and gave the team unprecedented spatial resolution across the ring structure.

The IAA-CSIC led every phase of the work, from the scientific design and occultation prediction to the ring modeling and physical interpretation, in collaboration with researchers from Spain, Brazil, France, Hungary and the United States. Santos-Sanz said the ability to catch rings in the act of changing "opens a new window for understanding the evolution of these systems and, possibly, that of other ring systems in the solar system."