Webb Spots Changes in the Rings of Chariklo

The two rings around Chariklo, an icy body that orbits between Saturn and Uranus, have changed since astronomers last watched them cross in front of a star, and they have changed in opposite ways. Pablo Santos-Sanz of the Institute of Astrophysics of Andalusia (IAA-CSIC) in Granada, Spain, and colleagues report that the inner ring now blocks significantly more starlight than it did in earlier crossings, while the outer ring blocks much less.

The measurements were made with the James Webb Space Telescope in near-infrared light and published Sept. 9 in Science Advances. The paper says the observations show that Webb can track how rings around small bodies change over time, and that little is settled about those rings: where they come from, how long they last and what they are made of are all open questions.

The inner ring is the denser of the two, and the change there is toward more material in the line of sight, not less. The team reads that as a sign of a ring being topped up with fresh material, or reshaped by the forces at work in the system.

The outer ring went the other way, giving a much weaker signal than it had before. That may reflect a loss of material, the authors say, and could mean the outer ring is short-lived. They also offer a second reading: how much light the ring blocks may depend on the wavelength it is observed at, in which case the ring itself need not have changed.

Rings have been found around several small bodies in the outer solar system, all of them during crossings like this one, which astronomers call stellar occultations. As the body drifts in front of a background star, each ring cuts the starlight for a fraction of a second, and the depth of that dip records how much material lies in the way.