The two narrow rings circling Chariklo, a small icy body that orbits the Sun between Saturn and Uranus, appear to have changed within just a few years, with the inner ring blocking more starlight and the outer ring blocking less. The finding, published in the journal Science Advances, is the first evidence that the ring system of a small solar system body can change over such a short period, according to the research team.

The result comes from a single, carefully timed observation by the James Webb Space Telescope. Researchers led by the Institute of Astrophysics of Andalusia (IAA-CSIC) in Spain compared that measurement with stellar occultation observations collected over the previous decade.

The team cannot yet say what caused the change. It may reflect real physical evolution in the rings, differences in the wavelengths of light used in different observations, or both. Each possibility points to a different story about how small worlds hold onto rings.

A Ring System That Upended Expectations

Rings were long associated only with the giant planets. That changed in 2013, when astronomers found two dense rings around Chariklo, an object only about 250 kilometers (155 miles) across that travels nearly 17 times farther from the Sun than Earth, according to the CSIC announcement of the new study.

Chariklo belongs to a group called Centaurs, icy bodies whose orbits lie among those of the outer planets. Its rings are so narrow and distant that no telescope, including Webb, can resolve them in an image.

Instead, astronomers rely on stellar occultations. When the rings pass in front of a distant star, they briefly block some of its light. Measuring those dips reveals how much material the rings contain and how it is arranged.

Webb's Close Look as the Rings Crossed a Star

The key observation took place on Oct. 18, 2022, when Webb watched as Chariklo's rings passed in front of a background star. It was the first stellar occultation specifically predicted and planned for Webb and then successfully observed, a feat described in an earlier report on the observation.

At the time, Chariklo was moving relative to Webb at just 2.5 kilometers per second (about 1.6 miles per second). That unusually slow relative motion gave researchers exceptionally fine detail on the rings' structure.

Yücel Kilic, a postdoctoral researcher at IAA-CSIC and a co-author, said the observation required knowing Chariklo's orbit, the star's position from ESA's Gaia mission, and Webb's own path around the L2 point "with extraordinary precision."

Comparing Webb's data with earlier occultations produced the surprise. "We discovered opposite changes in the two rings," said Pablo Santos-Sanz, the IAA-CSIC researcher who led the study. The inner ring showed significantly higher opacity, while the outer ring showed lower opacity.

According to an account of the paper from the Paris Observatory, whose researchers took part, the inner ring's opacity rose by about 50 percent, and the outer ring's fell by about 60 percent compared with 2017 measurements. The team's modeling suggested it was unlikely that Webb had simply sampled an unusually dense part of one ring and an unusually sparse part of the other.

Competing Explanations and No Verdict

The researchers have described several possibilities without settling on one. In a preprint version of the study, they suggest the inner ring's rise in opacity could point to active replenishment with new material, while the outer ring's weaker signal could reflect material loss, which would mean that ring may be transient.

There is also a measurement question. Webb observed in near-infrared light, while many earlier occultations were recorded at visible wavelengths. If ring particles block light differently at different wavelengths, part of the apparent change could reflect how the rings were observed rather than what happened to them. The authors argue that wavelength effects alone are unlikely to explain the inner ring's increase, but they leave that possibility open for the dusty outer ring.

That is why this is best described as a well-supported anomaly, not a solved case. The study is peer-reviewed, and the Webb data are among the most detailed ever obtained for Chariklo's rings. Even so, the comparison rests on one Webb occultation set against earlier observations made with different instruments.

"Our results force us to rethink how they form, how they evolve," Santos-Sanz said of small-body ring systems.

Broader Stakes for Rings Around Small Worlds

Scientists had generally regarded rings around small bodies as relatively stable structures. Chariklo now suggests they can change on short timescales through processes no one has fully identified. If that holds up, it would change how researchers think rings form and persist around small, icy objects in the outer solar system.

Researchers from Spain, Brazil, France, Hungary and the United States collaborated on the work. Webb is an international observatory led by NASA, so the result also shows a new use for a major U.S.-led science investment: precise occultation measurements of faint, distant objects.

The next test will require more occultations, ideally recorded at several wavelengths at once, so scientists can separate real change from wavelength effects. Readers should also be cautious of headlines suggesting the rings are disappearing. The data show a change in how much light each ring blocks, not that either ring has vanished.

What Readers Want to Know

What is Chariklo?

Chariklo is a small icy body about 250 kilometers (155 miles) across that orbits the Sun between Saturn and Uranus. It belongs to a group known as Centaurs and was the first small body found to have rings.

What changed in Chariklo's rings?

Compared with earlier observations, Webb data show the inner ring blocking more starlight and the outer ring blocking less. The Paris Observatory says the inner ring's opacity rose about 50 percent and the outer ring's fell about 60 percent compared with 2017.

Are Chariklo's rings disappearing?

No. The data show changes in how much light each ring blocks. They do not show that either ring has vanished.

How did scientists observe rings they cannot photograph?

They used a stellar occultation, measuring tiny dips in a star's light as Chariklo's rings passed in front of it during the Webb observation.

Why are scientists unsure what caused the change?

The shift could reflect real gain or loss of ring material, or it could partly reflect the different wavelengths used by Webb and by earlier ground-based observations.

What happens next?

Researchers say more stellar occultations, ideally observed at several wavelengths at the same time, are needed to confirm whether the rings are truly evolving.