A timed Webb occultation shows Chariklo's inner ring gaining opacity and its outer ring losing it
Webb watched Chariklo, the first small body found to have rings, cross in front of a star in 2022, and the ring opacities it measured do not match the ground-based ones from the previous decade. The paper offers three explanations.
The Scientist · Science desk
What happened
- Chariklo, a centaur roughly 250 kilometres across orbiting between Saturn and Uranus, has had known rings since 2013, when it became the first small solar system body found to have any.
- Ground-based telescopes measured those rings again in 2014 and 2017, and the comparison with a James Webb Space Telescope occultation from 2022 was published Sept. 9 in Science Advances.
- The authors say they do not know how that happened, and they put forward three candidate explanations spanning the telescope, the ring material and the optical behaviour of the grains.
- It was the first time a Webb observing campaign had been planned specifically around a stellar occultation, the brightness-dip method also used to probe exoplanet atmospheres.
Why it matters
- What it limits constraint Nobody can re-check this on demand. It takes another star passing behind Chariklo, so orbital geometry, not telescope time, sets how often the evidence can be updated.
- What doesn't add up contradiction The paper's own list of explanations weakens the simplest reading of it: two of the three would produce the same measured difference with rings that had not changed at all.
- What's now possible capability Webb can now be scheduled against a predicted occultation, so structures far too faint to image directly become measurable at its wavelengths.
- Who has to decide decision Modellers deciding whether ring systems far from any planet need active maintenance now have one measurement pointing that way, resting on a single new epoch.
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Opacity here is not an image. Chariklo's rings are too faint to photograph, so every measurement of them, from the ground and from Webb, comes out of a light curve: the brightness of a background star dipping and recovering as the body and its rings pass across it [16]. The depth of the dip is how much starlight the ring blocked. The same amount of blocking can come from different combinations of composition and grain size, and one of the paper's candidate explanations turns on exactly that, "grains with wavelength-dependent optical properties" [12].
"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," said Pablo Santos-Sanz, who is at the Institute of Astrophysics of Andalusia [7][8].
The comparison spans five years at its narrowest, from the 2017 ground campaign to Webb's 2022 occultation, and nine years from the discovery of the rings in 2013 [26][25]. Only one of the three explanations the authors offer describes something happening at Chariklo: a change in the ring material or in the grains themselves [11]. The other two describe what a better instrument would show of unchanged rings, either by separating dense and sparse zones that earlier data averaged together [10], or because the grains behave differently at Webb's wavelengths [12].
The observation was hard to arrange for reasons unrelated to the rings. Chariklo crossed the star slowly as seen from Webb, at 2.5 kilometres per second, and that low relative speed is what allowed fine sampling of the ring profile [18]. At that speed the relative motion covers a distance equal to Chariklo's 250 kilometre diameter in about 100 seconds [24][2]. Star positions came from ESA's Gaia mission, which charts more than 2 billion stars, and the prediction had to fold in Webb's own orbit roughly 1.5 million kilometres from Earth on the side away from the sun [19][20]. The researchers said the planning and execution required "extreme precision" [21].
"Our results force us to rethink how they form, how they evolve, and what mechanisms maintain their stability," Santos-Sanz said [13]. He said the work suggests that small bodies with rings, including those far from the gravitational influence of any planet, may change more than scientists had assumed [14].
What would settle the question is the next time Chariklo passes in front of a star, and the authors say such an observation would test both the drop in one ring's opacity and the rise in the other's [22]. Until it exists, evolving rings are one of three live options, and I would not yet report them as the result. "These observations would help disentangle temporal evolution from wavelength-dependent scattering effects, and clarify the physical processes shaping Chariklo's rings," the authors wrote [23].
What to watch
- Whether Webb's 2022 light curve is reprocessed at the effective resolution of the 2014 and 2017 ground data, which is the direct test of the resolution explanation.
- A single occultation recorded simultaneously from the ground and from Webb would separate wavelength effects from a real change in the rings.
- Whether the inner ring's opacity keeps rising at a later epoch or returns toward its 2013 to 2017 value.
Clarity's read
What the record supports and how the coverage leans. The claims behind it follow.
Reality
- Evidence62
- Adoption
- Not scored
- Hype gap+25
- Incentives30
- Confidence60
Claim ledger
Ranked by verification strength, evidence, and original report placement.
- [1]
Chariklo is a centaur, a type of small body orbiting between Saturn and Uranus that shows characteristics of both asteroids and comets.
- [2]
Chariklo is about 250 kilometres (400 miles) in diameter.
- [3]
Scientists first discovered Chariklo's rings in 2013, making it the first small solar system body found to have rings.
- [4]
Chariklo's rings were studied again with ground-based telescopes in 2014 and 2017.
- [5]
The James Webb Space Telescope observed Chariklo in 2022.
- [6]
The study was published Sept. 9 in the journal Science Advances, cited as Science Advances 12(37), eaeh4794.
- [7]
"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."
- [8]
Pablo Santos-Sanz is a researcher at the Institute of Astrophysics of Andalusia and first author of the study.
- [9]
The researchers do not know exactly how the changes took place, and the study authors offered three suggestions.
- [10]
The first suggested explanation is that JWST, which has fine resolution, was able to see both denser and sparser zones of the rings than previous observations showed.
- [11]
The second suggested explanation is that there were changes in the rings' material or in the grains themselves.
- [12]
The third suggested explanation is that JWST could see "grains with wavelength-dependent optical properties," meaning effects of composition and grain size that explain the variability.
- [13]
"Our results force us to rethink how they form, how they evolve, and what mechanisms maintain their stability."
- [14]
Santos-Sanz said the research suggests that small bodies with rings, even those orbiting far from the gravitational influences of planets, may have more changes in their systems than scientists previously thought.
- [15]
The Chariklo observations were the first time scientists planned a JWST viewing campaign specifically around a stellar occultation.
- [16]
All observations of Chariklo's rings, from the ground and from space, have used stellar occultations, because the rings are too faint to image directly; astronomers instead measure the dips and increases in brightness as Chariklo passes in front of a background star.
- [17]
Scientists use a similar brightness-dip method to study the atmospheres of alien planets with JWST's infrared instruments.
- [18]
Chariklo's speed relative to JWST was low, at 2.5 kilometres per second (1.5 miles per second), allowing fine imaging of the rings.
- [19]
To time the occultation the researchers used information from the European Space Agency's Gaia mission, which precisely charts the locations and movements of more than 2 billion stars.
- [20]
The researchers calculated the orbit of JWST at a location roughly 1.5 million kilometres (930,000 miles) from Earth in the direction opposite the sun.
- [21]
According to the researchers, the planning and execution of the observation required "extreme precision."
- [22]
The researchers said a follow-up observation taken the next time Chariklo passes in front of a star would help confirm the JWST results, especially the apparent decreased opacity of one ring and the increased opacity of the other.
- [23]
"These observations would help disentangle temporal evolution from wavelength-dependent scattering effects, and clarify the physical processes shaping Chariklo's rings."
- [24]
At a relative speed of 2.5 kilometres per second, the relative motion covers a distance equal to Chariklo's 250 kilometre diameter in about 100 seconds.
- [25]
Nine years separate the 2013 discovery of Chariklo's rings from the 2022 JWST observation.
- [26]
Five years separate the 2017 ground-based observations from the 2022 JWST observation.
Sources
2 independent publishers whose own reporting we read for this story.
- livescience.comStrange 'centaur' object between Saturn and Uranus has mysterious, changing rings, James Webb telescope study hints
1 article · September 15, 2026
- sciencedaily.comJames Webb reveals Chariklo’s mysterious rings are changing faster than expected
1 article · September 15, 2026
Topics and entities
Topics
- Stellar occultation
- Ring systems of small bodies
Entities
- Spanish National Research Council
- Pablo Santos-Sanz
- European Space Agency
- Institute of Astrophysics of Andalusia
- Science Advances
- James Webb Space Telescope
- GAIA