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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.
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Why it matters
- 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.
- 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.
- capability Webb can now be scheduled against a predicted occultation, so structures far too faint to image directly become measurable at its wavelengths.
- 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.
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 [18]. 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 [3][2]. 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 [20]. At that speed the relative motion covers a distance equal to Chariklo's 250 kilometre diameter in about 100 seconds [1][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 [21][22]. The researchers said the planning and execution required "extreme precision" [23].
"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 [15]. 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 [16].
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.