TL;DR: JWST watched Chariklo pass in front of a star on Oct. 18, 2022, and found its inner ring more opaque and its outer ring much fainter than in earlier occultations.

JWST catches Chariklo's rings in a changed state

A near-infrared stellar occultation recorded by the James Webb Space Telescope shows opposite opacity shifts in the two narrow rings of Centaur object Chariklo, according to Science Advances. The paper reports that the dense inner ring designated C1R has become significantly more opaque than during previous occultations, while the outer ring designated C2R produced a much weaker near-infrared signature than previously observed. The authors interpret the inner-ring result as possible replenishment or dynamical restructuring, and the outer-ring result as possible material loss, a transient character, or wavelength-dependent opacity. The work is presented as evidence for unexpected changes in the ring system and for the ability of JWST occultations to track evolving structure around small bodies.

Why Chariklo matters for small-body rings

Chariklo was the first small Solar System body found to host a ring system with two sharply confined components, according to Science Advances. Those components are C1R and C2R, orbiting at 390 and 405 km from Chariklo's center, respectively. The discovery came in 2013 from a ground-based stellar occultation, when Chariklo passed in front of a distant star, according to the Paris Observatory - PSL. That finding expanded the known domain of rings beyond the giant planets Jupiter, Saturn, Uranus and Neptune. Subsequent campaigns studied Chariklo's ring dynamics, but origin, composition and grain-size distribution remained poorly constrained more than a decade later, according to Science Advances.

A Centaur between the giant planets

Chariklo belongs to the Centaur population of small outer Solar System bodies whose orbits lie between those of the giant planets, according to the Paris Observatory - PSL. It orbits between Saturn and Uranus. Its distance is described as some two billion kilometers from Earth. Its radius is about 125 km, making it the largest known Centaur. Its diameter is therefore about 250 km. The rings lie only a few kilometers in width despite circling hundreds of kilometers from the body's center. In the discovery geometry, the two rings were located approximately 390 and 405 km from Chariklo's center.

First purpose-planned JWST stellar occultation

The Oct. 18, 2022 observation was the first stellar occultation specifically planned and successfully observed with the James Webb Space Telescope, according to the Paris Observatory - PSL. An international team led by the Andalusian Institute of Astrophysics, IAA-CSIC, carried out the measurement. Researchers from Paris Observatory - PSL, the CNRS and Sorbonne University participated. The project was developed in part within the ANR project Roche at the Laboratoire Temps Espace of Paris Observatory - PSL. It was also part of the ERC project Lucky Star, focused on stellar occultations and involving teams from France, Spain and Brazil.

How a stellar occultation probes invisible rings

A stellar occultation measures the decrease in a star's light when an object passes in front of it. The brief dimming reveals information about the occulting body's size, shape and surroundings. The technique has been used to detect tenuous atmospheres, place upper limits on atmospheres, and discover ring systems beyond the giant planets. Known examples beyond Chariklo include the Centaur Chiron, the dwarf planet Haumea and the trans-Neptunian object Quaoar, according to Science Advances. For Chariklo, the method detects rings that are otherwise invisible in direct images because the rings briefly block starlight as they cross the line of sight.

An exceptionally close miss of the main body

The JWST observing chord passed just about 7.4 km from Chariklo's surface, according to Science Advances. That geometry represented an exceptionally close approach to the main body while still intersecting the ring system. The reconstructed closest approach was 132.2 km, slightly larger than Chariklo's volume-equivalent radius of 124.8 km. That comparison confirmed that the main body itself did not occult the star. The full JWST time series showed no additional brightness dips above the two-sigma photometric noise level. The absence of other dips indicated no additional ring material or satellites along the JWST chord.

Two infrared colors at once

The observation was conducted simultaneously in two near-infrared bands centered at 1.5 and 3.2 micrometers, designated F150W2 and F322W2, according to Science Advances. It included the first stellar-occultation detection of a minor-body ring system at wavelengths beyond 3 micrometers. That wavelength regime is inaccessible from the ground. JWST can observe in the near-infrared up to about 5 micrometers, enabling study of the rings at wavelengths previously inaccessible from the ground, according to the Paris Observatory - PSL. Exactly the same JWST setup was later used to observe the occultation by the ring system of Quaoar, according to Science Advances.

Ring positions steady, opacities shifted

The radial locations of both rings in 2022 remained consistent with earlier determinations, while their occultation properties differed. The striking aspect is that the positions of the two rings are essentially unchanged but their properties appear modified, according to the Paris Observatory - PSL. The inner ring C1R now appears about 50% more opaque than before. The outer ring C2R exhibits a much weaker signal, with opacity apparently decreased by about 60% compared with 2017. The Science Advances paper likewise describes the inner ring as significantly more opaque than in previous occultations and the outer ring as much weaker in the near-infrared than previously observed.

Not simply a different slice of ring

The variations do not appear attributable simply to the fact that JWST observed different portions of the rings, according to the Paris Observatory - PSL. They instead indicate that an actual evolution of the system is taking place. Predicting such an event from a space-based observatory is inherently challenging, according to Science Advances. Despite extensive occultation campaigns and multiple studies of ring dynamics, the limited number of well-resolved observations and possible structural changes make wavelength-dependent work observationally challenging. The new comparison therefore provides a rare multi-year baseline from 2013 through 2017 to the 2022 JWST epoch.

What could make one ring thicker and one thinner

For C2R, the opacity decrease could reflect a gradual loss of material, according to the Paris Observatory - PSL. For C1R, the opacity increase could result from an influx of material. It could also result from collisions that broke larger particles into many finer grains. The Science Advances abstract frames the same possibilities as ongoing replenishment or dynamical restructuring for the dense inner ring and material loss for the outer ring. The outer ring could be transient. Alternatively, its weaker infrared appearance may arise from wavelength-dependent opacity rather than disappearance of material. The origins, lifetimes and compositions of rings around small outer Solar System bodies remain little known, according to Science Advances.

Grain size and wavelength remain open

Wavelength-dependent optical properties of grains produce different occultation opacities in visible and near-infrared light, according to Science Advances. Such differences may provide direct constraints on grain properties and grain-size distribution. The JWST data extend that test to longer infrared wavelengths. Future occultations, particularly in the visible spectrum, will make it possible to determine whether the observed changes are continuing, according to the Paris Observatory - PSL. Those future measurements will also help definitively establish whether the differences represent temporal evolution or effects related to wavelength. That distinction is central because a fainter infrared signal does not by itself prove that solid material has vanished.

A new tool for evolving small-body systems

The observations demonstrate the power of JWST occultations to probe the evolving structure of rings around small bodies, according to Science Advances. Stellar occultations offer exceptional precision for distant bodies and their environments. The Chariklo result adds a time-domain piece to the puzzle of the origin, stability and evolution of its compact rings, according to the Paris Observatory - PSL. It follows the 2013 discovery that revolutionized understanding of small bodies in the Solar System by showing that a body only about 250 km across could retain two dense rings. The 2022 JWST chord shows that such systems need not be static over timescales of a few years.

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