Webb sees Chariklo's inner ring thicken by 50 percent while its outer ring fades by 60 percent
A James Webb Space Telescope stellar occultation in October 2022 shows the two narrow rings of Chariklo, a Centaur about 250 kilometres across, changing in opposite directions: the inner ring blocks about 50 percent more starlight than in earlier occultations, while the outer ring's signal is about 60 percent weaker than in 2017. The rings kept their positions. The team published the comparison in Science Advances on 9 September and does not yet know whether material moved or the wavelength made the difference.
Science··Midday
The inner ring blocks more starlight, the outer ring less
On 18 October 2022 Webb watched Chariklo pass in front of a distant star, the first stellar occultation planned specifically for the telescope and observed successfully; the team was led by Pablo Santos-Sanz of the Institute of Astrophysics of Andalusia (IAA-CSIC), a Spanish research institute. Compared with ground-based occultations of the past decade, the inner ring is about 50 percent more opaque, while the outer ring's signal is about 60 percent weaker than it was in 2017; the radial positions of both rings were essentially unchanged. The comparison appeared in Science Advances on 9 September and documents changes in the rings of a small body over a span of only a few years.[1], [2]
A body 250 kilometres across with two narrow rings
Chariklo is a Centaur, one of the icy bodies on unstable orbits among the giant planets, and measures about 250 kilometres across. Its rings were discovered during a 2013 stellar occultation and were the first found around a small body; they orbit roughly 390 and 405 kilometres from its centre and are only a few kilometres wide, the inner ring C1R about six to seven kilometres and the outer ring C2R about two to four, separated by a gap of about nine kilometres. Astronomers measure the rings through the dip in a background star's light as the body and its rings pass in front of it. In 2022 Webb's near-infrared camera recorded the event through filters near 1.5 and 3.2 micrometres; the inner ring made clear dips in the starlight, while the outer ring was only marginally detected at 1.5 micrometres and not detected at all at 3.2.[1], [2]
Lost material, or a trick of the wavelength?
The authors do not yet know whether the rings actually gained and lost material or whether the difference comes from Webb observing at a longer wavelength than the visible-light occultations it is compared with. If the outer ring really is fading, the paper's estimate that an unconfined ring would spread out in a median of only 0.36 years implies that some process holds it at the radius where it was found in 2013. The other reading is grain optics: models reproduce the weak infrared signature if the outer ring is dominated by silicate grains about 0.2 to 0.5 micrometres across, which block visible light efficiently yet become far less apparent at Webb's wavelengths. The two scenarios can coexist, and a future occultation in visible light could separate them: if the outer ring again looks as opaque as in earlier visible observations, wavelength is the stronger explanation; if it stays faint, real material loss becomes harder to avoid.[1], [2]
