The star went dark twice on each side
On October 18, 2022, Chariklo crossed in front of a distant star and JWST caught the brief dips in the star's light at a relative speed of 2.5 kilometres per second. What was measured is simple and robust: how much light the rings block. For the inner ring C1R, the pre-JWST occultations give an average normal opacity of 0.303 ± 0.028; the JWST measurements alone give 0.431 ± 0.012. The gap is 4.2 sigma, a ring roughly 50 percent more opaque than anything seen before. The outer ring C2R went the other way: its equivalent width fell about 60 percent against 2017, it was not detected at all in the F322W2 filter, and only at the 1-sigma level in F150W2. One ring thickening and the other fading were measured on the same night, with the same instrument, against the same star.[1]
Put the scale in place. Chariklo's radius is about 125 kilometres; its rings sit roughly 390 and 405 kilometres from the centre and are only a few kilometres wide. From two billion kilometres away we are measuring how much light a strip a few kilometres across lets through, and that requires knowing Chariklo's orbit, the star's position from Gaia, and JWST's own path around L2 with extraordinary precision. This is the first stellar occultation planned specifically for JWST and observed successfully. The near infrared out to about 5 micrometres is a window no ground-based occultation has reached. That is exactly what makes it valuable: the same rings were seen for the first time in another colour. For the same reason, the interpretation gets harder.[1]
Three explanations and one wavelength
The authors set out three scenarios. The first is the most ordinary: the rings are azimuthally variable and JWST happened to sample a dense stretch of C1R and a sparse stretch of C2R. With a simple azimuthal density model they put the chance of drawing such a pair at both ingress and egress very low, below about 0.002 for C2R. The second is a genuine change in material: the total amount or the effective cross-section of the grains rose in one ring and fell in the other. The third is wavelength-dependent grain optics: if sub-micron grains whose scattering efficiency drops at longer wavelengths dominate, C2R would look fainter in the infrared than in visible light with no material lost at all. The scenarios are not mutually exclusive; the ordinary one fails the first test, the second and third are still standing.[1]
My reading is this: the wavelength explanation can carry C2R's fading on its own, but it struggles to carry C1R's thickening. Small grains blocking less light in the infrared makes a ring look fainter; the same effect is not expected to make the other ring look more opaque. So the plainest account of the 4.2-sigma rise in the inner ring is what the team proposes, replenishment or a dynamical restructuring. That is an inference, not a measurement; two rings with different grain populations, with larger grains in C1R blocking more infrared light, could produce the opposite trends without any change in material. The equivalent widths holding roughly steady between 2013 and 2017 and then moving between 2017 and 2022 points to a timescale of a few years, and the episodic activity and temporarily bound structures seen around Chiron suggest that ephemeral rings may form a class among small bodies.[1]
The observation that decides
In the authors' own words, a single near-future occultation measurement in visible light, different from JWST wavelengths, could confirm the fading of C2R and the strengthening of C1R. The test is clean: if the outer ring is also fainter than in 2017 in visible light, material is really leaving; if it is back at its old brightness, the problem lies in the colour of the grains. That measurement, which waits on a star, a rock and a telescope lining up, gives the first true time series of how rings arise and fade over years around the smallest ringed body in the Solar System. In 2013 we did not know these rings existed; a decade later we are asking whether one of them is disappearing. How long does a ring last?[1]