For decades, astronomers thought rings were something only giant planets had. That changed in 2013, when a small, dark body orbiting between Saturn and Uranus passed in front of a star and blinked twice on either side of the main event, revealing two narrow rings around an object barely 250 kilometres across.
"It was a surprise," says Pablo Santos-Sanz, an astronomer at the Instituto de Astrofisica de Andalucia in Granada, Spain. Ever since, the question has been what such rings are made of and how long they can last.
The object is now known as Chariklo. Its two rings, C1R and C2R, sit 390 and 405 kilometres from its centre. They are only a few kilometres wide and about 7 kilometres apart.
The observing technique is straightforward in principle. "We predict when a Solar System object passes in front of a star," Santos-Sanz said. The starlight dims for a moment, and the shape of that dip reveals the size, shape and surroundings of whatever caused it. "This is particularly challenging for minor bodies, and more challenging for distant minor bodies," he said.
Doing it from space is harder still. The James Webb Space Telescope sits at the L2 Lagrange point and has to be nudged every few weeks to keep its orbit stable, and observations of this kind must be planned at least 14 days in advance. Between the team's first prediction of the Chariklo event and one of the last, the projected line of sight shifted by about 110 kilometres, enough to move it off the body entirely. "We did this maybe a bit blindly, because we didn't know exactly where the line of sight was," Santos-Sanz said.
The occultation came on 18 October 2022. The reconstructed geometry shows the telescope's sightline to the background star skimmed 7.4 kilometres above Chariklo's surface, missing the body but catching its rings. It was recorded simultaneously at 1.5 and 3.2 micrometres, the first time anyone has caught a minor body's rings beyond three micrometres, a range Earth's atmosphere puts out of reach of ground telescopes.
The inner ring showed up unmistakably, with abrupt, distinct edges, but it was far darker than before. Averaged over roughly ten previous ground-based occultations, C1R's normal opacity, the fraction of starlight it blocks, sat at 0.303. Webb measured it at 0.431. "We didn't believe it at the beginning, so we fought a lot with the data," Santos-Sanz said.
The obvious alternative was geometry: rings are not uniform, and the telescope might simply have cut through a denser clump. To test that, the team built a lumpy ring model and ran 10 million simulated occultations. Reproducing an opacity as high as the one recorded came out at roughly a 1 in 1,000 chance at 1.5 micrometres, and 4 in 100,000 at 3.2 micrometres, for a single measurement. The telescope caught the ring twice, going in and coming out.
The outer ring did the opposite. C2R barely registered at 1.5 micrometres and did not appear at all at 3.2. "At the beginning we didn't even see the outer ring in the light curve," Santos-Sanz said. Either the infrared is showing grains that scatter light differently, or the rings have physically changed. "We are witnessing a real evolution of the rings with time," he said. "Of course it is not a certainty, but for me it is the preferred explanation."
The material does not balance. Measured as equivalent width, the inner ring gained about ten times more than the outer ring lost. "We don't know where the extra material is coming from, but there are some hypotheses," Santos-Sanz said. The leading one is a small shepherd satellite sharing the outer ring's orbit, which would also explain the rings' stability and sharp edges. "This satellite has not been detected yet, if it exists," he said.
The accounting is the part worth dwelling on. There are two rings. One shrank. The other grew by ten times as much. Had the material merely migrated inward, the books would balance. They do not. Chariklo has ended the decade holding more than it started with, and the surplus has no declared origin.
This newspaper regards it as an unusually clean result. A 250-kilometre rock with no atmosphere, no industry and no visible trading partners has posted a tenfold gain on a single line and declined to say how.
The leading explanation is a satellite that has never been observed. In the language of the ledger, that is an undisclosed supplier: an entity inferred entirely from the fact that the goods arrived.
The proposed audit is equally orthodox. Astronomers intend to wait until Chariklo passes in front of another star and watch again, which is to say they will establish that the supplier exists by observing the delivery route go dark for a fraction of a second.
The Department of Unnecessary Research notes one further detail from the survey. The rings orbit 390 and 405 kilometres from the centre of a body roughly 250 kilometres across. Chariklo's holdings are larger than Chariklo.