Webb occultation shows Chariklo's two rings changing in opposite directions

An occultation records one quantity: how much of a background star's light a ring blocks as it passes in front. For Chariklo that is the main way to see the rings at all, because they are too faint to observe directly and good alignments are rare ^[6]. The 2013 detection worked that way. The star dimmed twice before the asteroid covered it and twice after ^[4]. Later observations showed two thin, sharply defined rings only 14 kilometres apart, possibly containing water ice, around a body 302 kilometres across that is not very spherical ^[5][3].

Taking the measurement to Webb meant predicting three uncertain positions at once: the telescope's, Chariklo's, and that of a star drawn from the Gaia mission's catalogue ^[7]. "At the beginning I felt like a crazy guy proposing this," said Santos-Sanz, an astrophysicist at the Instituto de Astrofisica de Andalucia in Granada ^[9][18]. When he saw the results, he asked himself, "Oh, what has happened here?" ^[19]

The finding compares one Webb event with earlier observations. "The opacity of the outer ring is now lower than in the previous observations, but also the opacity of the inner ring was stronger," Santos-Sanz said ^[2]. The Eos account does not give a figure for the outer ring's decline, an uncertainty on the inner ring's 42% rise, or the number of earlier occultations in the baseline ^[11].

The two candidate explanations differ over the inner ring's material ^[22]. In one, particles there are colliding and changing how much light they reflect, while outer-ring particles drift away. In the other, an unseen satellite holds both rings in their orbits and supplies the inner ring with extra material ^[22].

Heikki Salo, an astronomer at the University of Oulu in Finland who was not involved in the work, thinks the change might not be so inexplicable after all ^[17][16]. "The perturbations induced by the highly nonspherical shape of Chariklo make most orbits in its vicinity highly unstable," he said ^[16]. The rings orbit in a particular gravitational resonance ^[17]. "The existing observations of such miniature ring systems are still very sparse, compared to the vast amount of data collected of their giant-planet analogues," Salo said ^[15].

Eos ran the result under the subhead "Rings Might Not Be Forever" ^[20]. The data support a narrower statement. Discovery and the Webb event are about nine years apart, so the observed change happened within that span ^[1]. A before-and-after comparison cannot tell a ring that is steadily dispersing from one that brightens and fades around a stable average. I think the result shows that Chariklo's rings change on a scale of years. How long they last is a question for a time series, and building one depends on future occultations from the ground or from space ^[14].