Webb caught Chariklo's rings changing in five years
Key takeaways
- Chariklo is a 250km Centaur orbiting between Saturn and Uranus, and in 2013 it became the first non-planet found to have rings.
- A Webb occultation from 2022 shows the dense inner ring roughly 50 percent more opaque than before, while the outer ring is about 60 percent less opaque than in 2017.
- Rings on small bodies were expected to disperse quickly or stay static, not evolve on timescales short enough to watch.
Chariklo is about 250 kilometres across, which is shorter than Tasmania is long. It is a Centaur, an icy body orbiting between Saturn and Uranus, and in 2013 it became the first object that is not a giant planet ever found to have rings. That was startling enough on its own.
Webb has now found the rings are not stable. A study led by the Institute of Astrophysics of Andalusia, published in Science Advances on 9 September, compared a 2022 Webb occultation against earlier measurements. The dense inner ring came out roughly 50 percent more opaque than before. The outer ring appeared about 60 percent less opaque than it did in 2017.
Five years is extremely fast for this
The expectation for rings around small bodies ran in two directions, and neither matches what Webb saw. With so little mass to shepherd the material, the rings should either disperse quickly or sit essentially unchanged for very long stretches.
Instead something is redistributing material around a rock the size of a small country, on a timescale short enough that two sets of observations five years apart disagree. Nobody has settled what that something is.
Why Chariklo rings changing matters
Ring systems were a giant-planet phenomenon, then a rare curiosity once Chariklo turned up with two of them, and now they look like an active process. Each revision has made the outer solar system stranger than the version before it, which is a pattern worth noticing. The same run of surprises produced liquid nitrogen apparently flowing on Pluto and the youngest planet ever found around Elias 2-24B.
What to watch
More occultations. Chariklo passing in front of a background star is the only way to measure these rings at all, and each event adds a data point to a series that currently has very few. The next few will show whether the change is steady, cyclical, or a one-off event caught by luck.
None of this is visible from a garden. Chariklo is far too faint and far too small. Saturn's rings, on the other hand, resolve in almost anything, and a pair of 7x50 astronomy binoculars is enough to see the planet as something other than a dot.
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