James Webb: Chariklo's Rings Are Changing in Years
Comparing new James Webb images with measurements from the past decade shows two narrow rings around the small icy body Chariklo changing in opposite directions within just a few years.
A team of scientists led by the Institute of Astrophysics of Andalusia (IAA-CSIC) has reported that the rings of the small icy body Chariklo are changing on a timescale of just a few years. Chariklo is a body roughly 250 kilometers across that orbits the Sun between Saturn and Uranus, at about 17 astronomical units, and is known for two narrow rings, a rare feature among such small objects in the solar system. The study was published on September 9, 2026, in the journal Science Advances, under the title “JWST stellar occultation reveals unexpected changes in Chariklo’s ring system.”
The discovery came from comparing new data from the James Webb Space Telescope with measurements collected over the past decade. In 2013, Chariklo became the first small body beyond the giant planets found to have rings, a finding confirmed by a paper published in Nature in 2014 after a star dimmed twice, symmetrically before and after the body itself, as Chariklo passed in front of it. Until then, rings were thought to belong exclusively to giant planets such as Jupiter, Saturn, Uranus and Neptune.
How Was the Occultation Captured?
On October 18, 2022, IAA-CSIC led an observation of a stellar occultation, a technique that measures the dip in a star’s brightness as a body and its rings pass in front of it. It was the first occultation ever predicted and planned specifically for James Webb. At the time, Chariklo was moving at 2.5 kilometers per second relative to the telescope, which sits at the L2 Lagrange point, about 1.5 million kilometers from Earth, on the side facing away from the Sun. The Near-Infrared Camera (NIRCam) captured the event through filters at roughly 1.5 and 3.2 micrometers. Of 14,827 images taken in total, the analysis relied on 1,646 taken around the predicted moment of occultation, according to SpaceDaily.
The research was led by Pablo Santos-Sanz of IAA-CSIC, with colleagues Yücel Kilic, Celia Navas and Nicolás Morales from the same institute, and teams from Spain, Brazil, France, Hungary and the United States.
“Comparing the James Webb observations with those obtained during other stellar occultations over the past decade allowed us to discover opposite changes in the two rings: while the inner ring shows significantly greater opacity, the outer one shows less,” Santos-Sanz said in an IAA-CSIC statement carried by El Debate.
What the Numbers Show
The inner ring, labeled C1R, is 6 to 7 kilometers wide and sits about 390 kilometers from Chariklo’s center. The outer ring, C2R, is 2 to 4 kilometers wide, about 405 kilometers from the center, with a gap of roughly 9 kilometers between the two. According to SpaceDaily, the only source among those available that cites specific figures from the paper, the inner ring became about 50 percent more opaque compared to 2017 measurements, while the outer ring became about 60 percent less opaque. The radial position of both rings, the same outlet reports, stayed unchanged within the margin of measurement. Neither the IAA-CSIC statement nor the coverage on Yahoo and El Debate cite those percentages, speaking instead in general terms of “significantly greater” and “lower” opacity.
A change in opacity does not necessarily mean there is physically more or less material in a ring. Opacity reflects how strongly the material blocks the light of the star behind it, and it is affected by the amount of material, the size of the particles, and the wavelength at which it is observed.
“Our results force us to reconsider how rings form, how they evolve, and what mechanisms keep them stable,” Santos-Sanz told Space.com.
The discovery is also a technological achievement. “To achieve that, we had to know Chariklo’s orbit, the star’s position, thanks to the European Space Agency’s Gaia mission, and the trajectory of James Webb itself around the L2 Lagrange point, all with extreme precision,” explained Yücel Kilic, a postdoctoral researcher at IAA-CSIC, in the institute’s statement.
What This Doesn’t Prove
The cause of the changes remains an open question, and the authors themselves point to three possible explanations that are not mutually exclusive: a genuine evolution of the material in the rings over time, a difference stemming from the fact that James Webb observed in infrared while earlier measurements were made mostly in visible light, or a combination of both effects. SpaceDaily also reports an estimate that, if the loss of opacity in the outer ring is real, it would mean the material is dispersing on a timescale of only a few months to a year without some mechanism sustaining it, which on its own does not explain why its radius has not changed since the 2013 discovery.
The result rests on a single study and a single new occultation, not on a series of repeated, independent measurements. The authors themselves stress that only additional observations, including a future occultation in visible light, can separate a real change in the material from a wavelength effect. Until then, it also remains an open question how the two narrow rings hold their shape at all, since there are only hypotheses for that, such as small shepherd moons or resonances tied to Chariklo’s own irregular shape.
