Astronomers using the NASA/ESA/CSA James Webb Space Telescope have captured the reflectance spectrum of the double-ringed centaur 10199 Chariklo and observed the shadows of starlight cast by Chariklo’s thin rings.
This illustration shows what Chariklo and its rings could look like, based on our current understanding. Image credit: NASA / ESA / CSA / Leah Hustak, STScI.
Chariklo is an icy, small body, but the largest member of a class known as the Centaurs.
It is 250-km in diameter and located between Saturn and Uranus about 2 billion km away.
In 2013, a team led by Dr. Felipe Braga-Ribas from Brazil’s Observatório Nacional/MCTI discovered that Chariklo hosts a system of two dense and narrow rings.
The astronomers had been watching a star as Chariklo passed in front of it, blocking the starlight as they had predicted. Astronomers call this phenomenon a stellar occultation.
To their surprise, the star blinked off and on again twice before disappearing behind Chariklo, and double-blinked again after the star reemerged.
The origin of these rings remains a mystery, but they may be the result of a collision that created a disk of debris.
In October 2022, Dr. Pablo Santos-Sanz from the Instituto de Astrofísica de Andalucía and colleagues found that Chariklo was on track for a similar occultation event.
They then used Webb’s Near-Infrared Camera (NIRCam) instrument to closely monitor a star called Gaia DR3 6873519665992128512, and watch for the tell-tale dips in brightness indicating an occultation had taken place.
The shadows produced by Chariklo’s rings were clearly detected, demonstrating a new way of using Webb to explore solar system objects.
The star shadow due to Chariklo itself tracked just out of Webb’s view. This appulse — the technical name for a close pass with no occultation — was exactly as had been predicted after the last Webb course trajectory maneuver.
The Webb occultation light curve, a graph of an object’s brightness over time, revealed that the observations were successful.
The rings were captured exactly as predicted. The occultation light curves will yield interesting new science for Chariklo’s rings.
“As we delve deeper into the data, we will explore whether we cleanly resolve the two rings,” Dr. Santos-Sanz said.
“From the shapes of rings’ occultation light curves, we also will explore the rings’ thickness, the sizes and colors of the ring particles, and more.”
“We hope gain insight into why this small body even has rings at all, and perhaps detect new fainter rings.”
Reflectance spectrum of Chariklo, captured by Webb’s Near-Infrared Spectrograph (NIRSpec) on October 31, 2022. This spectrum shows clear evidence for crystalline water ice on Chariklo’s surface. Image credit: NASA / ESA / CSA / Leah Hustak, STScI / Noemí Pinilla-Alonso, FSI & UCF / Ian Wong, STScI / Javier Licandro, IAC.
Shortly after the occultation, Webb targeted Chariklo again, this time to collect observations of the sunlight reflected by Chariklo and its rings.
The spectrum of the system shows three absorption bands of water ice in the Chariklo system.
“Spectra from ground-based telescopes had hinted at this ice, but the exquisite quality of the Webb spectrum revealed the clear signature of crystalline ice for the first time,” said Dr. Noemí Pinilla-Alonso, a planetary scientist at the Florida Space Institute.
“Because high-energy particles transform ice from crystalline into amorphous states, detection of crystalline ice indicates that the Chariklo system experiences continuous micro-collisions that either expose pristine material or trigger crystallization processes,” said Dr. Dean Hines, an astronomer at Space Telescope Science Institute.
Most of the reflected light in the spectrum is from Chariklo itself: models suggest the observed ring area as seen from Webb during these observations is likely one-fifth the area of the body itself.
Webb’s high sensitivity, in combination with detailed models, may permit us to tease out the signature of the ring material distinct from that of Chariklo.
“By observing Chariklo with Webb over several years as the viewing angle of the rings changes, we may be able to isolate the contribution from the rings themselves,” Dr. Pinilla-Alonso said.
