In 2013, Felipe Braga-Ribas and colleagues using ground-based telescopes discovered that Chariklo contains a system of two thin rings. Such rings were expected to occur only around large planets such as Jupiter and Neptune. The astronomers watched the star as it passed in front of Chariklos, blocking the starlight they had predicted. Astronomers call this phenomenon occultation. Surprisingly, the star flashed and turned on twice before disappearing behind Chariklo, and blinked twice again when the star reappeared. The glow was caused by two thin rings – the first rings discovered around a small Solar System object.

Pablo Santos-Sanz of the Instituto de Astrofísica de Andalucía in Granada, Spain, has an approved Opportunity Market (schedule 1271) for observing occlusion as part of Webb’s Guaranteed Time Observations (GTO) led by Heidi Hummel. From the Association of Universities for the Study of Astronomy. By extraordinary luck, we discovered that Chariklo was on the right track for such an occultation event in October 2022. This was Webb’s first attempt at obscuring the stars. Much work has gone into defining and improving predictions for this unusual event.

This video shows observations of the star (locked in the middle of the video) as it passes in front of Chariklo by NASA’s James Webb Space Telescope. The video consists of 63 individual 1.5 micron near infrared (F150W) Webb camera observations taken over ~1 hour on October 18. A careful analysis of the star’s brightness shows that the rings of the Chariklo system have been clearly detected. On October 18, we used the Webb Near Infrared Camera (NIRCam) to observe the star Gaia DR3 6873519665992128512 up close and observe the dips in brightness that indicate an eclipse is occurring. The shadows cast by the Chariklo rings were clearly detected, demonstrating a new way of using Webb to study solar system objects. A star shadow from Chariklo himself passed beyond Webb’s field of view. This blow (the technical name for the no-shield close pass) was exactly as predicted after Webb’s final maneuver in the trajectory trajectory.

A dimming curve from the 1.5-micron wavelength (F150W) Webb Near Infrared Camera (NIRCam) shows the star’s (Gaia DR3 6873519665992128512) decrease in brightness as the Chariklo rings pass in front of the star on October 18. As can be seen in the plot of the occultation, the star did not pass behind Chariklo from Webb’s point of view, but passed behind its rings. Each slope actually corresponds to the shadows of two rings around Chariklo, ~4 miles (6-7 kilometers) and ~2 miles (2-4 kilometers) wide, separated by a 5.5-mile (9-kilometer) gap. At each slope of this light curve, the two separate rings are completely indistinguishable. Image source: NASA, ESA, CSA, Leah Hustak (STScI). Science: Pablo Santos-Sanz (IAA/CSIC), Nicolas Morales (IAA/CSIC), Bruno Morgado (UFRJ, ON/MCTI, LIneA)

The rings are likely made up of tiny particles of water ice mixed with dark matter, formed from fragments of an icy body that collided with Chariklo in the past. Chariklo is too small and too far away for even Webb to directly image the rings separated from the main body, so the occlusions are the only tool for characterizing the rings themselves.