A stellar occultation that took place on October 18, 2022, made it possible to study asteroid Chariklo observing its rings as well, a truly peculiar feature for such a small object. A team of researchers used the James Webb Space Telescope to capture Chariklo’s passage in front of a star from Webb’s point of view. The NIRCam and NIRSpec instruments enabled images and spectrographic features of Chariklo to be obtained. In recent days, the NIRISS instrument had a software problem that blocked the transmission of the collected data but no hardware failures were found, therefore it’s possible that the data will be recovered later to complete an in-depth study of this asteroid.
Discovered in 1997, Chariklo is a centaur asteroid, which means that it belongs to a class of icy planetoids whose orbit is between Jupiter and Neptune. These orbits are unstable due to the proximity of the two planets, so it’s probable that after a few million years, a centaur will end up out of orbit.
The surprising discovery of two rings around asteroid Chariklo happened in 2013 using ground-based telescopes. This was possible thanks to a stellar occultation when Chariklo passed in front of a star from the Earth’s point of view. This was a stroke of luck, and in the absence of occultations, it’s now impossible to detect Chariklo’s rings even for the James Webb Space Telescope.
Help arrived on October 18, 2022, from a star so anonymous that it’s known only as Gaia DR3 6873519665992128512, a designation from the Data Release 3 of the map created thanks to ESA’s Gaia space probe. Asteroid Chariklo passed in front of that star from Webb’s perspective and its instruments were able to detect the asteroid and its rings’ transit.
The top image (NASA, ESA, CSA, Leah Hustak (STScI). Science: Pablo Santos-Sanz (IAA/CSIC), Nicolás Morales (IAA/CSIC), Bruno Morgado (UFRJ, ON/MCTI, LIneA)) shows the occultation light curve of the background star caused by the passage of asteroid Chariklo captured by the James Webb Space Telescope’s Near-infrared Camera (NIRCam) instrument. Shadows were cast by both Chariklo and its two rings as predicted by astronomers.
The bottom image (NASA, ESA, CSA, Leah Hustak (STScI). Science: Noemí Pinilla-Alonso (FSI/UCF), Ian Wong (STScI), Javier Licandro (IAC)) shows the spectrum captured by the James Webb Space Telescope’s Near-infrared Spectrograph (NIRSpec) on October 31, 2022, shortly after the occultation. This is an important result because the infrared emissions contain chemical ‘signatures’, in this case showing the presence of water ice on the asteroid’s surface and as an important constituent of its rings.
In 2013, seven astronomical observatories were used to exploit the occultation and in October 2022 the most powerful telescope ever built was used to exploit another phenomenon of that type and study asteroid Chariklo. This gives an idea of how difficult it is to examine such a small object several hundred million kilometers away. Chariklo is interesting because the presence of the rings is an anomaly, as they’re typical of planets.
Astronomers want to understand how rings are formed around an asteroid, so these preliminary results will be examined and in the coming months the researchers will publish a paper on this topic. The studies will continue in case of future occultations. Those events are also useful for studying other asteroids and dwarf planets that can be much more distant than Chariklo.
