In high-precision observations, scientists used a new technique from NASA’s James Webb Space Telescope to capture the starlight shadows cast by Charikle’s thin rings. Charikla is an icy small body, but the largest of the known centaur population, located more than 2 billion miles beyond Saturn’s orbit. Charikle is only 160 miles (250 kilometers) in diameter, or about 51 times the size of Earth, and its rings orbit about 250 miles (400 kilometers) from the center of the body.
We asked members of the scientific team observing Charikla to tell us more about this unique system, occultation techniques, and what they learned from observing Webb.
In 2013, using ground-based telescopes, Felipe Braga-Ribas and his colleagues discovered that Charikla contains a system of two thin rings. Such rings were expected only around large planets such as Jupiter and Neptune. Astronomers watched the star as Charikla passed in front of it, blocking out the starlight just as they had predicted. Astronomers call this phenomenon an occultation. To their surprise, the star blinked and turned on twice before disappearing behind Charikle, and blinked twice after the star reappeared. The twinkling was caused by two thin rings – the first rings ever discovered around a small Solar System object.
Pablo Santos-Sanz of the Andalucía Astrophysics Institute in Granada, Spain, has the “Possible Target” (Program 1271) program approved for the eclipse observation attempt as part of Webb’s Guaranteed Time Observation of the Solar System (GTO) led by Heidi Hamel. from the Association of Universities for Research in Astronomy. By an extraordinary stroke of luck, we discovered that Charikla was on track for just such an occultation event in October 2022. This was the first attempt at occultation of the stars made with the help of Webb. A lot of hard work went into defining and refining the predictions of this unusual event.
On October 18, we used Webb’s Near Infrared Camera (NIRCam) to closely observe the star Gaia DR3 6873519665992128512 and watch for telltale dips in brightness that indicate dimming. The shadows cast by Chariklo’s rings have been clearly detected, demonstrating a new way to use the Web to explore solar system objects. A star shadow from Charicle himself tracked beyond Webb’s field of vision. This apulse (the technical name for a close pass without occultation) was exactly as predicted after Webb’s last course trajectory maneuver.
Webb’s light occultation curve, a plot of an object’s brightness over time, showed that the observations were successful! The rings were captured exactly as predicted. Light occultation curves will provide interesting new science for the Charikla rings. Santos-Sans explained: “As we dig deeper into the data, we will investigate whether we are accurately separating the two rings. Based on the shapes of the light occultation curves of the rings, we will also investigate the thickness of the rings, the sizes and colors of the rings. ring particles and more. We hope to gain insight into why this small body has rings at all, and perhaps discover new, fainter rings.”
The rings are likely composed of tiny particles of water ice mixed with dark material, debris from an icy body that collided with Charikla in the past. Charikla is too small and too distant for even Webb to directly image the rings separated from the main body, so occultations are the only tool to characterize the rings themselves.
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