Rings around a tiny body have changed over the past decade

Text settings Story text Size Small Standard Large Width * Standard Wide Links Standard Orange * Subscribers only   Learn more Minimize to nav For decades, astronomers thought rings were something only giant planets had. That changed in 2013, when a small, dark body orbiting between Saturn and Uranus passed in front of a star and blinked twice on either side of the main event, revealing two narrow rings around an object barely 250 kilometers across. “It was a surprise,” says Pablo Santos-Sanz, an astronomer at the Instituto de Astrofísica de Andalucía in Granada, Spain. Ever since, the question has been what such rings are made of and how long they can last. In a recent study, Santos-Sanz and his colleagues used the James Webb Space Telescope to watch the same body, now known as Chariklo, pass in front of a background star again. They found one of its rings had grown denser and the other had almost vanished. We don’t know exactly why. Shadowing a star The technique behind the observation is simple. “We predict when a Solar System object passes in front of a star,” Santos-Sanz said. The starlight dims for a moment, and the shape of that dip reveals the size, shape, and surroundings of the object that caused it. “This is particularly challenging for minor bodies, and more challenging for distant minor bodies,” he said. The difficulty is that the target’s silhouette on the sky is minuscule, and knowing when it will cross a particular star requires very precise positioning data for both. Still, in 2013, we used ground telescopes to discern Chariklo’s two rings, C1R and C2R. These sat 390 and 405 kilometers from its center and were only a few kilometers wide and about 7 kilometers apart. Doing this with a space-based telescope, though, makes lining things up considerably harder. JWST sits at the L2 Lagrange point, and controllers need to nudge the telescope every few weeks to keep its orbit stable. “It’s a kind of tricky task,” Santos-Sanz said. His team identified a possible Chariklo occultation in August 2022 and redid the prediction every week. Between the first prediction and one of the last, the projected line of sight shifted by about 110 kilometers, which was enough to move it off the body entirely. Unfortunately, JWST requires observations like this to be planned at least 14 days in advance. “We did this maybe a bit blindly, because we didn’t know exactly where the line of sight was,” Santos-Sanz said. “I’m going to move one of the biggest, best telescopes in space, and we don’t know if finally we will catch this or not.” But it all worked out.