First discovered in 1997, Chariklo is an icy asteroid, one of the building blocks of the solar system, but it’s also known as a minor planet. Either way, it’s the smallest known planetary body to have rings.
About 146 miles/235 km in diameter—so about half as long as the Grand Canyon and twice as wide as the English Channel—Chariklo is a Centaur, an asteroid that orbits the Sun from way out beyond Jupiter and Neptune.
It’s the biggest Centaur we’ve found so far, but from a telescope’s point of view, it’s tiny, it’s distant and its dark. So there’s only one way to reveal it—starlight.
Astronomers have known for almost a decade that Chariklo has rings. A pair of narrow water-ice rings, one four miles/ seven kilometers wide and the other two miles/three kilometers. Its rings orbit at a distance of about 250 miles (400 kilometers) from the center of the body.
That was discovered using the telescopes at the European Southern Observatory (ESO) at La Silla, Chile, which observed Chariklo move in front of a star. That background star’s light lit up Chariklo for a moment, enough to reveal its rings.
A similar feat has just been achieved using JWST, which on October 18, 2022 captured the shadows of starlight cast by the thin rings of Chariklo when it drifted in front of a star. It’s the only way to study Chariklo, which is way too dim and distant to image directly.
Using JWST’s Near-Infrared Camera (NIRCam) instrument the shadows produced by Chariklo’s rings were clearly detected in a light curve, a graph of an object’s brightness over time. From the shapes of rings’ light curves the researchers hope to determine their thickness, size and colors—and perhaps even new fainter rings.
Like most Centaurs, Chariklo isn’t in a stable orbit, moving from comfortably beyond Saturn to the distance of Uranus during one journey around the Sun, which takes about 63 Earth-years for it to complete.