How Astronomers Weigh a Ring by Watching It Blink

The trick is called stellar occultation, and it is as low-tech in principle as standing in a doorway and watching someone pass in front of a lamp. Astronomers predict when a Solar System object will cross in front of a distant star; the starlight dims for a moment, and the shape of that dip reveals the size, shape, and surroundings of whatever caused it, according to Pablo Santos-Sanz of the Instituto de Astrofísica de Andalucía in Granada, Spain.

That is how a ring system turned up around an object barely 250 kilometers across in 2013, when a small dark body orbiting between Saturn and Uranus blinked twice on either side of its main passage in front of a star. For decades, astronomers had assumed rings were the preserve of giant planets; the discovery rewrote that assumption. Santos-Sanz called it "a surprise."

The same technique produced a new and stranger result. Using the James Webb Space Telescope, he and his colleagues watched the same object, now known as Chariklo, pass in front of a background star again and found that one of its rings had grown denser while the other had almost vanished.

Why that happened is still open. The appeal of occultation is that it turns a star into a backlight and lets a shadow do the measuring. Its difficulty is that minor bodies are small, and distant minor bodies smaller still, so the predicted passage has to be timed precisely. A brief flicker of light is carrying a lot of information.