This image shows Uranus’s very faint rings, which were discovered in 1977. Extremely dark, they may be made of countless fragments of water ice containing radiation-altered organic material. (NASA/JPL/Calvin Hamilton/Kinetikon Pictures)

Storms have clouded Uranus’s normally placid face. In the past year, the gas planet has played host to huge cloud systems so bright that even amateur astronomers can see them from Earth — and their cause is a mystery.

“We have no idea. It’s very unexpected,” says Imke de Pater of the University of California at Berkeley.

De Pater observed Uranus last August and was surprised to spot unusually bright features, the hallmark of clouds condensing in the planet’s upper atmosphere. “It was brighter than anything we had ever seen in Uranus’s atmosphere before,” she says.

The planet’s weather generally picks up at its spring and autumn equinoxes every 42 years, when the sun shines on its equator. But the last equinox was seven years ago, so the recent spike in activity is difficult to explain.

De Pater’s group spread the word, and amateurs around the globe trained telescopes on Uranus; some of them spotted a storm that de Pater had imaged at a different wavelength. Using the Hubble Space Telescope, de Pater and colleagues saw storms spanning a variety of altitudes, a phenomenon that may be linked to a vortex deep in Uranus’s atmosphere.