Usually there's not much to see on Uranus, says astronomer Imke de Pater of the University of California, Berkeley. But last year was its stormiest on record.

Ever since its equinox in 2007, when the Sun shined directly on its equator, the seventh planet has been becoming more active. Last year it hit a new peak.

When analysing infrared images of Uranus, Prof de Pater's team noticed eight large swirling storms in its northern hemisphere in August 2014. One of these storms was the brightest ever observed. It reflected 30% as much light as the rest of the planet, the team reports in the journal Icarus.

Nobody had expected it, says de Pater. It shows how little we understand even about planets inside our own Solar System.

The team analysed bright patches on images of Uranus. These spots of light represent clouds.

They deduced how thick the clouds were, and how high up in the atmosphere. From the altitude they could then infer what the clouds were made of.

The clouds they saw were extremely high up. As they rose ever higher, methane gas condensed into methane ice, causing the clouds to glow.

"The very bright one we saw high in the atmosphere must be methane ice," says de Pater. "Another one observed by amateur astronomers could be hydrogen sulfide."

Uranus takes 84 Earth years to travel around the Sun. For half this time one of its poles is in darkness. But during the 2007 equinox each pole was equally lit up, and astronomers expected that this change in illumination would cause a particularly stormy year.

While they did see some turbulent weather, it was nothing compared to the storms of 2014.

Mystery squalls

"We have no idea why this is happening right now," says de Pater.

The storms might be driven by the changing seasons, but to find out we would need to see if they also occur over the southern hemisphere. That will take many more years. "I don't think I will live enough to see the whole cycle of Uranus," she adds.

While we frequently see images from Mars, Jupiter and Saturn, Uranus has only ever been fleetingly visited by one space craft, Voyager 2. But that was in 1986 and it only observed a "featureless haze" of dense clouds.

That's why scientists rely on images taken at the ground-based Keck observatory in Hawaii. Increasingly, they also combine these with images taken by amateur astronomers, as their telescopes are powerful enough to see Uranus.

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