Image caption The weather on a brown dwarf: molten iron rain, hurricanes and lightning

The first weather forecast for a brown dwarf is in - and it's not looking good up there.

Astronomers predict it will rain molten iron and "snow" hot sand, with lightning and hurricanes likely.

New observations with Nasa's Spitzer telescope reveal surprisingly turbulent storm clouds circling these "failed stars".

The weather report was given at the 223rd meeting of the American Astronomical Society in Washington DC.

It is the most detailed ever for a world outside our Solar System.

"So let's all sing the forecast for our nearest brown dwarf: Let it snow (rocks) let it snow (sand) let it snow (gems)," said Prof Adam Burgasser of the University of California.

Brown dwarfs are "failed stars" which lack the mass to keep fusing atoms and blossom into fully-fledged suns.

They were known to be hot and hostile. But when astronomers pointed Spitzer at 44 different brown dwarfs, they were surprised to find almost half showed clear evidence of weather - dynamic storm systems with unpredictable patterns.

"This is not like Jupiter's Great Red Spot. The storms on brown dwarfs are much more violent and variable. This is weather, not climate," says Dr Aren Heinze, of Stony Brook University, New York.

Image caption Brown dwarfs are more massive and hotter than planets but lack the mass required to become stars

"The rain is too hot to be water. It is probably molten iron or silicates (sand)."

As the brown dwarfs revolved on their axes, the team used Spitzer to look for changes in the brightness coming from their surfaces - signs of patchiness in the cloud cover.

Their results show evidence for a "weather scaling law" which would allow forecasts to be made.

"This makes us 'astro-meteorologists'. We can tell you how cloudy it's going to be, what the temperature is and how windy it's going to be that day," said Prof Burgasser.

"We hope to expand this forecast to exoplanets - we might find weather laws there too."

His team picked one special brown dwarf system - Luhman 16AB, home of the closest pair of brown dwarfs to Earth.

At just 6.5 light-years away, it is the third-closest system of any kind to our Sun, and the closest to the nearby star of Alpha Centauri.

Their telescopes detected hurricane-force winds of 100-400mph, temperatures up to 1,227C (2,240F; 1,500 Kelvin), and found clouds covering 50% of the brown dwarfs' surfaces.

One storm cloud was so huge it covered 20% of its dwarf, whereas the Great Red Spot covers just 1% of Jupiter.

"It turns out the Great Red Spot is not so great after all," said Prof Burgasser.

Another interesting result was that some brown dwarfs rotate much more slowly than expected.

"This means it's possible they harbour undiscovered planets - you could have a planet orbiting a brown dwarf and slowing down its rotation," said Dr Heinze.