“It’s tentative,” he says, but “it’s the first evidence for water clouds” outside our solar system. Even within the solar system, observers can see water clouds on only Earth and Mars; the giant planets are so cold that ammonia ice clouds cover the water clouds on Jupiter and Saturn while the atmospheres of Uranus and Neptune block the view there.

Observers have previously discerned water vapor in the atmospheres of extrasolar planets, but Fortney says water clouds are a new phenomenon. “One of the things we don’t really know is how common partly cloudiness is,” he says. Venus, whose clouds consist of sulfuric acid, is totally cloudy, whereas Earth is partly cloudy. Faherty says the brown dwarf is also partly cloudy: About half is obscured by clouds.

Verifying the discovery will require spectra. Because the object is so dim, this will likely await the James Webb Space Telescope, which will be launched later this decade.

Water clouds tentatively detected just 7 light-years from Earth