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Even though brown dwarfs are completely covered in clouds, they're too far away for us to pick out individual clouds like we do on planets within our solar system. But we can still measure how long it takes for a group of clouds to do a lap around the atmosphere ; as clouds come in and out of view they change the brightness of the planet, this lap time depends on two things : how fast the brown dwarf itself is spinning, and how fast the wind is blowing on top of that.