Which planet is the windiest in the solar system?

Neptune, with winds measured at around 2,000 kilometres an hour - faster than the speed of sound on Earth, and by a wide margin the fastest in the Solar System. That is deeply counterintuitive: Neptune receives about a thousandth of the sunlight we do, so whatever drives the weather is not the Sun.

It appears to be internal heat. Neptune radiates more than twice the energy it receives, from a source not fully identified - possibly still cooling from formation, possibly heat released as material separates inside. Uranus, almost a twin in size and composition, radiates almost nothing extra and is correspondingly duller, and nobody is confident why the two differ.

Only one spacecraft has ever visited. Voyager 2 passed in 1989 and found a dark storm the size of Earth, which had disappeared entirely by the time Hubble looked a few years later - the atmosphere generates and destroys features on a timescale of years. Almost everything known about the planet comes from a few hours of flyby thirty-five years ago.