Which planet has the shortest day?

Jupiter turns once in about nine hours and fifty-six minutes, despite being by far the largest planet - the surface at its equator is moving at something like 45,000 kilometres an hour. Spinning that fast visibly deforms it: Jupiter is noticeably flattened, about seven per cent wider across the equator than pole to pole, and you can see it in a small telescope.

It also does not have one rotation period. Being a ball of gas, Jupiter's equator rotates several minutes faster than its higher latitudes, so astronomers use three official systems - and the one used as the real answer, System III, is based on the rotation of its magnetic field, since that is anchored to the interior rather than to the weather.

The spin drives the stripes. Fast rotation stretches the atmosphere into bands running parallel to the equator, with neighbouring bands moving in opposite directions and jet streams tearing along the boundaries between them. The Great Red Spot sits wedged between two of those opposing flows, which is a large part of why a storm can last for centuries rather than days.