Planets acquire their rotation when they are formed, taking angular momentum from the impacts that shape them. There is a relationship between the mass of a planet and the speed of its spin, except where tidal forces have decelerated planets and made their days longer.
Main Story: Length of Alien Planet's 'Day' Clocked for First Time, an 8-hour World
Exoplanet Beta Pictoris b is the fastest-spinning known planet.
Beta Pictoris b in Pictures: An Alien Planet Image Gallery
At between 8 million and 20 million years old, the Beta Pictoris system, 63.4 light-years away, is still in its formative stage. Young planets are still accreting as planetesimals collide in the debris disk that surrounds the star.
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