Which planet has the shortest day (fastest rotation)?

Jupiter has the shortest day of any planet because it rotates fastest, completing one turn on its axis in about 9 hours 56 minutes. That rapid spin is remarkable because Jupiter is also the largest planet in the Solar System.

Jupiter is a gas giant made mostly of hydrogen and helium, so it does not have a solid surface whose rotation can be timed in the same way as Earth’s. Astronomers use features in its atmosphere and radio emissions from its magnetosphere to establish its rotation period. Different cloud bands rotate at slightly different speeds, a phenomenon called differential rotation.

The fast spin produces a powerful equatorial bulge: Jupiter’s diameter across its equator is noticeably greater than its polar diameter. It also helps organize the planet’s atmosphere into alternating bright zones and dark belts, with jet streams reaching enormous speeds.

A frequent confusion is between a planet’s rotation period and its orbital period. Jupiter takes nearly 12 Earth years to orbit the Sun, but less than 10 hours to spin once. Saturn is also a rapid rotator, yet its day is longer, at roughly 10 and a half hours.