The planet with the shortest day in the solar system is Jupiter. It rotates on its axis once approximately every 9 hours and 55 minutes, making its day shorter than any other major planet in the solar system. Jupiter's rapid rotation is notable given its massive size, and this fast spin causes it to bulge at the equator.

Here is a comparison of day lengths for major planets:

Planet| Length of Day
---|---
Mercury| 1,408 hours
Venus| 5,832 hours
Earth| 24 hours
Mars| 25 hours
Jupiter| ~9 hours 55 minutes
Saturn| 11 hours
Uranus| 17 hours
Neptune| 16 hours

Among all, Jupiter spins fastest, making its day the shortest.