Jupiter, the biggest planet in the solar system, boasts the shortest day of every planet. It completes 1 full rotation on its axis in just under 10 hours— approx 9 hours and 55 minutes. The fast rotation is due to Jupiter's composition and size. Earth has a solid surface, but Jupiter is a gas giant which is primarily made up of hydrogen and helium. Its massive size and lack of a solid surface allows it smoothly to spin faster.
Jupiter's fast rotation is also influenced by its internal structure. The planet’s core likely rotates more slowly, but the outer layers, that are composed of gases, experience differential rotation. Which means that different parts of the planet’s atmosphere move at different paces. The equator of Jupiter, for eg. spins faster than the poles. This rapid rotation creates a powerful magnetic field, strong weather patterns, and the iconic bands of clouds that we observe.
The planet’s heavy size also plays a role in its fast spin. Jupiter's immense mass leads to a stronger gravitational pull, which results in a more enhanced conservation of angular momentum. In easier words, as the planet formed from a rotating disk of gas and dust, the conservation of angular momentum causes it to spin more quickly.
Despite its rapid rotation, Jupiter’s day is more short than Earth's by a significant margin. In comparison, Earth takes 24 hours to complete a rotation, while Jupiter does it in just under 10 hours—which truly makes it a unique and fascinating feature of our solar system.