Why Do The Gas Giants Spin So Fast?

Our solar system is divided into two types of planets: the inner rocky planets and the outer gas giants. Both types of planets are significantly different from one another in almost every respect. Rocky planets are small and contain higher amounts of heavy elements, while gas giants are gigantic worlds composed mostly of lighter elements such as hydrogen and helium. Rocky planets also have defined solid surfaces, while it remains unknown whether the gas giants have any solid surfaces at all. Another significant difference between the rocky worlds and the gas giants is their rotational speed. The rocky planets rotate rather slowly compared to the gas giants. For example, Earth, which spins faster than the other rocky worlds, rotates at a speed of 1,037 miles per hour (1,670 kilometres per hour). Meanwhile, Jupiter, the planet with the fastest rotational speed, spins at a rate of 28,324 miles per hour (45,583 kilometres per hour). That’s over 27 times faster than Earth. 

Reasons For Rotation 

Jupiter has the fastets rotation of any planet in the solar system. Image credit: NASA

The reasons behind the rotational velocity of the gas giants and the rocky worlds are somewhat different. For example, the rocky worlds likely rotate as fast as they do because of planetary impacts in the past. Although the rocky planets would have rotated prior to any collisions with other forming planets, their rotational velocities would have been slower, and planetary impacts would have given them more momentum. The gas giants rotate for a somewhat different reason. The gas giants likely began as rocky worlds with solid cores, yet the outer solar system contained a high amount of light elements such as hydrogen and helium, and so these rocky worlds gradually drew in more and more material. The sheer amount of material that went into the forming gas giants provided them with immense amounts of angular momentum, allowing them to spin significantly faster than the inner rocky worlds.