Abstract: Many planets have rotational motions. In a reference frame rotating with the planet，the mass point

of the planet is subjected to an inertial centrifugal force，which makes it appear as a flat rotating ellipsoid. The faster

the planet rotates，the greater the centrifugal force of inertia is. When the speed is greater than a critical value，

the planet will break. Based on the principle of minimum potential energy，the upper limit of planetary rotation

　　speed is strictly calculated.

Key words: planet, angular velocity, gravity, centrifugal force of inertia, principle of minimum potential energy