The motion of planets around the sun in the solar system is governed by the gravitational force, which acts as a central force directed towards the sun. Because the line of action of this force passes through the sun (the axis of rotation), the torque exerted by the gravitational force on the planet is zero ($\tau = \mathbf{r} \times \mathbf{F} = 0$). According to the principle of conservation of angular momentum, if the net external torque acting on a system is zero, its angular momentum remains conserved ($L = \text{constant}$). This principle also forms the basis of Kepler's second law of planetary motion.