Physics - Rotation Question with Solution | TestHub

For a planet revolving around the Sun, the gravitational force exerted by the Sun on the planet is a central force. A central force always passes through the center of rotation (the Sun in this case). Therefore, the torque due to this force about the Sun is zero.

According to the principle of conservation of angular momentum, if the net external torque on a system is zero, its angular momentum remains constant. Hence, the angular momentum of the planet about the Sun remains constant.