The plane of the orbit of a planet can never change on its own. Why ?

Explanation

As is known, every planet revolves arund the sun in an elliptical orbit. The centripetal force necessary for the purpose is provided by gravitational pull of the sun on the planet. As this force acts towards the centre of the sun, external torque due to this force is zero.
According to the principle of conservation of angular momentum, in the absence of external torque, angular momentum `overset rarr(L)` of the planet shall remain constant in magnitude and direction. Hence, orbit of every planet will be in a fixed plane. The plane of the orbit cannot change on its own.