You are asked to verify Kepler's Laws of Planetary Motion. For these exercises, assume that each planet moves in an orbit given by the vector valued function $\mathbf{r}$. Let $r=\|\mathbf{r}\|,$ let $G$ represent the universal gravitational constant, let $M$ represent the mass of the sun, and let $m$ represent the mass of the planet.
Prove Kepler's Third Law: The square of the period of a planet's orbit is proportional to the cube of the mean distance between the planet and the sun.