Just a small addition and a small correction to the Third Law..

Since planets travel in an elliptical orbit there is no constant radius, and for the purposes of Kepler's Third Law, R, the orbital radius takes the value of the semi-major axis of the elliptical path. K is a constant used in planetary mechanics usually denote GMm, in which G is Newton's Gravitational Constant, M is the mass of the larger body, and m is the mass of the smaller body. In this instance, the constant of proportionality referred to by WarMachine as K is defined as (4*pi^2)/GM, where G and M are defined earlier in my write-up.