With only two lectures in my back pocket, I still can't find a reasonable solution for this problem. Suppose that a planet were discovered between the sun and Mercury, with a circular orbit of radius equal to 2/3 of the average orbit radius of Mercury. The orbital period of Mercury is 88.0 days. What would be the orbital period of such a planet? and also, in general: Four identical masses of mass 500 kg each are placed at the corners of a square whose side lengths are 15. cm. What is the magnitude of the net gravitational force on one of the masses, due to the other three?