1. The problem statement, all variables and given/known data Two identical planets (equal masses, m) move in identical circular orbits around a star (mass M) diametrically opposed to each other (opposite sides of the planet). Find an expression in terms of m, r, M and G for the orbital period T. 2. Relevant equations T^2=(4pi^2/GM)r^3 F=Gm1m2/r^2 3. The attempt at a solution I haven't really gotten to far on this because I can't decide if the planets are affecting each other. It would seem that they are because all mass exerts a gravitation force on other mass. But they are not orbiting around each other...So the force between the two is irrelevant to the problem?