1. The problem statement, all variables and given/known data Suppose that we see a planet in our Solar System that we measure to have an orbital period (around the Sun) of 18.0 years. We look at it with a telescope and see that it has a moon. From repeated observations, when the planet is near or at opposition, we note that the orbit of the moon is approximately circular, with an observed radius of about 1.2 arcminutes and a period of 10 days. What is the mass of this planet? Pplanet = 18.0 years = 567024668 seconds Pmoon = 10 days = 864000 seconds emoon = 0 θmoon = 1.2 arcminutes 2. Relevant equations P2G(M1 + M2) = (4∏2)a3 L = mvr M = rv2 / G 3. The attempt at a solution I simply have no idea where to begin. I want to say that, mplanetvplanet = mstarvstar But that ignores the moon and the earth's positions and what not. Also, I'd have to assume that its orbit is circular, when it doesn't say so in the question.