天文既知識

No. the specialized Kepler's 3rd law, which is based on the fact that the mass of the Sun is so big that its motion is negligible, is not valid here. But in the case of Pluto and Charon, their masses are of similar order. They both orbit around their centre of mass. So, we cannot neglect the motion of Pluto. Now, with the information given, we can only calculate the combined mass of Pluto and Charon.

We should apply a generalized form of Kepler's 3rd law:

(P / 2π)2 = a3 / [ G (M + m) ]

where P is the period of the orbit, a is the semi-major axis, M and m are the masses of the two orbiting bodies.

Then the combined mass of Pluto and Charon can be calculated easily.

Reference: see http://en.wikipedia.org/wiki/Kepler's_laws_of_planetary_motion

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