• Anthing in the same position as Pluto and travelling at the same speed will trace out the same orbit, however big it is. But if you see Charon orbit about Pluto at a certain distance, with a certain period of orbit, you can compute Pluto's mass. If you can also see the wobble caused in Pluto's orbit, you can work out Charon's mass too. density is just mass over volume.

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