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Originally Posted by Blue Fire Hmmm... further on in the announcement article, they list Pluto's moon Charon as a Planet (Pluton), and yet they clearly state (in their proposed definition of a planet) that a planet cannot be any body that is a satellite of a planet. Perhaps this is because Pluto and Charon are a kind of "double planet" in that they both orbit around a point that is not at the center of Pluto? Or is the listing of Charon just a typo or some other mistake in the announcment? |
That's because the center of gravity of the Pluto-Charon system is between the two objects, above Pluto's surface. Thus, they together orbit the Sun, as opposed to our Earth-Moon system where the center of gravity is within Earth, so the Moon orbits Earth which orbits the Sun.
That's also how they don't count any of the Galilean satellites nor Titan.