re: A discussion from BA
...However, including Charon, Pluto’s moon, as a planet — even as a pluton – opens up a bigger can of worms. Here is the committee’s reasoning:
For two or more objects comprising a multiple object system, the primary object is designated a planet if it independently satisfies the conditions above. A secondary object satisfying these conditions is also designated a planet if the system barycentre resides outside the primary. Secondary objects not satisfying these criteria are “satellites”. Under this definition, Pluto’s companion Charon is a planet, making Pluto-Charon a double planet.
Via: Bad Astronomy