The Earth-Moon system has often been characterized as having some characteristics of a “double planet” system. It is demonstrated that while the orbital barycenter of the Earth-Moon pair lies inside the radius of the Earth, the Moon does meet all three requirements of the IAU definition for “planet” and therefore the Moon can correctly be identified as the Solar System’s 9th planet. In order to avoid confusion by this development it is necessary to add definitions for “double planet”, “double dwarf planet”, “satellite planet”, and “satellite” to complement the International Astronomical Union definitions for “planet” and “dwarf planet”. The Earth-Moon system meets the requirements of a “double planet” system while the Pluto-Charon system meets the requirements of a “double dwarf planet” system. In order to extrapolate sub-stellar taxonomy to exoplanetary systems, general formation mechanisms (star-like gas collapse in molecular clouds vs. proto-planetary disk formation) should be included in the definitions for the various classes of sub-stellar mass bodies.
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