Charon (moon) - Classification As A Moon or Dwarf Planet

Classification As A Moon or Dwarf Planet

The center of mass (barycenter) of the Pluto–Charon system lies outside either body. Since neither object truly orbits the other, and Charon has 11.6% the mass of Pluto, it has been argued that Charon should be considered to be part of a binary system. However, the International Astronomical Union (IAU) describes Charon simply as a satellite of Pluto.

In a draft proposal for the 2006 redefinition of the term, the IAU proposed that a planet be defined as a body that orbits the sun that is large enough for gravitational forces to render the object (nearly) spherical. Under this proposal, Charon would have been classified as a planet, since the draft explicitly defined a planetary satellite as one in which the barycenter lies within the major body. In the final definition, Pluto was reclassified as a dwarf planet, but the formal definition of a planetary satellite was not decided upon. Charon is not in the list of dwarf planets currently recognized by the IAU. Had the draft proposal been accepted, even Earth's moon would have been classified as a planet in billions of years when the tidal acceleration that is gradually moving the Moon away from the Earth takes the Moon far enough away that the center of mass of the system no longer lies within the Earth.

The other moons of Pluto, Nix, Hydra, S/2011 P 1 and S/2012 P 1, orbit the same barycenter, but they are not large enough to be spherical, and they are simply considered to be satellites of Pluto.