pluto-charon for binary planet

Peter @LowellObservatoryIn my Dec 3 facebook post of me next to the Pluto-discovering telescope at Lowell Observatory, I hashtagged #PlutoCharonForBinaryPlanet.  While facebook finds no other instances of that exact hashtag, the concept isn’t new.

Seeing the beautiful pictures of Pluto-Charon in, I did some more research.  In July ( and August (, had articles on whether or not Pluto-Charon is a binary planet; and Wikipedia has the Double planet article (I prefer “binary” to “double”, for parallelism to “binary star”).

Despite the Wiki article’s point that Earth-Luna would become a binary planet some in a few hundred-million years under the definition, I still think center-of-mass provides the best definition.  Maybe a modified definition, which includes center-of-mass and some ratio of the smaller mass to the larger mass: Charon:Pluto = 1/9, whereas Luna:Earth = 1/81, so you could probably find a dividing line.  Though, once again, it may be that when we have a statistically-valid selection of extra-solar planets to clarify our solar-system’s small sample size, we may find that both 1/9 and 1/81 would classify as double-planets.  Really, I’d rather classify Earth-Luna as a binary planet in 100Myr, rather than classify Charon as a moon of Pluto.