In 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 http://news.discovery.com/space/private-spaceflight/pluto-bound-nasa-probe-wakes-from-electronic-slumber-141207.htm, I did some more research. In July (http://news.discovery.com/space/is-charon-a-moon-of-pluto-or-a-binary-planet-120715.htm) and August (http://news.discovery.com/space/can-we-call-pluto-and-charon-a-binary-planet-yet-140808.htm), discovery.com 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.