“Pluto is something much cooler than a mere planet,” argues Mika McKinnon. “It’s the largest dwarf planet we know, and one half of the first binary planet system. Pluto didn’t get demoted, it got promoted.”
When it comes to stars, any time the barycenter of two stars’ orbit is beyond the surface of the primary object, and is instead out in space somewhere, that’s enough to declare them a binary star system. The same is true for asteroids — we’ve found asteroid pairs with barycenters outside both rocks, and declared them binary asteroid systems. Since the barycenter of Pluto and Charon is an empty point in space, surely that means that Pluto-Charon a binary planetary system. This would make Pluto and Charon not only the first binary planet system in our solar system, but the first one we’ve found among the literally hundreds of Kepler exoplanet worlds. […] One final argument in favor of listing Pluto and Charon as a binary dwarf planet system is that they are the undeniable pair dominating all the little moons. Nix and Hydra are the larger of the remaining moons, but are just a tiny fraction of a percent of the size of Charon. Styx and Kerberos are even smaller yet. This family of tiny moons doesn’t even orbit Pluto directly: they all orbit the barycenter between Charon and Pluto.
(Here‘s some Wikipedia background to the double planet issue.)