Discovery’s Ray Villard reports that the massive relative size of Pluto’s largest moon, Charon, might indicate that it is not a moon at all. It is possible that Charon and Pluto are actually “binary planets,” orbiting a central mass at the center of the system.
Charon is 12 percent as large as Pluto, which is huge for a moon. (Our moon is only one percent as large as Earth.) Pluto’s four other moons, on the other hand, are only a fraction of that size.
Orbit also adds something to the argument. Pluto and Charon waltz across space, pivoting as they orbit, around the center of Pluto’s system. Villard points out that Earth and the moon do this also, but the two bodies in that case pivot around the center of the Earth system, which exists INSIDE Earth’s planetary radius. In the case of Pluto, the two bodies dance around a black center that exists between Pluto and Charon.
Binary planets would be a revolutionary concept. Binary star systems are very common — about half of the stars in the sky consist in co-orbital pairs. There are also binary asteroids. It’s possible that Pluto and another icy planet collided eons ago, spawning Charon and the other four (so far discovered…) moons.
The four smallest of Pluto’s moons were discovered by the Hubble Space Telescope over the past seven years. Charon was discovered in 1978.
This is not the first time scientists have proposed classifying Pluto-Charon as a binary planet. In 2006, the International Astronomical Union briefly considered it when they demoted Pluto to dwarf planet status.