"Binary planet" is a term often used to describe any pair of worlds that are similar in mass. Each orbits the other around a gravitational balance point that is between the two - a location called the center of mass. When one object has a much bigger mass and the objects are far apart then the center of mass is close to the center of the bigger object and the bigger object hardly moves. This is the case of the Earth orbiting the Sun - the Sun's moves only 0.0003 of its diameter due to the gravity of the Earth in its yearly orbit. In the case of Pluto and Charon, separated by 17 Pluto radii, the ratio of their masses is 8:1 so that the center of mass is outside Pluto.

Other Binaries in the Solar System

Most satellites in our solar system are too small, compared to the planet they orbit, to put the balance point very far from the center of the primary body in the system. But the mass ratio of Pluto to Charon is just 8:1 (compared to typical planet:satellite mass ratios of 10,000:1), and the balance point of Pluto-Charon lies a few thousand kilometers above Pluto, toward Charon.

Although Pluto-Charon is the only recognized binary planet in the solar system, there are binary asteroids and binary Kuiper Belt Objects. Of course, there are also many binary stars in the galaxy as well. New Horizons is expected to be the first mission to visit any kind of binary object.

More graphics and animations can be found at http://pluto.jhuapl.edu/science/everything_pluto/10_binary_planet.html