"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.

Below is a diagram and two movies that show the motion of Pluto and Charon in their orbits. These diagram shows the orientation of the orbits around the time in its 248-year orbit when Pluto was closest to the Sun (in 1989).

For further discussion of this period of "mutual eclipses" click here. For orientations at different locations and times in Pluto's orbit, click here.

Most Planets have north poles that point roughly up and out of their orbit planes - except Uranus and Pluto, which rotate on their "sides." Like most satellites, Charon orbits above Pluto's equator. From 1985 through 1990 Pluto's equator and Charon's orbit plane were aligned with the line of sight from Earth. Charon would pass in front of Pluto every 6.4 days.

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.

Click here to make a model of the Pluto/Charon system