Aug 23, 200602:24 PMThe Life

Is Pluto-Charon a double planet?

If tomorrow's vote goes well, there may be something new under the sun.

Or over the sun.

Or somewhere around the sun. Don't confuse me.

From the International Astronomical Union site:

Q: Why is Pluto-Charon a "double planet" and not a "planet with a satellite"?
A: Both Pluto and Charon each are large enough (massive enough) to be spherical. Both bodies independently satisfy the definition of "planet." The reason they are called a "double planet" is that their common centre of gravity is a point that is located in free space outside the surface of Pluto. Because both conditions are met: each body is "planet-like" and each body orbits around a point in free space that is not inside one of them, the system qualifies to be called a "double planet."

[Finally, here's a fascinating Pluto-Charon orbit simulation, courtesy of George Greenstein and his students at Amherst College.]