Pluto and Charon

Pluto has satellites, or more correctly, objects that co-orbit the Sun with Pluto. The largest of these objects is Charon, which is half the size of Pluto. Why do I say it co-orbits around the Sun rather than orbiting around Pluto? It has f\to do with something called the barycenter of the two bodies.

The easiest way to explain barycenter is it is the balance of mass between the two objects. Think of a seesaw (or teeter-totter) with a person on either end. If both people are the same weight, the seesaw is perfectly balanced with the center of mass being at the fulcrum of the seesaw. However, if one person weighs more then the other person, the seesaw is no longer in balance, and the center of mass shifts towards the heavier person. To get the seesaw back in balance, the heavier person needs to move closer to the fulcrum. Therefore the center of mass is closer to the heavier person.

The same thing is true in orbiting objects. In the Earth-Sun system, the center of mass is closer to the Sun. In fact, the Sun is so much more massive, that the barycenter is within the Sun itself. This is true for all planets orbiting the Sun. In the Earth-Moon system, the Earth is about 83 time more massive than the Moon, so therefore the barycenter for the Earth-Moon is about 4700 km from the center of the Earth (about 75% the Earth's radius). In general, you can look at all the planets and their moons and find the same thing: the barycenter for a planet-moon system will lie within the radius of that planet. (The Jupiter-Sun system is an exception. However, the barycenter is just outside the surface of the Sun.)

However, Pluto and Charon are different. Charon is about half the mass of Pluto. Therefore, the barycenter for Pluto and Charon is a third of the total distance between the two planets, well outside Pluto's radius. This gives us another argument against Pluto being a planet. Charon is not truly a satellite of Pluto since their center of mass is not within Pluto. Pluto and Charon are more like co-orbiting bodies around the Sun.