# Effect of Charon on Pluto

Even though Pluto is not known as a planet anymore, theroetically it has/had a moon, called Charon. I've heard about something that their size are so close to each other that while Charon rotates around Pluto, it also rotates around a central point.

How does it work exactly?

• Of course, it also has Styx, Hydra, Nix and Kerberos - which also have an effect, just a much smaller one. Commented Sep 30, 2013 at 20:05
• @RoryAlsop Never heard of them. My knowledge is a bit old maybe Commented Oct 2, 2013 at 14:48
• Has a moon, or five. In 1994 a smaller asteroid was found orbiting a lsrger one, and ever since a "moon" does not have to orbit a "planet". So Charon (and now, others) is a moon of Pluto, period. Commented Jan 16 at 0:23

Charon is 11.6% the mass of Pluto, and is (on average) 19,571 km away from Pluto (Source 1, 2). The barycenter is the point at which the masses of the two bodies "balance", and for the Pluto-Charon system it lies at a distance of $$\frac{m_{\text{Charon}}}{m_{\text{Pluto}}+m_{\text{Charon}}}\times\text{distance}=\frac{0.116}{1.116}\times\text{distance}=0.104\times 19,571\,\text{km}=2,034\,\text{km}$$ from the center of Pluto. As Pluto is only 1,153 km in radius, the barycenter lies ~900 km above its surface. The Pluto-Charon system is the only known (minor or major) planet for which this is the case, although the Earth-Moon system will likely satisfy this criteria billions of years in the future (see Is the Moon a Planet? on Physics SE). There are some instances of binary asteroids, however, for which the barycenter lies outside the surface of both bodies.