They're going to kill plutoFollow

It's just that the Earth is so much more massive that the center of mass of the Earth-Moon system is inside Earth. With Pluto and Charon, apparently the center of mass is not inside Pluto, and by this new definition, that makes it a double planet and not a moon plus planet.

The definition works great for rocky planets, but for balls of gas, I'm sure they need some additional criteria such as the radius of the gassy planet is the distance from its center of mass which contains, say, 99% of its mass.

Warning: Math Content Below!

To compute the distance to the center of mass (of the system) from the *larger* object compute:

m*r / (m+M)

where m is the mass of the small planet, and M is the mass of the big planet, and r is the distance *between* planets. Compare this to the radius of the large planet, R, to see if the small planet is a planet or moon.

m*r / (m+M) < R --> small "planet" is called a moon.

m*r / (m+M) > R --> small planet is a planet.

Examples:

Earth and Moon:

m = 7.3E22 kg
r(E-M) = 383000 km
M = 6E24 kg
R = 6400 km

m*r/(m+M) = 4600 km < R --> the Moon is a moon.

Pluto and Charon:

m = 1.5E21 kg
r(P-C) = 40000 km
M = 1.3E22 kg
R = 1500 km

m*r/(m+M) = 4100 km > R --> Charon/Pluto are a double planet.

Both objects orbit the center of mass (in addition to any other motion they may undergo, such as orbits around the local star, let's say and influcences of other planets, etc). Look at the numbers for the Earth-Moon system. Ignoring all else for the moment, it means (so long as I didn't strike a wrong number) that Earth wobbles significantly due to the presence of the Moon. I would never have guessed that.