From SpaceflightnowSOUTHWEST RESEARCH INSTITUTE NEWS RELEASEPosted: June 14, 2006

Each of our Solar System's outer gaseous planets hosts a system of multiple satellites, and these objects include Jupiter's volcanic Io and Europa with its believed subsurface ocean, as well as Titan with its dense and organic-rich atmosphere at Saturn. While individual satellite properties vary, the systems all share a striking similarity: the total mass of each satellite system compared to the mass of its host planet is very nearly a constant ratio, roughly 1:10,000.

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Jupiter's four Galilean satellites are each roughly similar in size, while Saturn has one large satellite together with numerous much smaller satellites. Even so, the total mass in both satellite systems is about a hundredth of one percent (0.0001) of the respective planet's mass. The Uranian satellite system structure is similar to that of Jupiter, and it also exhibits the same mass ratio. In contrast, the large satellites of solid planets contain much larger fractions of their planet's masses, with the Moon containing 1 percent (0.01) of the Earth's mass, and Pluto's satellite, Charon, containing more than 10 percent (0.1) of its mass.

Ona side note:
One more bit of info to perplex the what is a planet and what type.