Classification by hydrogen content

Jupiter/Saturn: mostly H/He

Uranus/Neptune: ~ 10% H/He, rest of H bound with C,N,O (~20% total)

Earth/Mars: ~ 0.1% H, bound with O

Venus/Mercury: ~ 0%, dehydrated

...take Ceres, recently shown to be ~ 25% H2O, or ~ 2.78% H, bound. Very similar to Triton/Pluto, but not as fluffy as comets like Tempel 1. Ceres is a borderline planet IMHO, the end product of a series of accretional events. Comets and similar cosmic debris aren't accretion products - something else enabled them to come together as porous masses. Perhaps they're gigantic "dust bunnies"?

Ganymede, Titan and Callisto typically get called planets if not for their inconvenient bond with a more massive primary. They're roughly ~ 65/35 ice/rock, by mass, and so are ~ 6.8% bound hydrogen. If they orbitted in independent solar orbits would they be planets???

Alan Stern is right that the scattered oligarchs are different - they'd've accreted into bigger planets if left unperturbed - but according to some theories so would've Uranus and Neptune. Do we demote them because they didn't pull in huge H/He envelopes???

Adam