Giants, Worlds, and Rocks

"Planet" doesn't really have a definition. Any definition that
includes Pluto but excludes Ceres, Chiron, and Varuna -- or Ganymede
or Titan -- would be pathetically tortured, simply attempting to
define a category with the a priori goal of including the nine
"planets" in a group and excluding everyone else.

Our other definitions could use some work too. Just as it's absurd to
lump Jupiter into the same category as Pluto, it's equally absurd to
put Titan into the same category as Sinope or Cordelia. Both "planet"
and "moon" are arbitrary terms of accidents of history and celestial
cartography instead of being categories of real object.

But there is a sense that we have a couple different kinds of thing in
the solar system. I'd like to suggest a movement start to separate
the objects of the solar system into three categories:

- Gas giant
- World
- Rock

Gas giant is self-explanatory; the division between "world" and "rock"
is that a world is a body with a solid surface large enough such that
its gravity forces its solid shape into a sphere.

The number of "worlds" in the solar system is roughly thirty (there
are perhaps a dozen asteroids that from measurement of mass and
brightness might qualify, but we don't have sufficiently close
pictures of them to know whether they're worlds or rocks.)

The worlds of the solar system include:

Mercury, Venus, Earth, Moon, Mars
Ceres (assumed), Vesta (probably), Pallas (probably), more?
Io, Europa, Ganymede, Callisto
Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Iapetus
Chiron (maybe?), other centaurs?
Miranda, Ariel, Umbriel, Titania, Oberon
Proteus (probably), Triton, Nereid (maybe)
Pluto, Charon, 2000 EB173, Varuna, almost certainly other plutinos

Honorable mention to Hyperion, which is big enough to be a world but
is a shattered piece of something bigger.

eyelessgame