Quote:
Originally Posted by Leiningen
I had a rather radical idea: Why not say that a moon can be a planet? Right now we say that something like Luna or Charon is either a moon or a planet. But we don't even have an official definition of the term "moon". What if we said: The term "planet" is referring to the physical nature of the object itself, and the term "moon" is referring to its role within a system of interacting bodies? This would have the following consequences, for example:
1) Luna is a terrestrial planet, because of its physical composition and mass. And it is a moon, because it has the terrestrial planet Earth as a planetary senior-partner.
2) Io, Europa, Ganymede, Callisto are terrestrial planets. And they are moons of the jovian planet Jupiter.
3) Triton is a dwarf planet. And it is a moon of the neptunian planet Neptune.
4) Charon is a dwarf planet. And it is a moon of the dwarf planet Pluto.
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I'd be perfectly fine if we use the term planet to define any object that has pulled itself into hydrostatic equilibrium shape and is not, or never was, a star.
It can be divided based on orbit...
Primary planet: orbits star
Secondary planet: orbits planet
Rogue planet: drifts freely in space
And also divided based on mass/size...
Giant planet: massive enough to be gaseous
Terrestrial planet: solid, massive enough for gases in its hill sphere to form atmosphere, not massive enough to be a giant planet
Dwarf planet: solid and round, but not massive enough to be a terrestrial planet
So Earth is primary terrestrial planet, Enceladus is a secondary dwarf planet, etc. But like you said, we can still use "satellite" when referring to all objects orbiting a common body.