Classifying Planets

When I was a kid, I learned that there were two kinds of planets: rocky planets and gas giants. Rocky planets were small, and had solid surfaces and molten cores. Gas giants were big, had no solid surface, but a solid core. I learned that there were nine planets, that Earth had a moon, and that some of the other planets had moons and oh look it's time for math.

I never had much confidence in the school system's ability to get students excited about space travel. By the time I was twelve, I felt I knew more about space than any class would teach me. (In middle school, you just start to get a taste of the enlightenment every high schooler is convinced they possess. None of them actually have it.)

I think the moons of Jupiter and Saturn deserved at least a day of attention sometime in those twelve years. A volcano moon, an ice moon that may have an ocean underneath, a moon covered in yellow fog with oceans of methane! It would blow a kid's mind.

We're discovering a lot of new things, and discoveries tend to force revisions of existing classification schemes. This happened to Pluto, but that's not what I'm talking about.

Planets like Kepler-22b break the old idea of terrestrial and gas planets. Here's a planet that may have an ocean hundreds of miles deep. Do we still call it a terrestrial planet?

"Great, now we have to revise all the textbooks, and all the students will just assume we're trying to milk them for more money. Again." (Image source: NASA)

When I was in high school, I played a demo for one of those ridiculously complex "4X" space strategy games. I remember the game had not two but three categories of planet: rock, gas, and ice. With that, and water planets like Kepler-22b, and some time wasted browsing Wikipedia again, I got an idea for a new scheme:

Type I: Gas

The large planets with which most are already familiar. I'd like to see more creativity with gas planets in sci-fi, as the idea of airborne life forms in a gas planet isn't completely impossible...

Type II: Ice

Different from rocky planets, in that they are solid, but composed primarily of volatiles, like water ice, or solid methane or ammonia. They might have thin atmospheres, allowing seas of liquid nitrogen or something.
Since Pluto's not a planet, there aren't any "ice planets" in our solar system, though there are lots of minor planets that fit that description. Notably the Centaurs, which come in a surprisingly wide color palette.

"TheKuiperBelt Albedo and Color.svg" by Chesnok. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:TheKuiperBelt_Albedo_and_Color.svg

Type III: Pelagic

Planets like Kepler-22b, covered entirely, completely, in oceans many miles deep (I won't attempt to set a specific threshold). These planets could look very much like Earth's oceans, or they could be supercritical, lacking a defined surface between sea and sky. The high pressures on the ocean floor could also result in exotic forms of "hot ice".
These oceans don't have to be water. They could be another volatile substance, like ammonia, for example.

Type IV: Terrestrial

The vast majority of fictional planets would be this type, as would any real planets capable of supporting life as we know it, Jim. If an ocean planet has any landmass above the surface, any at all, it is probably terrestrial rather than pelagic.

Type V: Vulcanian

This is a planet where the surface is entirely lava, not just a planet with volcanic activity. Mustafar in Star Wars would not fall under this classification, due to having a lot of solid ground. In fact, I can't think of any planets in sci-fi that fall under this classification. We don't yet know for sure if there are any real planets of this type exist, although candidates exist, such as Kepler-10b.

Good luck having a climactic lightsaber duel here. (Source: NASA)

Of course this classification scheme already breaks down with the idea of ice giants, which are considered by some to be distinct from gas giants. Maybe we can call these Type Ib.

And a final word on the concept of a "planet". The word is derived from the Ancient Greek word for "wanderer", so in a sense you could apply the word to any object orbiting a star, be it an asteroid, Earthlike planet, gas planet, or protostar. When Pluto was demoted to "dwarf planet", I was told it was not a "planet". How is a "dwarf planet" not a planet?
We've long had the term "minor planet" to denote asteroids. I suggest we refer to the major 8 planets of our solar system as "major planets", and put major, dwarf, and minor planets all under the super-category of "planet".

Of course, the more we discover, the more we'll have to revise our scientific categories. So if my systems described here turn out to be completely wrong, I'm ok with that.