Planetary Classification

Tony Harris asks:

It sounds as though you have some kind of planetary classification scheme set up. Care to share how it works? :please:

Well, yes, yes I do.

(Let me start out by saying that it does not, however, define what a planet is, except inasmuch as the classification scheme’s largest category stops at 2.5e28 (i.e. 2.5 x 10²⁸) kg, which is about 13.2 Jupiter masses, at which point it gets kicked over to the stellar classification scheme at the bottom end of dwarves, brown.

Insofar as there is a definition of a planet used there, it’s dreadfully informal and would run along the lines of:

• Orbits a star (other than a brown dwarf that is itself part of another star system);
• Masses less than 2.5e28 kg;
• Masses enough to be approximately spheroidal;
• Is cared enough about by travelers to be listed in Leyness’s Worlds, or some other casual reference that isn’t an astronomer’s catalog or a space pilot’s ephemeris.

So, Pluto, despite being a gelidian-class planetesimal by the scheme, is a planet because it has enough historical significance and popular interest that it’s undoubtedly mentioned in the hypothetical Leyness’s Worlds: A Guide To Places No-One’s Discovered Yet, Except The Natives Who Obviously Don’t Count, while the other plutoids or whatever we’re calling them these days aren’t, because they don’t.)

As for the scheme itself: well, it’s kind of long and complicated and unfinished, especially since the universe keeps surprising us with New Facts About Planets that need to be fitted into it as if they’d been there all along, so I hope you’ll forgive me for not sharing the whole thing. But I don’t mind talking a bit about it, so I’ll do that.

The primary distinction it uses when classifying – and it classifies everything from small rocks through moons and on up, not just “planets” – is mass, which has the advantage of encompassing a fair bit of other useful information. Using mass, it defines five basic categories: small bodies (< 6e20 kg; too small to maintain hydrostatic equilibrium); planetesimals (6e20 kg to 9e23 kg; can maintain hydrostatic equilibrium); lithics (9e23 kg to 3e25 kg; tend to clear out their orbits and self-sustain geology); helians (1.8e25 kg to 1e26 kg; big enough to retain helium, but not yet gas giants, and yes, this overlaps with the one before it); and gas giants (5e25 kg to 2.5e28 kg, and so does that). There’s also a sixth class, wanderers, which defines those odd planetary-massed objects that aren’t gravitationally bound to any star and just drift about in the deep.

The overlaps, incidentally, are because none of these top-level categories are intended to be particularly strict. The Imperial Grand Survey learned long ago that the universe is a very complicated place that doesn’t fit itself into neat boxes for their convenience – so the boxes are a mite fuzzy, and worlds are assigned to the class that bests fits them even if technically they’re a little too massive for the top-level category that world sits within.

So, the categories: small bodies are asteroids, comets, and similarly-sized moons, for the most part, subcategorized primarily by composition and solidity (aggregate-class rubble piles vs. silicaceous-class stony asteroids, for example). The comets are also divided based on whether they’re icy bodies orbiting peacefully in the outer system, or which are actively plunging through the inner system.

Planetesimals are mostly large asteroids and moons, divided up by age (are they just forming?), composition (rocky or tarry or icy), level of geological activity and its source (mostly passive, due to eccentric orbits and epistellar heating, due to tidal flexing, due to internal heating, etc.

Vesta, Ceres, Luna (selénian-class), Europa, Titan (galínilacustric-class), even Mars (eutalentic-class), they all fall somewhere into this classification.

Lithics are the big rocky ones with substantial atmosphere. Their subclassifications are much the same as the planetesimals, plus additional ones: age (forming or dying), water content (xeric through thalassic to pelagic – a global ocean, in these terms) and their methane and ammonia equivalents, presence of a biosphere, presence of halogens, and so forth. There are more lithic subclasses than there are subclasses of anything else, just about, partly because they’re more varied, and partly because they also attract a great deal of interest.

Earth (sylithotectonic-class) fits in here.

Helians get the aborted gas giants, plus a variety of superterrestrial rocky worlds with thick, helium-rich atmospheres. Its number of subclassifications is relatively limited, simply because there’s a very fine line between retaining helium at all and ending up among the…

Gas giants, divided principally by mass (subgiants, dwarf giants, mesogiants, supergiants), and then principally by their orbital positioning (epistellar, within the snowline, beyond the snowline) which defines most of their composition and behavior.

Around here, they would include, for example, Jupiter (melíeréan-class) and Neptune (déiran-class).

So, I hope that was an interesting peek into how the IGS planetary classification system works. I’m happy to answer a bit more, if anyone’s interested, but as I said, I don’t really want to put the whole thing out there until it’s suitably finished and polished.