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Old January 30 2013, 09:16 PM   #13
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Re: planetary classes

Back in 2006, I posted a thread in this forum that critiqued Star Charts's version of the planetary classification scheme and offered an alternative suggestion based on modern understanding of planetary sciences. It doesn't seem to be on the forum anymore, but fortunately I saved a copy, so I'll repost it here:


Rethinking STAR CHARTS planet classification?

The other day, I was looking through the planetary classification scheme covered on pp. 22-29 of Star Charts (which is more or less a mix of the conjectural schemes from tie-in publications over the decades and the few letter classifications that have been established canonically), and I came to realize it has some flaws in its assumptions, particularly where gas giant planets are concerned.

I realized this when I discovered it has no classification specifically for Neptune-class planets. Neptune and Uranus are a distinctly different class of planet from Jupiter and Saturn, sometimes called "ice giants," because they have a different internal composition and structure including a greater proportion of water, ammonia and methane in ice and/or liquid form. But there's no class in Star Charts for that kind of planet. Indeed, Class J, the class including Jupiter and Saturn, is the smallest of the four proposed gas-giant types in SC. Class I is called a "Gas Supergiant" and is said to be considerably larger than class J, ranging up to 10 million kilometers, while classes S and T are said to be even more immense, ranging from 10-120 million km -- which doesn't even make sense, since the Sun is only 1.4 million kilometers across!

Also, when planets get as big as Jupiter or bigger, their cores are so compressed that they collapse into degenerate matter (the stuff white dwarfs are made of). So the more mass you pile onto them, the more you compress their cores, and that shrinking cancels out any size increase you'd get from adding more stuff. That means there aren't any gas giants or even brown dwarfs that are significantly bigger in diameter than Jupiter. 140-150,000 km is the biggest that just about any Jovian or brown dwarf is ever going to get. The only exception is if a Jovian is really close to its star, which would heat its atmosphere and cause it to expand.

This brings up the remaining flaw in SC's categorization of gas giants -- it assumes they would all be in the so called "Cold Zone" beyond the ecosphere. But in recent years we've discovered many "hot Jupiters," Jovians orbiting close to their stars, in other star systems, so we now know that Jovians can be found anywhere in a star system, not just in the outer reaches.

And a fellow named David Sudarsky has come up with a classification scheme for extrasolar Jovians as a function of their distance from their primary stars and thus their atmospheric temperatures, which would affect their appearance and composition.

So instead of the four Jovian classes in Star Charts, three of which are physically impossible, we probably need something like six -- one for each of the Sudarsky classes, plus one for Neptune-class ice giants. Since classes U, V, and W are undefined in the book, those could perhaps be used. Unfortunately, we're canonically stuck with Classes H and K being terrestrial planets and Class J being Jovian, so we can't put those six classes together.

Here are some other Star Charts classifications I have thoughts about:

Class A: Geothermal: A young, partially molten rocky planet. Cools to become Class C: Geoinactive, a dead, frozen planet with Pluto and Psi 2000 listed as examples. Now, the problem here is that Pluto is now classed as a dwarf planet, and that bodies like it are composed as much of ice as of rock.

Class B: Geomorteus (huh?): A hot rocky world close to a star, like Mercury -- except said to be "partially molten" on the surface, which Mercury isn't.

Class D: Asteroid/Moon. The problem here is that there should be more than one class of this. There are several distinct types of asteroids, including silicate, carbonaceous, icy/carbonaceous, or a mix. Luna is a mostly rocky moon, while most large moons in the outer system are a mix of rock and ice (indeed, Saturn's moon Mimas is almost entirely made of water ice). Some moons are bigger than some planets, while others would qualify as dwarf planets if they weren't satellites.

Class E through G: all intermediate stages in the cooling process of young worlds that become class K through P. Do we really need three of them?

Class O: Pelagic: a world with water over more than 80% of surface area. This is a reasonable category to have; recent findings suggest that the inward migration of "hot Jupiters" or the presence of a red dwarf companion could send tons of comets into the inner system to bombard the planets, possibly resulting in terrestrial planets with as much as 100 times as much water as Earth. So worlds that are mostly or entirely covered in water are quite possible. Although I'd set the dividing line between M and O somewhat higher, maybe 90%.

Class Q: Variable: Allegedly something that changes "due to eccentric orbit or variable output of star," but its example of the Genesis Planet is pretty iffy and the category is vague. It might make more sense to say that a planet changes from, say, Class L to class P conditions as it moves in its orbit.

Class R: Rogue, a planet expelled from its system like Dakala in ENT: "Rogue Planet." Actually more scientifically credible than it seems. This class might also encompass what are being called "planemos," planetary mass objects that formed independently in deep space. The problem is that the class doesn't distinguish between terrestrial and Jovian rogues.

Now, canonically, only ten letter classifications have been used: Classes D, H, J, K, L, M, N, P, T, and Y. That leaves the rest up for re-evaluation. And some of the canonical ones could have their details tweaked too without violating onscreen material.

So here's my tentative proposal for a revised classification scheme:

A: A young, molten planet or dwarf planet. (Gothos, Excalbia)

B: A moon or dwarf planet with an outer layer of water ice and perhaps a liquid-water mantle, found in the middle portions of a system. (Ceres, Europa)

C: A moon or dwarf with an outer layer of water, methane, ammonia and/or carbon dioxide ice, found in the outer part of a system. (Pluto, Triton)

D: A dry, silicate moon or dwarf planet. (Luna, Regula I)

E, F, G: Maybe make these lifeless rocky planets of varying sizes, from Mercury-sized to Earth-sized to super-Earth-sized.

H: Canonically, the H-class world Tau Cygna V is habitable but subjected to levels of hyperonic radiation that humans can't survive. Perhaps generally this could be a world that's habitable but too irradiated for humanoids.

I: A Neptune-class ice giant.

J: A Jupiter-class (Sudarsky I) gas giant.

K: Canonically, "adaptable with pressure domes." Say, something reasonably close to Earth gravity but lacking a dense or breathable atmosphere.

L: An almost Earthlike world, but with relatively little oxygen in the atmosphere. Essentially what Earth would've been about 400-500 million years ago.

M: Duh.

N: Let's stick with Mandel's idea of making this a Venus-like world, superdense atmosphere. Fits with the earlier classes being distinguished by their thin or oxygen-poor atmospheres.

O: Pelagic, again sticking with Mandel. Actually I'd like to have N and O the other way around for a better progression, but N is already sort of canonical.

P: Glaciated -- essentially the "Snowball Earth" scenario.

Q: Let's call this a carbon planet of the kind that's been conjectured.

R: I'm willing to stick with Mandel's classification for a rogue terrestrial planet.

S: Call this a rogue Jovian or a sub-brown dwarf. Outside of a star system, it would have different properties from any of the Sudarsky classes.

T, U, V, W: Sudarsky classes II-V.

X: Maybe a hyperthermic environment, hotter than the standard boiling point of water but with enough atmospheric pressure to raise that boiling point and allow hyperthermophilic life to exist.

Y: Demon class, as seen in VGR: "Demon."

Z: Let's call it "other."
Written Worlds -- Christopher L. Bennett's blog and webpage
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