From: nuny@bid.nes   
      
   On Thursday, March 2, 2017 at 11:09:22 AM UTC-8, Adam Warnock wrote:   
   > I'm working on a planetary system generator, and I've come to the realization   
   > that there's not a formal classification system for planets, or at least not   
   > one that I have found. Naturally, I felt this needed addressing, so I've come   
   > here to get some feedback on my first stab at coming up with one.   
      
     Huh. Who's the system for? A warship commander tasked with slingshotting   
   around a planet won't care about composition, temperature, or atmosphere, just   
   mass and radius. That's two dimensions for any planet.   
      
     A colony planner needs a system with many dimensions. Mass, radius, surface   
   gravity, distance from primary, overall composition, magnetic field strength   
   (if present), spin, surface temperature, atmosphere composition and surface   
   pressure (if present),    
   presence/absence of liquid water, number (and type) of moons, compatibility of   
   native life (if present) with Earth life...   
      
   > Mercurian - Rocky planet little to no atmosphere, no surface ice, and no   
   > icy/hydrous mantle.   
   > Terrestrial - Rocky planet with an atmosphere.   
   > Gaian - Terrestrial that is habitable by life as we know it.   
   > Ceresian - Rocky planet with no surface ice/water, but has an icy/hydrous    
   > mantle beneath the crust.   
   > Kuiperian/Plutonian - Icy/rocky planet with surface ice.   
   > Uranian - A gas/ice giant that is not massive enough to produce metallic   
   > hydrogen.   
   > Jovian - A gas/ice giant that is massive enough to produce metallic hydrogen,   
   > but not massive enough to start nuclear fusion.   
      
     This is clearly a Sol-system-biased classification system. In a multi-system   
   setting, I doubt the fifth-generation colonists on a planet in another system   
   with political independence will use those names, especially if they have   
   planets that don't fit    
   any of those types.   
      
     What about helium planets, hypothesized ordinary gas giants close enough to   
   their stars to lose their hydrogen but not their helium?   
      
   > Brown Dwarf - a body that is massive enough to start nuclear fusion, but is   
   > not undergoing nuclear fusion.   
      
     ITYM "not massive enough to sustain fusion throughout its core". IMO those   
   should be classified as stars (or stellar companions) but not planets.   
      
   > And it'd probably be useful to define some things which might come up.   
   >    
   > Planet - An object massive enough to be rounded by it's own gravity.   
   > Protoplanet - An object that isn't massive enough to be fully-rounded, but is   
   > massive enough to have produced a differentiated interior.   
      
     Does that happen? ISTM differentiation in such bodies depends on what came   
   together to make them.   
      
   > Double (Proto)Planet - A pair of planetary objects that are gravitationally   
   > bound to each other and where the barycenter of the system is beyond the   
   > surface of either object.   
      
     Like Roche and Eau? Gets around the "clears its orbit" gotcha.   
      
   > Defining things this way neatly avoids having to write a few hundred words to   
   > define the difference between a planet and a dwarf planet. If Pluto was in an   
   > orbit where its "neighborhood" was clear of similar sized bodies, would it   
   > still be a dwarf planet? Anyway, I'm getting off topic in my own opening   
   post.   
      
     Nicknames can help avoid data dumps. Like Ceres is a "microplanet", Pluto is   
   a "miniplanet" or something.   
      
   > So, applying this to the Solar System, Mercury is a Mercurian. Venus and   
   Earth   
   > are Terrestirals, but Earth is, to be more specific, a Gaian. Mars is also a   
   > Terrestrial, but I'd say it's close to a borderline Mercurian. Basically, the   
   > dividing line is whether the atmosphere is substantial enough to have a   
   > practical effect on something passing through it.   
      
     NASA is having a hell of a time figuring out how to land large cargoes on   
   Mars last I heard. The atmosphere's there, but it's too thin to glide deeply   
   enough to lose enough velocity to safely open a parachute like the Space   
   Shuttle, so it would need    
   way bigger wings than anyone is willing to draw up plans for.   
      
   > Ceres is a Ceresian.   
   > Vesta is a Mecurian protoplanet, though I might be wrong about it being big   
   > enough to have some differentiation. Jupiter and Saturn are Jovians. Uranus,   
   > Neptune, and the hypothetical Planet 9 are all Uranians.   
      
     I always liked "ice giants".   
      
   > Pluto, Makemake,   
   > Hamuea, and all the other Kuiper Belt objects that are planets as defined   
   > above are Kuiperian/Plutonian.   
      
     What about moons? Luna, Mercury, many of the gas giant moons and Pluto are   
   similar except for water content.   
      
     How would you class Io if it orbited Sol instead of Jupiter? A mini-Venus?   
      
     And what about Titan? It's bigger than any of those and has an atmosphere   
   and oceans (just not water). How is it classed? A pseudo-Terrestrial moon?   
      
   > If we wanted to apply this to some fictional planets, then Hoth would be a   
   > Kuiperian/Plutonian Gaian because it is a icy/rocky planet with surface ice   
   > and is habitable by life as we know it. Tatooine and Arrakis are Gaians,   
   > because while they may have had water underground, they didn't have   
   > icy/hydrous mantles.   
      
     What's Mesklin? Jinx?    
      
   > So, any suggestions or comments? I thought about adding classes for Hot   
   > Jupiters, but I think it's better to define the planet by its characteristics   
   > that are at least somewhat independent of its orbit.   
      
     Like I said, who's going to be using it? Which characteristics matter most   
   and which can be tacked on as appropriate?   
      
     Does everybody in a given story *have* to use the same system? Might be fun   
   if somebody forgot to convert, feet-to-meters-style, and sent a colony to a   
   furriner's idea of an "M Class World".   
      
      
     Mark L. Fergerson   
      
   --- SoupGate-Win32 v1.05   
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