Register Register

Author Topic: ideas for designers of game worlds  (Read 1943 times)


  • Master Sergeant
  • *
  • Posts: 277
  • ex scientia, ad astra
ideas for designers of game worlds
« on: 04 October 2017, 07:02:16 »

Habitable Zones

The inhabited IS comprises less than one tenth of one percent of all the stars within the same volume -- thousands (of inhabited systems) out of millions (all systems).  Thus, BT star systems are inherently exceptional, affording a rather wide latitude of artistic license.  However, the following guidelines are probably applicable to the statistical majority of systems within several hundred light-years of Earth.

Planets & Moons
  • "blue giant" stars (O,B) have no planets
  • white stars (A) have at most one "Super Jupiter" gas giant planet (100s-1000s of earth masses) with many many moons (?)
  • green stars (F) have at most several gas (mini-)giants (10s-100s of earth masses), widely spaced (at most 1 in the HZ), with many moons (?)
  • yellow & orange stars (G,K) have the most extravagent planetary systems (much like ours), up to 1-2 in the HZ, with up to 1-2 moons each around the larger stars of this group
  • "red dwarf" stars (M) also harbor extensive planetary systems, however...
    • the stars are fully convective, with stellar "thermals" rising all the way up from the fusion core to the surface, whereat powerful super-flares frequently erupt, showering everything within ~0.1AU with high doses of x-ray & particle radiation
    • the stars are so dim that their HZs reside within easy reach of those super-flares
    • planets within those HZs would also experience powerful Tidal effects, quickly becoming "Tidally Locked (TL)" to their parent star, much as the Moon is to Earth, or Mercury is to the Sun
    • so one face of the planet would be frequently fried by flares, and the other face would be perpetually dark and frozen nitrogen & CO2 cold
    • recharging Jumpships or Docking Stations would also be continually blasted by those super-flares, at almost point-blank range

Stellar Lifetime & Planet "geologic age"

The biggest potentially-habitable stars (A,F) only shine for 1-3Gyr before "self destructing" in a "slow-motion super-nova" (dispersing themselves into a mis-named "Planetary Nebula" surrounding a central white dwarf stellar core remnant).

But it takes an earth-mass planet ~2.5Gyr to cool sufficiently to be easily habitable.  Most of that heat "budget" derives from Uranium and other radioactive elements in the planet's core.  The cooling time of a rocky, terrestrial type planet increases with that mass of heavy heat-generating metals (MZ), which in turn is proportional to the total planet's mass; and decreases through radiation from the exposed surface area of the world (4pi R2):

T ~ MZ / 4pi R2 ~ M / R2 ~ g = surface gravity

For the planets in our Solar System, the expected "geologic lifetime" of scales as its surface gravity.  Thus, Mars (g = 0.38) ages about ~2.5x as quickly as Earth, and only lives ~40% as long.  Its sequence of geological ages (Pre-Noachian / Noachian / Hesperian / Amazonian) -- defined & bounded, respectively, by the progressive freezing of the Crust / Mantle / Core -- occurred about two-and-a-half times faster than the corresponding series of ages on Earth (Hadean / Archaean / Phan-proterozoic...).  (Earth's core is only half frozen, still has another billion years of geologic life in it, or so.)

Colonizing and terraforming a "Hadean" world could be impossible (the Crust has not even completely solidified), and an "Archaean" planet possibly impractical (incessant seismic, volcanic and other geologic activity on Crustal "rafts" floating on a churning subterranean sea of molten Mantle magma).  So, there is plausibly a "minimum age requirement" for settleable planets.


In sum, the usual scenario for settleability may well be the following:

A,F stars
  • cosmologically "young" systems <1-3Gyr old
  • habitable "Moon-to-Mars-mass" moon(s) of a "Uranus/Neptune or bigger" gas giant
  • 1/6 - 1/2 g
  • "Pandora / Endor / Warm Europa" scenario
  • primary planet's orbital is 2-3 Earth years

G,K stars
  • cosmologically "mature" systems up to 5-10Gyr old
  • habitable "Mars-to-Earth-mass" planets
  • 1/2 - 3/2 g
  • maybe a moon or two circling the larger planets orbiting the larger stars of this class
  • "regular Earth" scenario
  • orbits take 6-18 months

M stars
  • frequent super-flares
  • marginal colonies ?
  • orbits last weeks to months
  • Mercury-like orbital resonances, with 20-day days (spin, rotation) & 30-day years (revolution) such that the apparent day is 45 Earth days long, 3 weeks of sub-Arctic night and 3 weeks of perpetual day?

Dry Deserts vs. Water Worlds

Note that there are very few "super-Earths" more massive than 2-3 Earth masses with surface gravities greater than 1.5-1.7 g because such worlds have strong enough gravity to retain a massive atmospheric envelope and so become the cores of a Neptune-like mini-giant instead.  But "never say never" obviously :)

Moreover, another consideration is vertical terrain contrast -- how high are the "highs" and how low are the "lows" on some world, and what is the difference in elevation between them?  Scaling up the Moon or Mars would ceteris paribus make for taller mountains and deeper valleys.  However, eventually, the stronger gravity of more massive planets tends to pull mountains down, and push material into valleys.  In our Solar System, Mars actually has the optimal combination of "size without high gravity", such that Mars has both the tallest mountains (Olympus Mons) and deepest valleys (Vallis Marinaris).

Meanwhile, more massive worlds tend to have more & more water, which inundates the surface more & more deeply.  Mars in its heyday had >30 km of vertical relief between mountain peak and valley trough, and perhaps enough water to theoretically submerge the whole surface to a depth of ~1km.  So, Mars was mostly land, with a few large lakes and shallow regional seas.

Whereas Earth is mostly water, with a global ocean system ~5km deep on average, inundating ~half as much vertical terrain contrast.

The chart above estimates average "vertical contrast (km)" (green line) & "ocean depth (km)" (blue line) as a function of planet size (earth radii).  The normalized radius of the Moon R~1/4 (M~1/100, g~1/6) & Mars ~1/2 (M~1/10, g~1/2).

Observationally, smaller worlds are more common than larger ones, much as smaller stars are more common than more massive ones.


  • most IS worlds (MKG system planets & FA system moons) are like Mars
    • surface gravity ~0.5 g
    • mass ~0.1 Mearth
    • radus ~0.5 Rearth
    • 1/4 the surface area
    • mostly land with a few large lakes & shallow seas ("Great Lakes & Black Seas, Mediterranean at most")
    • rain clouds never reach 20-30% of the surface, which remain dusty deserts
    • "very vertical" with towering sheer-sided mountains & deep cliff-encased chasms
  • many IS worlds are like Earth (~1.0 g)
  • most "super Earths" (~1.5 g) are featureless & fully flooded "water worlds" with deep oceans, no land, and dense atmospheres
  • almost all "hyper Earths" (>2 g) have retained thick atmospheres to become the cores of (mini-)giants resembling Neptune / Uranus / Saturn / Jupiter
« Last Edit: 04 October 2017, 22:31:14 by Thunderbolt »


  • Corporal
  • *
  • Posts: 66
Re: ideas for designers of game worlds
« Reply #1 on: 05 October 2017, 03:36:08 »
Useful info for GMs, thanks.

Although I'm pretty sure our star isn't yellow, it's white (at least that's what they say, and that's what it mostly looks like too.)
« Last Edit: 05 October 2017, 03:40:06 by Dakkon »


  • Master Sergeant
  • *
  • Posts: 277
  • ex scientia, ad astra
Re: ideas for designers of game worlds
« Reply #2 on: 06 October 2017, 07:21:10 »
well, yes, "yellow... from the ground"  ;D

gives me one more color name to work with to label the stellar classes :)

There's one more issue I know of:

Mass vs. Radius for rocky terrestrial worlds

For a given Earth-like (essentially Solar-like sans H,He) bulk composition, known terrestrial worlds obey a pretty tight relation of overal mass (M) as a function of radius (R) -- the numbers in parenthesis are corresponding surface gravities:

The disk of the Milky Way galaxy is ~10Gyr old.  The earliest generations of stars >7-8Gyr old had low levels of "metals" (everything > H,He), especially iron-peak elements.  The few (terrestrial) planets which did form would have fallen "below the line" on the plot:
  • less iron, less dense
  • less concentrated, lower surface gravity, slower spins
  • shorter-lived & weaker magnetic fields
  • "felsic" surfaces with lots of light-colored silicates, e.g. sand, and few "mafic" iron/magnesium compounds
Conversely, the latest generation of stars <2-3Gyr old have higher metallicities, and their numerous rocky worlds would lie "above the line" on the plot:
  • more iron, more dense
  • more concentrated, higher surface gravity, faster spins
  • longer-lived & stronger magnetic fields
  • "mafic" surfaces with lots of dark iron/magnesium compounds, e.g. olivine
So you could imagine a dark, iron rich, rocky world that (say) spins 3x as fast as Earth, has a similar mass but is somewhat smaller with a higher surface gravity, and a surface magnetic field ~10x stronger than Earth's -- as strong as Jupiter's !!  Wreak havoc on Gauss Rifle rounds  ;D
« Last Edit: 06 October 2017, 07:23:00 by Thunderbolt »


  • Master Sergeant
  • *
  • Posts: 277
  • ex scientia, ad astra
Re: ideas for designers of game worlds
« Reply #3 on: 11 May 2019, 23:22:45 »
FYI, multi-planet exo-planetary systems with many planets all tend to obey a Titus-Bode like law, similar to our own Solar System:
  • planets tend to be reasonably spaced apart -- very few have orbital periods ("planetary years") within 50% of each other, e.g. one Mars year is about 1.9 earth years, which (in turn) is about 1.7 Venus years
  • planets tend to gravitate (literally, relatively quickly) into orbital resonances (given enough time) -- rational period ratios of 3:2, 5:3, 2:1 etc. account for the statistical majority of all pairings of planets drawn from the same systems
Might plausibly be the case that younger planetary systems, circling bigger, brighter but shorter-lived stars (F,A-class), having had less time to regularize orbits, might possibly tend to be less orderly and more random in their distribution of orbits & orbital periods (though larger stars tend to have fewer planets anyway)...

bearing in mind that in all planetary systems, resonant or random, closely packed planets with very similar years, are always highly unlikely -- perhaps because they are too close, interacting gravitationally too strongly, too frequently, to be stable?  Such pairs would tend to coalesce in the proto-planetary disk, and/or eject each other onto wild (even interstellar) trajectories after the central starlight clears the disk ??

again, just trying to provide useful guidelines for writers, designers  :D


  • Lieutenant Colonel
  • *
  • Posts: 6692
Re: ideas for designers of game worlds
« Reply #4 on: 12 May 2019, 00:48:02 »
I dunno if there's suppose to be more pictures, there's the icons for 'em but none are loading.  But DAMN this is useful stuff!
Ghost Bears: Cute and cuddly. Until you remember its a BLOODY BEAR!


  • Major
  • *
  • Posts: 5099
  • Pirates and C3 masters are on the hitlist
Re: ideas for designers of game worlds
« Reply #5 on: 12 May 2019, 08:16:32 »
Thanks for the useful information.
Herb: "Well, now I guess we'll HAVE to print it. Sounds almost like the apocalypse I've been working for...."

Fan XTRO: The Society


  • Lieutenant Colonel
  • *
  • Posts: 6692
Re: ideas for designers of game worlds
« Reply #6 on: 12 May 2019, 08:54:51 »
A system with a Red Dwarf or a really big star could make for some interesting battles none the less. You could have battles taking place on airless rocks.  you could be fighting on the surface of a tidally locked world that's as cold as the dark side of Mercury but within its stars flare zone.  Why go there? Minerals!  *INSERT HOUSE OR COMPANY* has established a mining facility on the world to mine up Jellybeans or what ever and your employer or Commander has sent you in to take it off them.

Also don't this raise issues with systems like Canopus? Because IIRC Canopus is a bloody HUGE star.  Unless the Canopians went to Canopus, found the system basiclly a highly radioactive hell with a star this massive

in it. Thought "Screw that!" found another more habitable system and said "This is Canopus now!" or named it after the bigger star because its 'nearby' at least in Spacial terms.

Then again considering that there's more than a few systems in lore that are in systems that either have red dwarfs or really big stars. And these were swapped out for more normal stars (so Sol type) for ease of story telling. 

I do recall this really awesome star map that someone did which showed the position of stars and what it might really be like in those regions of space due to dust etc.  There was also a really good map showing various nebulae and the like. Dunno where they are now.
« Last Edit: 12 May 2019, 09:15:39 by marauder648 »
Ghost Bears: Cute and cuddly. Until you remember its a BLOODY BEAR!


  • Master Sergeant
  • *
  • Posts: 277
  • ex scientia, ad astra
Re: ideas for designers of game worlds
« Reply #7 on: 21 July 2019, 06:55:35 »
Could consider the NatGeo documentary "Alien Worlds" (aka "Extraterrestrial" in the UK), specifically the episode "Aurelia" about just such an M-dwarf orbiting tide-locked world:

cp. other episode "Blue Moon" about an Avatar Pandora type planet-sized moon orbiting a Gas Giant