Extremes and medians of the dimensions of habitability

Continuing the discussion from Any favourite colonies?:

Habitable planets & moons

When people from Earth were colonising Space those who were content to live in artificial habitats built them in Earth’s solar system. Pioneers undertook the trip to another star only if they wished to live on a planet or moon. Most worlds have been fixed up a bit, but none of them could have been settled at all unless pioneers could live there during terraforming. So every colony world has a natural shirtsleeve environment. Every colony is on a rocky world, with oxygen, nitrogen, and water vapour in the air, with oceans and land, with sunshine and rain. But they do vary a bit.

Size, density, and gravity

The heaviest gravity of any colony is 1.58 g♁ on Huangdi, a planet 1.49 times the diameter of Earth. Humans can work in up to 2.0 g♁, but are reluctant to settle in more than 1.5 g♁. Golconda is larger (1.67 times as wide as Earth) but less dense, so it has has only 1.43 g♁.

The smallest inhabited world is Surikate (0.50 D♁) and the lightest gravity is on Hylas (0.45 g♁). If these worlds were any smaller or lighter, or if they were warmer, water vapour would have escaped from their atmospheres and they would be desiccated like Mars.

The median diameter of colony worlds is 10 500 km (0.85 D♁); the median gravity 0.81 g♁.

The colour & brightness of sunlight

The colonies with the reddest sunlight are Hennah and Kaiyen, which orbit M0V “red” dwarf stars. Their light is actually not as red as the light of a “warm white” fluoro, but, consisting mostly of invisible IR, such sunlight is also dim. The dimmest sunlight is on the colony Aurochs, which orbits a slightly warmer K9V. Visible illumination on Aurochs is 24% as bright as Sunlight on Earth: twice as bright as the lighting in a TV studio. Human vision barely notices this dimness and redness, but it does make photosynthesis slow. If their sunlight were any dimmer these worlds would not yet have developed oxygen atmospheres.

The brightest daylight is on New Macedon, 85% brighter than on Earth. The bluest sunlight is on Ardor, which orbits a B9V star. This light is only slightly bluer than a “daylight white” fluoro. Ardor is an anomaly: most stars this hot burn out before their planets can develop an oxygen atmosphere.

90% of colonies orbit F1V to K2V stars, the median is G1V.

Surface temperature

The coldest colony is Coldharbor. At a global average of -12°C it is 27K cooler than Earth, and only its equatorial belt is unfrozen. The warmest is Boleslav, at a global average temperature of 60°C. Settlement on Boleslav is confined to the polar zones, where the average temperature is in the high twenties.

The median global temperature of inhabited worlds is 16°C, 1K warmer than Earth.

Atmospheric composition and pressure

The colony with the thinnest air is Gough Island, with 0.24 bar of 40% oxygen. Any less oxygen would make for hypoxia; any less nitrogen and things would burn too easily. The thickest O₂-N₂ atmosphere is 5.68 bar of 10% oxygen on New Cincinnati, where a person who ventures down to sea level flirts with both oxygen toxicity and nitrogen narcosis.

3% of colonies are on worlds so large/cool/high-gravity that they retained several bar of helium against thermal escape. The thickest air of all is 11.5 bar of 85% helium on Salalah.


The driest inhabited planet is Aurelius, which is tide-locked to its star. Its dark side is covered with ice kilometres thick; 8% of its sunlit side is ocean and most of the rest desert. Tide-locked colonies have usually 15–25% but up to 46% ocean on their sunny sides.

Worlds that have day and night range from 50% to 100% covered by oceans. The colonies Bohemia, New Polynesia, Nuada, and Wakashu are confined to islands covering less than 0.1% of their surfaces; Bohemia also has inhabited sea ice at its poles.

Day length

The colony with the shortest day is Magsaysay, with 10.4 hours. The median day is 18.6 hours long, and 90% of colonies have days less than 48 hours long. 10% are tide-locked planets with infinite days. One colony, Toutatis, is in a spin:orbit resonance that makes its day twice as long as its year: 8159 hours. On Toutatis, day and night are effectively seasons.

Human populations

Disregarding brand new worlds whose first pioneers landed only a few weeks ago, the least populous world in Flat Black is Lowrie, a reasonably pleasant planet in the outermost Periphery. Lowrie received no settlers to follow its terraforming crew, and is now occupied by 9.9 million nomads and poor farmers at a population density of 0.044 people/km².

Worlds nearer to Earth were settled sooner, and received more immigrants. The oldest colony is Tau Ceti, which was settled 850 years ago and received 3.9 million immigrants over 250 years. Tau Ceti now has a population of 5.9 billion at a density of 97 people/km².

But seniority is not the only factor. The most populous world of all is Margulis, 89.5 light-years from Sol in the inner Periphery. Margulis was settled 630 years ago, receiving a total of 57 thousand migrants. But it has vast lands, coped well with the sudden loss of Earth, and has social features that encouraged population growth. Margulis’ population is 21.5 billion and its population density 71 people/km².

Of the 68 worlds in the Core, the average population is 2.34 billion and the median 1.32 billion. 42 worlds have more than a billion population and three have more than 10 billion (Tian Longshan, Lahar, New Athens).

Of the 557 worlds in the Periphery, the average population is 1.1 billion and the median 453 million. 170 worlds have over a billion people and two have over 10 billion (Ursula and Margulis). 62 worlds in the Periphery have fewer people than the least populous world in the Core (Lahar; 74.4 million).

Meanwhile, the Fringe is a gigantic dark horse. Without making much impression on minds in the old worlds, the Empire has developed 375 worlds, and sold land and passage to 93 billion migrants. The most populous new world is Florida, which has 2.65 billion residents. 26 new worlds have populations of more than one billion.

Crowding and overpopulation

No planet or moon is anywhere near being covered with a cityscape. The most densely-settled is Iter, which has 132 people/km² of land. That’s more that 2.5 times Earth in the early 21st century, but less than one eighth of solid suburban sprawl. Iter has in fact vast high-rise conurbations, which stretch as far as the eye can see even from the top of a thousand-metre skyscraper, but between them it has great expanses of high-biotech farms, and between those tracts of desert, mountains, and ocean.

As for very low population densities, on new worlds and marginally habitable planets they usually represent small patches of settlement in the midst of unoccupied, perhaps infertile, land. On worlds that are salubrious but economically backward they reflect dispersed, rural settlement structures.

Dry worlds, hot or cold worlds, dimly-lit worlds, or worlds where the terraforming is superficial or incomplete often have low carrying capacity, or may be fragile. They can be overpopulated even at modest population densities. 40% the worlds in the Core and 60% in the inner Periphery suffer from scarcities or environmental degradation because their populations exceed their sustainable carrying capacity with the tech they are using.

1 Like

For my purposes as someone theoretically running the setting or deciding where my PC comes from, most of this could be compressed to a table. I want to know what the place is like more than I want to know that it’s the driest inhabited planet. Does Aurelius have very low atmospheric humidity? Great moisture-extraction mines on the ice fields with planetary water pipelines? Ferocious recycling?

Oh, indeed. That is legacy text from the abandoned draft of a document that was entirely cut for length. I posted it because it included some data that @whswhs had indicated interest in.

I think the thing I’m working on now (Forty Exotic Worlds for Flat Black) will have a bit at the beginning about what the dimensions of variation are, what units they have if any, and an indication of the range and median values. But won’t give the names of the extremal worlds.

On planets where some resource essential to human survival is extremely scarce, to the point of limiting the population to a tiny fraction of Earth’s, what arrangements are made to produce and/or distribute that resource? Is it provided by the state, funded by whatever fiscal arrangements support the state, as a monopoly? Is it provided by a private monopoly, either a commercial enterprise or a nonprofit organization? Is it provided by a variety of competing private bodies?

The ideological arguments that surround such public choices are likely to be interesting. There may be arguments that something so essential can only safely be entrusted to the state, under public oversight, especially in societies that have some analog of voting. There may be arguments that it must be provided by a trusted private organization insulated from political trends and patronage. There may be arguments that any monopolistic arrangement is too much at risk of falling victim to bad management that threatens everyone’s survival, and that safety requires competing providers, perhaps ensured through stringent antitrust regulations. (I could imagine someone publishing Burroughs’s story of Barsoom having all its oxygen supplied by a single “atmosphere plant” that can be the target of mad plots. Or maybe Isaac Asimov’s rather ugly story “Strikebreaker.”)

Conflicts over such arrangements would have a particularly intense quality if player characters got involved in them.


Eeek! That’s a world where a bit of decompression is necessary when you leave it, and visitors really should take training on surviving in the atmosphere.

Are local dialects developing rapidly there? The effects of that much helium on speech are probably quite noticeable.

I think the frequency shift will be downwards, given the pressure.

Does air pressure affect the ability of the atmosphere to transmit different parts of the spectrum? I don’t remember that from Halliday and Resnick (my introductory physics textbook, which I’ve kept copies of ever since).

1 Like

I wish I had. But back when I was a first-year student Australia had an enormous import tariff on books (ostensibly to protect Australian publishers, authors, and kultur — but they didn’t publish STEM textbooks anyway), so Halliday & Resnick cost A$111.95 (equivalent to A$408–$528 today) and had a corresponding value on the second-hand market. I sold it because needed the money.

I recall from physics classes, though, that the speed of sound in an ideal gas is dependent on its composition and temperature, but independent of temperature.

Just as a note, about 1% of Earth’s food production comes from the seas, which make up about 71% of its area. A planet with effectively 100% ocean surface could have maybe 1.5% the population of an Earth of the same size, in the tens of millions.

Depending on how the productivity of the oceans varies with tech level (and on how much of the ocean cover is shallow water).

“Breathe in through the nose-pegs and out through your mouth, never go anywhere without a buddy who knows that they are your buddy, and don’t go below 2200 metres altitude until you get your breathing licence. Download the dytter app for your vademecum, and never go anywhere without it. You’ll be fine.”

Indeed, but suppose that the campaign is open to a bit of development in play, and you have some liberty to make up a planet for your character to have come from. Then knowing what’s typical and where the extremes are provides a welcome benchmark. Even if you’re being a little bit vague about your origins, some games systems require you to commit to some specifics about your growing-up: for example, GURPS requires you to commit to a specific value of “home gravity”.

Furthermore, such material as this can help with characterisation and reactions by providing context. Even if you never go to Surikate or Hylas, This can save you from the embarrassment of reacting to a world as an anomaly because it is 0.75 times as wide as Earth with a gravity of 0.79 g♁. Having information like his available helps you calibrate a character’s sangfroid and brag. It’s a lot more fun to say “Don’t patronise me, sonny. I spent three weeks in combat on New Cincinnati!” than “I have a PCS of 25 in EVA”.


To me – and this is where I put in my standard disclaimer that I’m not a typical player/GM – I’d rather get that information quickly from a table than have to parse it out of prose.

Parameter Min Max
Water cover 8% Aurelius (light side) 100% Bohemia, New Polynesia, Nuada, Wakashu

Then, separately, a list of one-para summaries of worlds would include

New Polynesia: settlement is confined to islands covering less than 0.1% of the surface. Land ownership is strictly hereditary, to the point that being adopted into a family is the only way to gain title to land; adoption for pay is a significant business.


But, emphasis, this is what Roger would want of a book, not a recommendation for a wider audience.

I find it easier to figure out such things as Aurelius’s water cover being on its light side, or New Polynesia’s actually having 99.9% rather than 100%, from Agemegos’s short prose descriptions than from table entries with table footnotes.