Habitable planets and moons


#1

I am pleased with the text below, which is much more succinct than this web page I wrote a while ago. But it is 930 words and the budget for this topic was 600. Brutal cuts are needed.

The Colonies

Habitable planets and moons

When people from Earth were colonising Space those who wished to live in artificial habitats built them in Earth’s solar system. Pioneers only undertook the trip to another star if they wanted to live on a planet or moon. Most worlds have been fixed up a bit, but none could be settled at all unless the pioneers could live there during terraforming. Thus every colony in Flat Black is on a world that has a natural shirtsleeve environment.

Suitable stars

Except in bizarre circumstances, worlds orbiting cool stars develop oxygen slowly, whereas worlds orbiting hot stars don’t get time to develop it at all. Subgiant and giant stars have only recently thawed out their current Goldilocks zones. So 98.8% of colonies in Flat Black are found orbiting F-type, G-type, and the hotter K-type main-sequence stars. The outliers are two M0V stars, nine A-types, and a single wildly anomalous B9V — Lambda Aquilae, the sun of the colony Ardor.

Surface temperature

Humans have never made a permanent settlement where the average temperature is above 30°C or below 0°C, and their agriculture needs extensive land in that range that is watered and sunlit. But the equatorial regions of a freely-rotating world are often about 15°K warmer than the global average: worlds may be habitable with average temperatures as low as -15°C. The coldest colony is Coldharbor, at -12°C global average. Similarly the polar regions of a rotating planet can be about 35°K cooler than the planetary average. Boleslav has an average temperature of 60°C and settlements at it poles. The median average temperature of inhabited worlds is 16°C, 1°K cooler than Earth,

On tide-locked planets the subsolar region is about 30°K warmer than average, but the areas cooler than average are in darkness. Such worlds may be habitable at average temperatures of -30°C to 30°C.

Diameter, density, and gravity

The size and mass of a world determine its surface gravity. Diameter, gravity, and the temperature of the upper atmosphere determine what gasses it will retain against thermal escape. Any world that is too warm/small/low-gravity will lose water vapour and desiccate. The smallest inhabited world in Flat Black is Surikate (0.50 D♁) and the lowest surface gravity is on Hylas (0.45 g♁).

Humans cannot work where the gravity is over 2.0 g♁, and have proved reluctant to settle in more than 1.5 g♁. The heaviest gravity on any colony is 1.58 g♁ on Huangdi, which is 1.49 times as wide as Earth. The largest inhabited world is Golconda, which is 1.67 times as wide as Earth but has a surface gravity of only 1.43 g♁ owing to low density.
The median diameter of inhabited worlds is 10 500 km (0.85 D♁) and the median gravity 0.81 g♁.

Atmospheric composition and pressure

Permanent settlements of unmodified humans need oxygen to breathe at a partial pressure between 0.1 bar and 0.55 bar, and can endure up to 3 bar of nitrogen, besides if necessary large partial pressures of helium. Worlds that retain water vapour always retain nitrogen and traces of argon; large, cool, high-gravity worlds may also retain helium without affecting respiration. The air on habitable worlds also contains water vapour, and traces of carbon dioxide equilibrating the temperature through the geochemical carbonate-silicate cycle.

The thinnest breathable atmosphere in Flat Black is 0.24 bar of 40% oxygen on Gough Island. The thickest is 11.5 bar of 85% helium on Salalah. Only 3% of colonies have a lot of helium in the air: median barometric pressure at sea level is 0.89 bar.

Oceans

Habitable worlds that are freely rotating have 50% to 100% of their surface covered by ocean basins (parts of which may be frozen over). The drier ones tend to have uninhabitable expanses of arid continental interior; the wettest ones afford very little land for occupation or farming. The colonies New Polynesia, Nuada, and Wakashu are confined to islands covering less than 0.1% of their surfaces; Bohemia has such islands and also inhabited sea ice at its poles.

Tide-locked planets cool enough for human habitation have large ice-caps on their dark sides. When that leaves no significant water on the sunlit face the planet do not develop oxygen atmospheres. So the inhabited examples of tide-locked planets usually have 15%–25% of their lit faces covered by water, sometimes up to 50%. The driest colony is Aurelius: 8% of its sunlit side is ocean.

Visible illumination

Cool K-type stars produce a large proportion of their solar energy as IR radiation, which warms a planet effectively but does not drive photosynthesis nor light things up. The human visual system adapts too well to notice: the dimmest and reddest K9 sunlight is still bluer than and twice as bright as TV studio lighting. But it does affect the rate of plant growth and oxygen production. The most dimly-sunlit colony is Aurochs, 24% as bright as Earth. On the other hand the sunlight on Ardor is 80% brighter than Earth’s and can cause discomfort.

Day length

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


#2

ObPeeve: there’s no such thing as a °K. It’s just a K. (Though in practice I’d stick with °C rather than switching anyway.)

A note on high-ocean worlds: aquaculture, if the world is otherwise interesting enough to settle? I know, that makes things longer.


#3

I am relieved to see you say so, since I had always thought so until someone recently persuaded me otherwise.

The use of °C for differences of temperature is standard but controversial. K is accepted.

Maybe, but I’m not sure that I’d emigrate to one through a flinger.

Also, my personal preference is to have a bit of silicate rock exposed to weathering, just in case I need a carbonate-silicate cycle some time.


#4

Suggestion of things to edit out…

  1. We don’t need to know how wide Golconda and Huangdi are - just tell us their gravity.

  2. Why are you telling people the median for various things like gravity, barometric pressure, etc? If those are facts the GM needs to know, stick it in the Planet Designing chapter. If the PCs get bonuses or penalties because their homeworld differs from the planet they are currently standing on, that goes in chargen and/or the Planet Design chapter.

And things to add… altitude affects partial pressure, so there might be planets where the Andean altiplano has a pleasantly thick atmosphere. Or ones where you can barely breathe at sea level.

Edit: presumably the median day would be infinite, not 18.6 hours if it was calculated including the 10% of planets with infinite day length! :slight_smile:


#5

Hmm, that would be the mean? The median is from the planet half-way down the list. :slight_smile:

If I were generating a GURPS character, I’d want to know what range of gravities I might be expected to work in, but that’s as much as I need to know for character design. It’s interesting to have these ideas of ranges, but as Dr Bob says it’s perhaps not necessary.


#6

What I’m trying to do here is to indicate the variety of characters’ homeworlds. In what ways do they vary? What’s typical? In the case of very asymmetrical distributions the middle point (median) gives a better idea than the average (mean): in the case of day length the average would be “infinite”, which is uncommunicative.

Some of these data are mechanically significant to character generation. For example the gravity of a character’s homeworld makes a difference in ForeSight and GURPS. Others are simply there to let players know whether to expect, say, giant worlds like Vance’s Big Planet or Silverberg’s Majipoor. Can I have come from a huge metal-poor world like Bug Planet? From a water world like those in Vance’s The Blue World or the RPG Blue Planet? These are the details of which this setting consists. They are the trees that make up the forest. I don’t want to spend more than a page on it, but I do think it is worth one page.

There isn’t going to be a character generation chapter, at least in this book. I’ve run or played Flat Black games under ForeSight, Star Hero, and GURPS. I’m contemplating FUDGE or Fate, and I suppose that other GMs, if there ever are other GMs, may favour Stars Without Number, N.E.W., Thousand Suns, or some such RPG that I wouldn’t touch with a barge-pole. This is the information that a user would have to use to produce a character generation screed for whatever system they used.

There isn’t going to be a planet-designing chapter either. I have already designed all thousand planets using a modified version of the GURPS rules (though I am going to have to do it over). There will be a gazetteer and a planet-generating app instead. I’m only planning to describe 125 planets or so, leaving room for GMs and character-players to insert the ones that their plots and character ideas need, but I expect users to select values from ranges in some sort of app, not wrestle with the intricacies of planet generation.


#7

Okay. I’ve re-written that so that it is less about general principles more about specific worlds. I think this draft is more engaging and more obviously relevant to the reader. But at 743 words it is still longer than I would prefer.

The Colonies

Habitable planets and moons

When people from Earth were colonising Space those who wished to live in artificial habitats built them in Earth’s solar system. Pioneers only undertook the trip to another star if they wanted to live on a planet or moon. Most worlds have been fixed up a bit, but none could be settled at all unless the pioneers could live there during terraforming. Thus every colony in Flat Black is on a world that has a natural shirtsleeve environment. Every colony is on a rocky planet, with oxygen, nitrogen, and water vapour in the air, oceans and land, 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, 1.67 times as wide as Earth, is larger but has a surface gravity of only 1.43 g♁ owing to low density.

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 inhabited worlds is 10 500 km (0.85 D♁) and 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. Such sunlight is also dim: most of it is invisible IR. 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 photosynthesis is slow. If the light were any dimmer these worlds would not yet have developed oxygen atmospheres.

The bluest and brightest sunlight is on Ardor, which orbits a B9V star. This light is only slightly bluer than a “daylight white” fluoro, the illumination is 80% brighter than on Earth. 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, median G1V.

Surface temperature

The coldest colony is Coldharbor. At a global average of -12°C it is 29K cooler than Earth, and only its equatorial belt is unfrozen. The warmest is Boleslav, which has an average temperature of 60°C. Settlement on Boleslav is confined to the polar zones, where the temperature is in the high twenties.
The median global temperature of inhabited worlds is 16°C, 1K cooler than Earth.

Atmospheric composition and pressure

The colony with the thinnest air is Gough Island, with 0.24 bar of 40% oxygen. Any less O₂ would make for hypoxia; any less nitrogen and things would burn too easily. The thickest O₂-N₂ atmosphere 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.

Oceans

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 New Polynesia, Nuada, and Wakashu are confined to islands covering less than 0.1% of their surfaces; Bohemia has such islands and also 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, and 90% of colonies have days less than 48 hours long. 10% are tide-locked planets with infinite days.

One colony, Toutatis, is on a planet in a spin:orbit resonance that makes the day twice as long as its year: 8159 hours. On Toutatis, day and night are effectively seasons.


#8

I’m going to say that “global average temperature” is a notion I find hard to make use of.

The starting point for planetary temperature is the blackbody temperature for a given stellar luminosity and orbital radius. That has to be adjusted for the physical properties of the planet, of course. But the usual calculation seems to assume that solar radiation is normal to the relevant surface. Elsewhere the incident radiation comes in at an angle and is thus spread over a wider area. I tend to think in terms of the temperature under direct illumination, with other temperatures usually being lower. That’s clearly not an “average.” I’m not sure what I would called it, though.


#9

Global average surface temperature seems pretty straightforward to me. Earth’s global average surface temperature (from satellite measurements calibrated by weather reports) is 288 K (15°C). That’s the figure that tells you what conditions are like to live in. The tropics are about 15 K warmer. The polar regions are about 30 K colder. Yes, it’s colder in winter and the small hours of the night, warmer in summer and the early afternoon. But the average isn’t a figure I find hard to use.

On the other hand, a planet’s effective black body temperature is important for understanding the physics, but albedo and greenhouse effects are so variable that it doesn’t tell you very much about what conditions are like for living on the surface. Earth’s effective black body temperature is 252 K. It was 252 K at the height of Pleistocene glaciation. It will still be 252 K in a hundred years even if we get over five kelvins of global warming. Habitability depends on the surface temperature, not the temperature at the stratopause (50 km altitude).

As for the temperature of a black surface under perpendicular insolation, around here summer sunshine will melt the bitumen on the roads. That doesn’t tell you very much about life on Earth.


#10

For the purposes of determining whether humans will choose to colonise, what you need is “temperature range on the major land masses”. You can proxy that with a nominal “average surface temperature” (which is the term used in GURPS Space) and some handwavy assumptions about where the land masses are and how much actual temperature varies from that average. This doesn’t mean that you expect it to be that temperature at any specific point.


#11

Well, what you really need is “how much arable land has an average annual temperature between 0°C and 30°C, with an average temperature below 40°C in its hottest month?”, but that is getting really fiddly. As you say, average surface temperature is the proxy.