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.
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 starsExcept 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.
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.
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 illuminationCool 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.
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.