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.
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 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.