It is perfectly true that the people who emigrated from Earth to found the colonies in the 22nd to 24th centuries did not need habitable planets to live on. They had technology that was quite capable of building sustainable habitats in space and on uninhabitable rocks, and they could have foraged in orbit for supplies. Indeed, more people left Earth for orbital habitats in the Solar System than ever emigrated to the stars. It is for exactly that reason that colonies are found only in star systems that do have habitable planets in them; people who were content to live in space stations stayed in orbit around Earth or Sol, only those who particularly wanted a world to live on chose to bear the cost and dangers of interstellar emigration. Terraformation would have been possible in principle. Indeed, almost every inhabited world has been fixed up a little, even if only by the establishment of an ecosystem compatible with life from Earth. But such terraforming as might take a world from uninhabitable to substantially habitable takes hundreds or thousands of years, and the plenty of naturally habitable planets made it unnecessary.

Like happy families according to Leo Tolstoy, habitable planets all have a certain sameness about them. They all have equable temperatures, oceans of water, moist atmospheres consisting mostly of oxygen, nitrogen, and perhaps helium, surface gravity within a certain range, illumination within a certain range. They are found orbiting stars that are neither much larger and brighter nor much colder and dimmer than Sol, in orbits not very eccentric, with obliquity not extreme. Each inhabited world is of course unique, and many are as varied as Earth was, just as much sprinkled with wonders. But with the exception of a trivial proportion who live and work in space, these are the backgrounds that characters in Flat Black come from, these are the settings where adventures take place. Habitable planets and moons where humans can farm in shirtsleeves; not floating cities in the oxygen layers of anomalous gas giants, integral trees in celestial smoke-rings, airtight warrens in airless moons or asteroids, colossal orbitals, ringworlds, or Dyson spheres.

Suitable stars

Habitable worlds are almost all found orbiting main-sequence dwarf stars with spectral type between about A7 and K7. Large bright stars are short-lived and therefore young. O, B and early A stars evolve into giants faster than any world can evolve an oxygen atmosphere; even around late A-types a planet has to evolve unusually fast to develop oxygen before the star goes giant. Small dim stars are long-lived, but they put out most of their heat as infrared radiation, which is efficient at warming planets but not at driving photosynthesis. On planets orbiting M and late K stars the accumulation of oxygen begins late and proceeds slowly; such planets seldom have breathable atmospheres. The worlds of K8 and cooler stars may eventually evolve a sluggish ecosystem of native life, but even with an oxygen atmosphere they are too dimly-lit for Earth crops and inhospitable to human agriculture. Furthermore planets that huddle close to dim stars are subject to strong tidal effects. These slow the planets’ rotation, leading to a state of tidal locking in which a planet has one side in permanent daylight and the other in permanent night. That’s not completely incompatible with habitability, but does make it less likely.

A scant handful of habitable planets have been found in the systems of class IV subgiant stars that very recently started developing towards their giant phase. These were presumably very cold worlds for most of their life and evolved life in frozen-over oceans, then thawed and developed photosynthesis as their star brightened. Such systems are most unusual.

Habitable worlds are sometimes found in multiple systems. A planet can orbit a pair of stars if they are close companions and its distance from their common centre is more than three times their maximum orbital separation. If stars are distant companions, planets can orbit either or both of them at distances closer than one third of their minimum orbital separation. If such orbits leave the planets in them warm but not too warm, those planets are just as likely to be habitable as similar planets orbiting solitary stars.

No habitable worlds have been found in systems that have a white dwarf, neutron star, or other supernova remnant in them. Such systems have had supernovas in them.

Suitable orbits

To be habitable, a planet has to orbit its star close enough that the surface does not freeze over, but sufficient far away that at least part of the surface is both sunlit and cool enough for human habitation. The bounds of this “Goldilocks zone”, where planets are not too hot and not too cold, depend on the luminosity of the star and the inverse-square law: the habitable planets of cool K-type stars are close in and have short years; the planets of hot A-type stars are more distant and have long years.

The inhabited planet with the shortest year is New Firenze, which orbits a K6 star with an annual period of 600 hours (25 Earth days); it is tidally locked.
The inhabited planet with the longest year is Nijenhuis, which orbits a G8 IV (subgiant) star with a period of 28.4 Earth years.
Of worlds orbiting main-sequence stars that with the longest year is Tania, which orbits an A1 V star with a period of 13.7 Earth years.

The heating effect of solar radiation varies inversely with the square of distance from the star, but tidal effects go inversely with the cube of distance. That means that tides from the star are not significant in the Goldilocks zones of A, F, and G type stars. Planets in warm orbits around early K stars rotate very slowly or are tidally locked; the effect reaches all the way to the cool edge of the Goldilock zone by about K7, and every planet with an equable temperature orbiting a K8, K9, or M star tide-locks long before it develops an oxygen atmosphere. Tidal locking does not preclude habitability, but it does make it less common, since planets with a modest endowment of water can get most of it trapped in an ice-cap on the dark side, leaving their day sides predominately desert. Such a planet has only small areas of vegetation, photosynthesis is slow, and the oxygen atmosphere develops late and slowly.

The orbit of a habitable planet must be near-circular. A planet in an eccentric orbit gets hot around its perihelion and cold around its aphelion. Agriculture can endure a very cold season, but a very hot season sterilises the soil, and any planet that periodically reaches a temperature at which its water vapour escapes to space will lack water and therefore oxygen, and be uninhabitable.

Spinning, tidally-locked, and spin:orbit resonant worlds

Most inhabited planets spin fast enough that their day-night cycles are shorter than about 50 hours. On such planets the areas that cool during the night rotate into sunlight before night-time ice-sheets can form, and the Coriolis effect twists their winds so that the climate forms latitudinal bands, with warm equators and cold poles. The shortest day of any inhabited world is 10.4 hours, on Magsaysay. The median day-length of inhabited planets is 18.6 hours.

Planets that are close to the stars that they orbit are subject to intense tidal effects, which tend to slow their rotation with respect to the star. The usual end-point of this process is that the planet becomes tidally locked: it rotates at the same angular speed that it orbits, so that one side always faces the sun and the other the depths of space. The dark sides of such planets are permanently and uniformly cold, and thick ice-caps form there. The sunny sides are permanently warm, warmest at the subsolar point. The weather pattern is dominated by cold dry winds from the dark side to the light at the surface, and moist warm winds from light side to dark at altitude. The polar regions do not trap cold air, and are no colder than other parts of the terminator. About 10% of inhabited planets are tidally locked, but they tend not to be very populous.

Moons are almost invariably tidally locked to the planet they orbit, which gives them a day equal to their month. Planets in eccentric orbits that are subject to intense and prolonged tidal braking under some circumstances settle into a spin:orbit resonance, which gives them a day that is longer than their year, typically precisely twice as long. On such worlds the night is hundreds or thousand of hours long and as cold as the night-side of a tide-locked world. A blanket of snow forms, and the surface of lakes and seas may form a crust of ice. But before an ice-cap can form it is carried back into sunlight by the world's rotation, warms up, and melts. On such worlds the temperature at the poles is stable, while the temperature in the tropics varies through about 50 K over a period of some hundreds or thousands of hours. The only such world that is inhabited is Toutatis, which has a daylength of 8159.2 hours (0.93 Earth years).

Constraints on habitability

Surface temperature

Owing largely to the requirements of agriculture, human settlement is confined to areas where the average annual temperature is at least 273 K (0 C) and no more than 303 K (30 C). Bioengineered crop plants extend this range a little, but no world was settled unless it had extensive areas of its surface with climates in that range. Earth's global average temperature is about 290 K (17 C), which leaves the polar regions distinctly too cold for human occupancy and the tropics close to the limit.

On worlds that rotate fast enough to have latitudinal bands of climate that means that the global average surface temperature must be at least about 258 K (-15 C, allowing agriculture in warm areas in the tropics) and not more than about 328 K (55 C, allowing agriculture in cool areas near the poles).
Planets that are tide-locked to their stars must have global average surface temperatures no less than about 244 K (-29 C) (which is the minimum for the sub-solar region to be ice-free), and no more than 298 K (25 C) (which allows agriculture in twilight zone just sunwards of the terminator).
Worlds with very long days, for example habitable moons, and planets in spin:orbit resonances, have a fairly uniform average temperature, and are habitable from pole to pole provided that their global average temperature is between 273 K and 303 K. The polar regions are a constant temperature, the equatorial belt ranges from about 20 K below the global average at night to about 30 K above average in the middle of the day, which may allow seasonal agriculture.

The median global mean surface temperature of inhabited worlds in Flat Black is 287 K (14 C), approximately 3 K cooler than Earth.

Even if its annual average temperature is equable, a region may be uninhabitable if its seasonal variation of temperature is too extreme. Humans do not settle areas where the daily average temperature exceeds 313 K (40 C) in the warmest season nor where it is less than 263 K (-10 C) in the coldest season.

Large obliquity (axial tilt) is found more often on planets where the solar tides are weak, i.e. the systems of hot A and F stars, and in orbits far from the star. It produces the classical seasonal pattern, in which one hemisphere is hot while the other is cold, and the equatorial regions are comparatively stable. It precludes or reduces habitability of hot planets (their equatorial regions are too warm for agriculture and the polar areas become too seasonal), and is more tolerable on cool ones, where habitable annual average temperatures are found in the stable tropics. Tidally-locked planets, habitable moons, and planets in spin:orbit resonances have negligible obliquity.
Large eccentricity of the orbit produces global winters at aphelion and global summers at perihelion. These can make a planet uninhabitable if its “summers” are so hot that water vapour escapes to space from the upper atmosphere, or if there is no part of the world that is continuously between 263 K and 313 K.

Size and gravity

Humans can live and work in gravity up to 20 ms^-2 (2.0 g_♁), but settlers have proven reluctant to endure gravity higher than 15 ms^-2 (1.5 g_♁) unless the world is very attractive in other ways. The habitable world with the heaviest gravity is Huangdi, which has surface gravity of 15.5 ms^-2 (1.58 g_♁). The lower limit of gravity for habitable worlds is set not by human tolerance, but by the ability of the world to retain water vapour in its atmosphere. The world’s diameter, temperature, albedo, and the thickness of its atmosphere are also involved, making the theoretical limit a bit lower, but the lightest gravity on any inhabited world is 4.4 ms^-2 (0.45 g_♁) on Hylas. The median surface gravity of inhabited planets is 7.94 ms^-1 (0.81 g_♁).

A world’s diameter and its surface gravity are related by way of its density. Together the two determine its escape velocity, which is √(diameter × surface gravity). Escape velocity and the temperature of the upper atmosphere determine what gases the atmosphere will retain against thermal escape. If a world does not retain water vapour (molecular weight 18 a.m.u.) it will desiccate like Mars; having no oceans it will not support photosynthesising life and will have no oxygen. If it does retain hydrogen (molecular weight 2 a.m.u) it will become a giant planet with excessive gravity and a reducing atmosphere, with which any oxygen produced would react. Whether or not a planet retains helium (molecular weight 4 a.m.u.) is of little significance to habitability. The smallest inhabited planet in Flat Black is Surikate, which has a diameter of 6,400 km (0.51 D_♁). The largest inhabited planet is Golconda, which has a diameter of 21,260 km (1.67 D_♁). The median diameter of inhabited worlds is 10,800 km (0.85 D_♁).

Atmospheric composition and pressure

Oxygen

Permanent settlements of unmodified humans need oxygen to breathe at a partial pressure of at least 10 kilopascals (0.1 bar). Partial pressure of oxygen exceeding 55 kPa (0.55 bar) is toxic. These limits can be extended somewhat by humans or parahumans with modified physiology, but in Flat Black no planet nor moon was settled that required such modification of the initial pioneers. Atmospheres with more than 30% oxygen result in wildfires that recur until either the oxygen level is reduced or all terrestrial vegetation is consumed, so the air must also contain at least 70% of something other than oxygen. In 97% of cases the something else is nitrogen, and in the remaining 3% is is helium together with nitrogen. The minimum atmospheric pressure on a habitable world is 33 kPa (one third of a bar at 30% O₂). The maximum pressure of breathable nitrogen-oxygen is 355 kPa (3.55 bar), on the edge of both nitrogen narcosis and oxygen toxicity. The median atmospheric pressure of inhabited worlds is 89 kPa (0.89 bar).

Oxygen in sufficient quantities to breathe is found only as a by-product of photosynthesis, and in particular of the kind of photosynthesis that releases excess oxygen. The world has to be large and cool enough to retain water vapour in its atmosphere (in which case it will necessarily retain nitrogen, oxygen, and carbon dioxide), well-lit with light of suitable wavelengths of light, and old enough for photosynthesising life to have evolved and converted an entire atmosphere of CO₂ to oxygen. On Earth the evolution of an atmosphere containing free oxygen took 2.2 billion years. That was moderately fast for a planet of Earth's type. The process goes faster in systems where the star is hotter and the light is moderately more blue, slower in systems where the the star is cooler and the light more infra-red. Human intervention can cut out the need to wait for evolution and drastically accelerate the accumulation of oxygen by introducing a succession of highly efficient engineered photosynthetic organisms. Even so, under ideal circumstances it take at least a thousand years for a biosphere to produce a breathable minimum of oxygen. The Imperial Terraforming Service has begun such programs; none has yet borne fruit. The implication is that every inhabited planet had native, at least as sophisticated and photosynthesising bacteria. In most cases it has been eradicated to make way for biomes that are compatible with human crops, or hangs on only in isolated pockets.

Nitrogen

Humans don't need nitrogen in their breathing mix, but a habitable planet does need a nitrogen cycle to provide nitrate nutrients to crops, and besides, nitrogen is always found in the air of planets with oxygen because it is cosmically abundant and retained by any atmosphere that can retain water vapour. Nitrogen is narcotic at partial pressures above 300 kPa.

Helium

Hydrogen is so abundant that any world large and cool enough to retain it ends up as a gas giant consisting mostly of hydrogen. Gas giants never develop breathable atmospheres. But some worlds are small and warm enough to shed hydrogen but large and cool enough to retain helium (as well as water vapour, nitrogen, oxygen, carbon dioxide). Many of those are too cold or too heavy-gravity for human settlement, but there are a few that thread the needle. About 3% of settlements are on such planets. They have atmospheric pressure of up to 1000 kPa at about 90% helium, 8% nitrogen, and 2% oxygen without producing any ill effects. It is difficult to get fuel to burn or water to boil on such worlds.

Carbon dioxide

Humans do not require carbon dioxide in their breathing mixture, though they do exhale it. They can breathe a mixture with up to 1 kPa of CO₂ without ill effect. A small trace is required by plants, but 1 pascal partial pressure is ample.

Nevertheless, some planets do require CO₂ in their atmospheres to remain habitable. CO₂ is a greenhouse gas capable of warming a planet to a certain degree. On planets that retain enough internal heat to have tectonic or volcanic activity it is outgassed steadily. So long as there is a water cycle CO₂ is removed from the air at a rate that depends on the temperature. Thus (except on old planets with no volcanism or tectonics, on planets so well-heated that the CO₂ level is driven to near zero, and on planets so ill-heated that their oceans freeze over even when the CO₂ is thick enough to be opaque at the relevant wavelengths) surface temperature is stabilised by the carbonate-silicate cycle at a point where the is liquid water at the surface, an equable temperature. Because of that, habitable planets of which the other characteristics would make them hot have little or no CO₂ in their air, and those of which the other characteristics would make them warm have CO₂-rich atmospheres, unless they have no volcanoes or tectonic activity.

Argon & other traces

Atmospheres usually contain a significant trace of argon, which is not significant. It's a noble gas like helium, but less abundant and more easily retained.

Visible illumination

The degree to a planet or moon is warmed by its sun depends on the distance between them and the total output of radiant energy from the star, its bolometric luminosity. But small cool stars put out a great deal of their power in the form of invisible infra-red radiation, and very hot blue stars put out most of theirs in the form of equally invisible ultra-violet. Those forms of radiation heat a world efficiently, but they don't light the place up. The visible illumination of a world depends on its distance from its star and the star's visual magnitude. That means that similar worlds at similar temperatures are most brightly-lit if they orbit F0 white stars. On the worlds of A-class stars the light is slightly more blue and slightly less bright — more heat is arriving as UV. On the worlds of G, F, K, and M stars the light gets progressively yellower and dimmer. The planet of a K7 star otherwise equivalent to Earth is only 47% as well-lit as Earth; the planet of an M4 only 10%.

Now, the human visual system is so adaptable that that doesn't make a big difference to visibility. At 3,300 K an M4 star is whiter than an incandescent lamp or “warm” fluoro light; at 10% as bright as sunlight it is as intense as TV studio lights and twice as bright as office lighting. But the effect does make a big difference to photosynthesis. Breathable atmospheres are opaque to most wavelengths of UV and IR, so the energy is absorbed in the atmosphere. As for that which reaches the surface, UV and IR (except very near UV and very near IR) are not very suitable for photosynthesis. Individual photons of UV are too energetic in comparison to the strength of molecular bonds in organic molecules: they damage photosynthetic pigments. Photons of IR have too little energy for a single one to reform a chemical bond, and pigments that can accumulated the energy of several are complicated and inefficient. Photosynthesis depends on the availability of visible and near-visible light.

The rate of photosynthesis determines the primary productivity of a biosphere, which is the rate at which it can generate oxygen. So brightly-lit planets are developmentally accelerated and dimly-lit ones retarded. Where it took Earth 2.2 billion years to reach its oxygen catastrophe, a similar planet orbitind an F0 star would do it in only 2 billion, but the planet of a K7 would take 4.4 billion years. The planet of an M3 would take 15 billion years to evolve a breathable atmosphere, which is longer than the age of the Universe.

The rate of photosynthesis — the visible illumination — also determines the productivity of human agriculture. Dimly-lit planets support smaller populations than brightly-lit ones.

The most dimly-lit inhabited planet in Flat Black is Aurochs, which orbits a K8 V star and is 28% as brightly-lit as Earth. The most brightly-lit is New Macedon, which orbits an F0 V star and is 85% brighter-lit than Earth.

Volcanic and tectonic activity

Plate tectonics and volcanism are distinct but related phenomena that are ultimately driven by the transport of heat from a planet's interior to its surface. Significant sources of heat include the latent heat of fusion released as a planet's core solidifies, radioactive decay, and in unusual circumstances frictional heating from tidal kneading. Because of the physics of the first two processes large planets are more active, or remain active longer, than small planets of similar composition.

Volcanic and tectonic activity build mountains, they also drive the carbonate-silicate cycle, which to a certain extent stabilises the temperature of a planet by adjusting its greenhouse effect. Once the volcanic and tectonic activity on a planet stop its climate becomes unstable on a scale of millennia and its mountains start to wear down.

Hydrographic cover

If only a small proportion of a world is covered by oceans, then it will have a great deal of land surface, but much of that will be desert. On the other hand, if it is entirely covered by water there will be no dry land to farm. Living in artificial habitats that float on the surface or rest on the ocean floor is technically feasible, but even more than living in orbital habitats it has not, in Flat Black attracted settlers to any star systems that lack arable land.

Freely-rotating worlds with breathable atmospheres range from being completely covered with deep water down to about 50% coverage by oceans (including floating ice); moons and worlds in spin:orbit resonances are similar. On habitable worlds that are tidally-locked a large proportion of the available water freezes out as a deep ice-cap on the night side. Glacial flow and the melting of the base of the ice-cap by geothermal heat allows water to flow back to the lit side. Nevertheless, the daylit sides tend to be left rather dry.

The driest inhabited world is Aurelius, a tide-locked planet with “oceans” covering only 8% of its sunny side. The wettest is Wakushu, an ocean-covered world dotted with islands than make up less than 0.5% of its surface. The median settled world in Flat Black is 75.5% covered by water.

Systems with multiple habitable planets

It is possible for a star system to contain multiple habitable planets. 7.3% of inhabited systems have a second habitable planet or a habitable moon. There is even one settled system with three habitable worlds in it. But it is uncommon for a system to have more than one highly habitable world, and so usually only one world per system is inhabited. 54 Piscium was an exception: close to Earth it was settled at a time when only fourteen destinations for emigration were available, and it held two of them. Two colonies were established, Vasileo and Orinoco. Orinoco was rendered uninhabitable in the Formation Wars.

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Flat Black

The Thousand-year Raj

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Naturally habitable worlds