[Space]: Habitability modifiers for tidelocked and resonant worlds

[Agemegos]

In his classic (but now dated) Habitable Planets for Man, Stephen Dole argued that no tidelocked planet, nor any planet with an apparent solar day longer than about 96 hours, is likely to be habitable to Mankind. Dole's view was that in the case of tidelocked worlds the oceans and maybe the air would all freeze out into an ice cap on the dark side, leaving the dayside and the crepuscular band arid, and that in the case of slow-rotating worlds the daily temperature variations would at some stage become so great that no latitude would be warm enough at night and cool enough during the day for crop plants to survive.

Current views are a little less pessimistic. Atmospheric modelling of one-face worlds suggests that the atmosphere can transport enough heat from day-side to night-side to prevent the air and water from freezing out. And thinking of planets in spin-orbit resonance, with apparent days perhaps thousands of hours long, we perceive that the polar regions may be stable enough for a permanent growing season, while in the lower latitudes morning and evening can be growing seasons, with deciduous or "annual" plants surviving the day and night as seeds or in a dormant state. In the case of human crop plants that would require genetic engineering, but that seems less problematic than it did when Dole wrote, in 1971.

Nevertheless, it does not seem likely that tide-locked worlds, or worlds with thousand-hour-long days and nights would be quite as salubrious for human life as planets with a more homelike alternation of night and day. In the case of tide-locked worlds the dark side will be unsuitable for agriculture because of the perpetual darkness, while at least a large part of the middle of the day side will be unappealingly hot and perhaps dry. Plant growth will be confined to a narrow belt just on the sunny side of the terminator, and even there crop growth and photosynthesis will be restricted by the low light levels.

The Star System Generation Sequence in GURPS Space is remarkable and admirable in that it treats tidal locking and orbital resonance at all. It estimates the dayside and nightside temperatures of tidelocked worlds and the reduction in their hydrographic cover and atmospheric pressure produced by any freezing-out of air and water: with a bit of initiative the same tables can be used to estimate the "daily" temperature range of spin:orbit resonant worlds. And with a similar exercise of initiative the world's Habitability score can be calculated after temperature, hydrographics, and atmospheric pressure are adjusted.

That said, it seems clear that even if parts of its surface are watered, lit, and at least a large part of the time at an equable temperature, a tide-locked planet will not be able to support as many people as a freely-rotating one with the same average temperature--agriculture &c. will be confined to a small proportion of the surface--and a spin:orbit resonant one will be less attractive to settlers because of the extreme climatic variations of its long diurnal cycle. In short, there ought to be a negative modifier to the Habitability score of a world with a long or infinite day. This is important because, in a recent sample of 1,000 randomly generated systems, I found that 65% of planets with Habitability scores of 4 or above either were tide-locked to their star or had an apparent day longer than 96 hours. (In 1,000 systems, 69 had 'habitable worlds', of which 24 had apparent days less than 96 hours.)

How much of a tide-locked planet's surface is typically well-lit and a suitable temperature for plant growth? I find it hard to imagine that the figure would be 50%, so at least a -1 to Habitability for a tide-locked world must be called for. I would think that a -2 or even -3 might be appropriate (each -1 to Habitability halves carrying capacity).

How much should the ~ 160°F "daily" ranges of temperature in the equatorial regions of spin:orbit resonant planets affect Habitability, taking into account 1,000+ hour nights?