A post on the Sci-Fi Ideas page got me thinking about face-locked habitable planets again.
My previous posts on Yaccatrice kind of cheated on the problem of tidal face-locking by placing the habitable world in a lunar orbit of a much larger planet to which it was face-locked. The Yaccatrice solution still appears plausible, but it was always a low-probability state of affairs. It’s a pretty delicate situation that isn’t likely to be seen very often. Any story-world where I situated a Yaccatrice-like world it would be unique unless I was positing a galaxy-wide or intergalactic civilization(even then there wouldn’t be many examples). I decided I needed to look at the more common forms.
The Orion’s Arm worlds of Twilight and Dante are pretty run-of-the-mill tidally-locked habitable planets with the habitable zone situated on the terminator in a perpetual state of twilight. Dante was particularly interesting because it rotated quickly enough to develop two moist habitable zones at the east and west poles(where the terminator intersects the rotational equator) due to coriolis-driven circulation. I disagree with the author of Orion’s Arm about the circulation pattern. He makes the assumption that warm moist surface air will flow across the east and west poles and return as a cold dry surface flow across the north and south poles. Coincidentally, the east and west poles in my model would still be wet, because this is where habitable temperatures will coincide with the rotation-driven inter-tropical convergence zone(ITCZ). This surface-level convergence causes a massive uplift of air which cools adiabatically as it rises till the temperature drops below the dew point and the moisture falls out as precipitation. This will create a band of precipitation all around the equator of the planet, although the precipitation will be at its weakest near the anti-stellar(midnight) pole and its strongest somewhere on the dayside to sunward of the east pole. If the planet has a rotation period on the order of a day(only possible for a habitable planet orbiting a star with a mass of much less than 0.1 solar mass, more like 5-7% of solar mass or less; generally a tiny red dwarf or brown dwarf) you could get a circulation with more bands like the Earth. This could lead to four “temperate” regions on the terminator between about 20 and 60 degrees latitude. The terms “temperate” or “tropical” will both be misnomers as the actual temperature in the habitable band will vary from very hot nearer the substellar point to very cold farther from the substellar point. As the “tropical” moist zones will be wetter on the west pole due to the prevailing trade winds, the eastern “temperate” zones will be moister due to the prevailing westerlies at their latitude carrying warm moist air off the dayside. The north and south poles will be dry due to the prevailing high pressure zone fed by the falling cold dry high level airflow.
Surprisingly, planets with very slow rotation might develop a simpler flow from cold side to hot side driven entirely by thermal differentials. My first naive assumption was that surface winds would flow outward from the sun pole toward the terminator and beyond. A momentary look at the thermal flows shows that to be completely wrong. Heat at the substellar point will cause air to rise causing a strong low pressure zone which drives surface winds towards the sun pole. This rising air will cool adiabatically and dump its moisture. For planets like this, then, the moistest region will be right over the substellar point and the best option for a habitable planet would be a fairly cool planet such that temperatures over the substellar point are reasonable.
The Orion’s Arm world of Bullseye looks similar to this, but the reason is different. The Orion’s Arm article on Vesperian Worlds assumes that worlds with denser atmospheres will tend to have more habitable dayside territory(as well as nightside) because of greatly reduced temperature extremes due to the moderating effects of stronger heat transport. This is completely true, and could potentially extend the habitable region well across the dayside and even into the nightside(for organisms that don’t rely on photosynthesis). On the other hand planets with thinner atmospheres(less than, as the page says, a few bar) could have prominent dayside and even substellar habitable zones due to being farther from their primary. This is important as an otherwise habitable planet where the only region with tolerable temperatures is in perpetual darkness is going to be a difficult place for Earthlike life to arise or prosper. We depend too much on our photoautotrophs…
Because the band of habitable temperatures is no wider than any other eyeball world, this implies some cold sunlit regions. I suspect the continuous sunlight, even if it doesn’t raise the temperature above freezing will gradually sublimate, leaving most of these areas looking like the Dry Valleys of Antarctica on Earth. There might even be a transition zone with rivers of meltwater from the farside glaciers.
Unless the planet is a nodder, like Chris Wayan’s Libratia with sufficient axial tilt and orbital eccentricity to move the substellar point at least about 5 degrees off of the equator, a hurricane simply can’t sustain itself over the
substellar point, no matter how hot the water is under it. Hot water would develop some interesting tropical depressions and storms that would spawn a permanent collection of hurricanes to the north and south of the SS point. Interesting…
Libratia is, itself an interesting case of the eyeball world. The axial tilt and eccentricity of that planet would have to be maintained by tidal interactions with other bodies since the stars tide will be trying to force the planet into a circular orbit with a fixed substellar point. Moving the tidal bulge around every year takes a lot of work(force times distance), but it shouldn’t be too unusual. Mercury, in the Solar system, is a nodder, probably driven by Jupiter and other planets. Perhaps the nicest part is the time variation of weather conditions. As the substellar point follows its ellipse across the surface most of the planet enjoys some kind of seasons. These aren’t your motherworld’s climates.
Raphael from Orion’s Arm is an interesting case. It orbits a red dwarf star that it is face-locked to, which in turn orbits a larger star which provides the bulk of the planet’s insolation. This case is similar to Yaccatrice, although I suspect that the red dwarf will provide sufficient insolation to make the area around Focus noticeably warmer than the far side.
There is a wide variety of different kinds of eyeball worlds ranging from almost earthlike to disturbingly exotic. Given the number of little stars out there, probably all the variations exist somewhere.