If the planet is tidally locked, the main determining property of the planet will be the heat transport from the warm to the cold side. There are two main heat transport mechanisms: Air currents (wind) and ocean currents.

To simplify the writing, I'll define principal directions as follows (this is different from the conventional definitions, but since there's no relevant rotation anyway "” unless the planet is very close to the star, but then the planet would probably not be inhabitable anyway "”, the normal definitions would be pretty useless anyway):

North is the direction away from the sun, south is the direction towards the sun. That is, it gets colder and darker as you go northwards.

East and west are the directions perpendicular to that, as usual. That is, in east-west direction, the brightness will be constant (except for terrain influences).

Air flow

The basic mechanism will be that the air/water will heat up and therefore rise on the southern side, and cool down and therefore sink on the northern side.

For air, what you'd experience is the lower part of the air flow. Therefore there would be a constant cold wind flowing from the cold to the warm side. The main influences on that wind would be mountains and oceans. Mountains can block winds if they are in east-west direction. They will not block the wind in north-south direction, but if shaped right, they might increase the speed, and therefore would allow the wind to get further before it warms up.

Also note that if the air goes over the ocean (especially if the ocean is warmer than the air), it will contain more water. If it then is forced to go over an (east-west) mountain, the water will condense and rain/snow down there, leaving condensation heat in the air; when the wind goes down it will therefore end up warmer than when it started (so-called Foehn wind). Of course if it is too cold north of the mountain, there will be no open ocean (free of surface ice) to draw water from (that might locally be helped using volcanism; even if the heat itself is insignificant, it might be enough to provide a significant open water area).

In the north, the extra cooling by the wind is unwelcome (it's pretty cold anyway). Therefore you'd preferably have east-west mountains north of the habitable zone; a well-placed east-west mountain in the north might be able to extend the habitable zone there, ideally with open water north of it. On the south side, the extra cooling would be most welcome; north-south mountains would be ideal. However very much in the south (ideally just at the end of the extended habitable zone), you'd again want east-west mountains in order to collect the water from the air before it goes off into the southern desert.

Also note that mountains could generate local wind patterns that differ from the predominant north wind. Eddies could even transport warmer air a bit to the north.

Other effects of mountains

Another point about habitability is the height. The air gets colder the higher you get, and therefore you could live in the mountains in the south where at sea level it already would get too hot. That's another reason why you'd want mountains in the south. Also note that in the shadow of a mountain (that never moves!) it will be colder than in the direct sun, so also those shadows might increase the habitable zone. Especially note that in/near the twilight zone you'll have very large shadows.

Air composition

Another thing to consider is the air composition. On one hand, you want greenhouse gases. Greenhouse gases not only make the planet warmer that it would be otherwise; by keeping the heat longer in the atmosphere, it also allows for more equal distribution. That is, a tidally locked planet which has less incoming radiation, but a higher greenhouse effect to achieve the same average temperature will have a larger habitable zone.

On the other hand, if there's much dust in the air (possibly because of lots of volcanism), the light will get more scattered, and therefore you'll get a more evenly distributed lighting (and a more impressive coloured sky/sun). Note that more dust will likely also reduce the average temperature.

Finally, there's also the refractive index of the air: The sunset on earth appears to be later than calculated from pure geometry because the air bends the light downwards. Therefore on the tidally locked planet, the twilight zone would be moved slightly to the north, leaving a larger area of the planet illuminated. The illuminated area would be the larger, the larger the refractive index of the air is.

Ocean currents

For ocean currents, you experience the upper side of the ocean, so the main effect of the ocean currents will be to transport heat from the south to the north. For example, Europe is much warmer than to be expected from the latitude thanks to the Gulf stream. Note that ocean currents are much more complex because of the continents, but as general rule you'd want to have oceans in north-south direction to enable northwards ocean currents. Also note that an ocean current could also be the reason why some northern oceans are ice free to provide a water source for the Foehn wind.

Orbital movement

Another point to consider is that the orbit might be slightly elliptic (most planet orbits are). In that case, the tidal lock will not be perfect, but the planet will apparently "oscillate" a bit around the lock position (because the planet's rotation is constant speed, but due to the elliptic orbit the revolution is not). Note that this is also the case for the earth's moon: It doesn't always show exactly the same side to the earth.

Such a slightly elliptic orbit would then cause seasons in some parts of the twilight zone (it also would have a seasonal effect on the total incoming radiation due to the varying distance to the central star). Basically, the sun would rise/lower a slight bit in the course of a year.

Since that apparent rise would also distribute the incoming stellar radiation over a larger area on average, it might also increase the inhabitable zone.

posted over 9 years ago

celtschk‭

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