If a planet is too close to its hosting star, it will be tidally- locked. Such planets should be found around M - type stars, White dwarfs and Brown Dwarfs. The climate pattern around a tidally-locked planet is different from other planet models. This material describes possible climate models on a terraformed tidally-locked planet.
Around some celestial bodies, the Habitable Zone is so close that a planet will experience massive tidal forces. In that case, just like majority of satellites in Solar System, it will show the same face to its hosting star. If it rotates faster or slower, the gravitational stress from the planet will most likely generate strong volcanism, powered by the planet's own rotation speed. At some point, rotation will slow down.
On a tidally-locked planet, one side will face permanent light, while the other side will be in eternal darkness. So, life will be found mainly on the day side. Winds will constantly blow, exchanging hot and cold air. As they do so, they will create a stable climate. There will be no seasons and no day-night fluctuations. Climate will be very predictable. Settlers will have strong and permanent winds to use for energy generation. Rivers will also have constant flows, only with occasional variations. Since the amount of water is constant, the rivers on such a planet will be perfect for irrigation and power generation.
Life will find a tidally-locked planet to be a good place. Without seasons, plants and animals will not migrate. They will stay in the same area. Without seasons, nature will not follow the same cycles we see on Earth. In a temperate climate, flowers may bloom all the time and slowly evolve into fruits. Without seasons, crops can be planted, grown and harvested all year-round. As a direct result, settlers will not need to store their harvested goods to feed all year; a grain storage silo would be useless. They will have fresh vegetables all the time.
There will be, however, a small problem. The closer you get to the twilight side, the more tilted the sun will be. Plants have a tendency to grow directly towards the light (as houseplants are always turning in direction of the window). So, you will see trees growing not straight up, but tilted.
Wildlife will also be different. Since there will be no winter, hibernation will not exist. Animals will breed all year and you will see their young all time around.
If there are settlers, they will need time to adapt to this endless day, but sooner or later they will. A similar problem is seen with people living in Alaska or Northern Siberia.
On the night side, life can still exist, limited around thermal springs. Tidally-locked planets have a high chance to have active volcanism. Also, some marine plankton can be driven by oceanic currents into the night side, where hungry animals are waiting for them. Human settlements may exist on the dark side, as well. Mining and other industrial activities can be done without sunlight. Keeping industrial centers at distance will reduce pollution to the illuminated side of the planet.
Would alien life adapt to these conditions? Some would, some would not. Some animals need seasons to survive; without them, their breeding cycle does not start. However, in many cases, random cycles can start. Most probably, the majority of species could adapt.
A smaller planet will experience lower climate variations, since its air will need less time to travel across it. Larger planets will have stronger differences between climate zones.
This is the most optimistic planetary model. It will have all climate patterns found on Earth, but without seasons.
In the center, we expect temperatures to rise to 158° F (70° C), hotter than anywhere on Earth. This could create a very inhospitable desert. If water is present, it will be host only for thermal organisms.
Surrounding the very hot area, depending on rain, there should be equatorial forests, savanna and deserts. It all depends on air circuits. If winds blow over a sea, they will bring water with them. If they blow over land, they will be dried.
Further away, are expected to be temperate regions. Again, depending on rain average, they could look like Mediterranean coasts, wetlands or deserts, like the Aral Sea basin on Earth. In cooler regions, we might encounter wet, foggy places like in England or even endless cold forests like in Siberia.
Along warm air currents, temperate climate will stretch even into the dark hemisphere. Along cold currents, things will be different. We will find tundra and permafrost. So, a tidally-locked planet could host Polar life forms.
On the night side, there will be ice. There will be a delicate balance to keep glaciers at a constant size. If they start to accumulate ice, this will lead to an eyeball planet. On opposite, if they lose water, we know from Earth what can happen.
An eyeball planet will be created when almost all water is stocked in glaciers on the night side. The day side is a desert, while the night side is all covered with ice. Water is melting and flowing through the twilight zone. Some rivers could flow deeper into the warm, but eventually they will dry-up. On an eyeball planet, settlers will look for land on the ring that forms the twilight zone. We can imagine a planetary highway and railway connecting settlements along the ring.
The eyeball model does not talk about disruptions. It is impossible that this ring will be perfect. There will be an air circuit. Warm currents, blowing hot air, will push desert maybe even inside the dark hemisphere, while cold currents will force ice deeper inside light hemisphere. But even so, the wet zone will not be a perfect circle, but will still be a ring-shaped area on the planet.
What happens if a planet has large oceanic surfaces and continents are small and diffuse? For example, imagine a tidally-locked Earth, with many small continents the size of Greenland or the Arabian Peninsula. In that scenario, the ocean can flow through any direction and will help modulate the climate.
Having both global air and water circulations, heat from the light hemisphere will travel faster to the night hemisphere, while cold air and water will return. Our planet will have strong winds, but also strong oceanic currents.
The direct result will be less ice on the dark side. Glaciers can accumulate only on dry ground. On water, they will only get at maximum a few meters thick. Ice that will accumulate on the dark side will never be that much to transform the planet into an eyeball model. As the water advanced into the dark side, it slowly cools. Fish and plankton brought by currents will still be in significant amounts, so that they could feed local animals and human colonies. Water entering the light hemisphere would be covered with icebergs that will slowly melt as they move forward.
The oceanic model suggests that in the center of light hemisphere an eternal hurricane will form. Having enough heat and wet air, nothing will be able to stop it. It is possible that small hurricanes will separate from it, affecting the surrounding land.
Main article: Oceanic Planet.
A special eyeball model is of a planet with high amounts of water. On the dark side, the planet will be covered with a thick ice crust, but on the day side, the ice would melt. Without natural barriers (mountains), air currents will have nothing to stop them. So, we expect in the center of the day hemisphere there will be a powerful, never ending hurricane.
If the ocean is deep enough, the ice will always be floating on a layer of water. This way, it will be impossible for all the water to condense on the dark side. Also, warm currents will slowly melt the ice and keep an equilibrium. Icebergs will be common and they will continuously depart from the dark side, but will melt in warm water currents. An equilibrium will form.
If the planet has some islands on the day side, they will be able to support human colonies. To see how would such a planet look, increase Earth's ocean level to 4000 meters. From Asia, a small continent will survive, composed of the Himalayas, Tibet and Pamir Mountains. From South America, the Andes will form a long and sharp continent. From all the other continents, what will not be submerged will be islands, formed by the higher mountains: the Alps, the Caucasus and a few other mountain ranges.
On the remaining dry land, climate will be wet, with enough rains to sustain agriculture. However, most of the food will come from the oceans. The heart of the day hemisphere will not be habitable, but all islands located on the day side will be suitable for colonization. On the night side, islands will be permanently covered with ice. Glaciers will form there, but will not grow large enough to strongly influence the sea level.
At the border between day and night, there will be both ice and exposed ocean. In the absence of continents, the ice shelf can be used as an alternative for building settlements, but only for a limited time, since ice sheet will move slowly.
Tidally-locked with Greenhouse Gasses
Let's look at White dwarfs. Around the coldest ones, there is a possibility for a planet to be tidally-locked, but at some distance. Before terraforming, that planet would experience temperatures like those found on Jovian moons. Afterwards, greenhouse gasses would keep it warm.
Energy output from the star is minimal. As a result, the light hemisphere will heat slower, while the dark hemisphere will cool more slowly. Air circulation could then keep temperatures above freezing for the entire planet. We would have, for example, a maximum temperature of 86-104°F (30-40°C), with a minimum temperature of 50-68°F (10-20°C). However, air should contain more moisture.
M - type stars sometimes produce violent flares that are able to blow away atmosphere and water. In that case, all water would be frozen on the dark side. If water is not enough, settlers would need to divert comets and other Kuiper belt objects to create oceans. But if that star has no Kuiper belt equivalent (for example, after a close encounter with another star), settlers will have severe challenges in order to terraform such a planet.
After terraforming, water will only condense on the dark side, but plants will mainly grow on the day side. In order to survive, settlers would need to transport water. On a dried planet, we could see trains carrying ice and liquid water. It might look like Frank Herbert's Dune, where water is scarce.
Planet with Two Suns
Many star systems are binaries. A good example is Omicron (2) Eridani or EZ Aquarii. Both systems are made of 3 stars and in each system two stars have habitable zones in places where a planet would be tidally-locked.
On the light side, extra light brought by the other sun wouldn't be as important. It could potentially heat the air an extra 32.18-32.9°F (0.1-0.5°C). On the night side, the extra light also would not influence much of the climate. The main difference is that the little extra light might give some plants a better chance to live. Plankton in the oceans can use that light to survive until currents will take them back on the light hemisphere. On land, some plants can better survive and keep some ecosystems alive.
Tides from the extra sun could potentially be enough to allow for monthly cycles. Many animals depend on tides to feed and breed.
The extra light and extra tides can affect climate on the planet, but only producing low seasonal variations. On light hemisphere, they are too faint, but on night side they can be considered seasons.
If a planet is tidally-locked but has an elliptical orbit, it will face massive volcanism. Just imagine Io located close to a red dwarf. If settlers did manage to terraform it, by changing atmosphere and bringing oceans, there would be massive problems. Earthquakes are expected to happen and be very violent. Fissures would form on the planet's crust. Volcanoes could blast lava across large areas. Gasses can erupt from the interior and from the volcanos, changing atmospheric composition. Still, this harsh environment could become a home for some people.
History has proven that life can thrive in harsh conditions. If a sentient race existed on such a planet, they would likely be a hardy and resilient people.
There may be other types of tidally-locked planets yet to be theorized.