If a planet is too close to its hosting star, it will be tidal locked. Such planets should be found around M - type stars, White dwarfs and Brown Dwarfs. The climate pattern around a tidal-locked planet is different from other planet models. This material describes possible climate models on a terraformed tidal-locke planet.
Around some celestial bodies, the Habitable Zone is so close, that a planet 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 planet's own rotation speed. At some point, rotation will slow down.
On a tidal-locked planet, one face will face permanent light, while the other face 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 of such a planet will be perfect for irrigation and power generation.
Life will find a tidal-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 are blush all time and slowly evolve into fruits. Without seasons, crops can be planted, grown and harvested all year around. As a direct result, settlers will not need to store their harvested goods to feed all year. A grain storage silo will be useless. They will have fresh vegetables all the time. There will be, however, a small problem. The closer you get to the twilight, the more tilted the sun will be. Plants have a tendency to grow directly to the light (as house plants are always turning in direction of the window). So, you will see trees growing not straight up, but tilted.
Wild life 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.
First, there will be a problem with settlers, until they will adapt to this endless day, but sooner or later they will do. The same problem is seen with people living in Alaska or Northern Siberia.
On the night side, life can still exist, limited around thermal springs. Tidal locked planets have a high chance to have an active volcanism. Also, some marine plankton can be driven by oceanic currents into the night side, where hungry animals are waiting for them. Also, human settlements should exist on the dark side. Mining and other industrial activities can be done without light. Keeping industrial centers at distance will reduce pollution to the illuminated side of the planet.
Will life adapt to these conditions? Some will, some not. Some animals need seasons to survive. Without this, their breeding cycle does not start. However, in many cases, random cycles can start. Most probably, majority of species will 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.
Equivalent Earth planet Edit
This is the most optimistic planetary model. It will have all climate patterns found on Earth, but without seasons.
In the center, we except temperatures to rise to 70 C, hotter then 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 raining, 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 tidal locked planet will host even Arctic and Antarctic 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.
Eyeball planet Edit
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.
Oceans into equation Edit
What happens if a planet has large oceanic surfaces and continents are small and diffuse? For example, imagine a tide-locked Earth, with many small continents, on the size of Greenland or Arabic 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 to 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.
Tidal locked with greenhouse gasses Edit
Let's look at White dwarfs. Around the coldest ones, there is a possibility for a planet to be tidal locked, but at some distance. Before terraforming, that planet experienced temperatures like those found on Jovian moons. Now, greenhouse gasses keep it warm.
Energy income from the star is small, but also energy outcome (radiation heat) is small. As a result, light hemisphere will heat slower, while dark hemisphere will also cool slower. Air circulation could then keep temperatures above freezing for the entire planet. We would have, for example, a maximum temperature of 30 to 40 C, with a minimum temperature of 10 to 20 C. However, air should contain more moisture.
Dried planet Edit
M - type stars sometimes produce violent flares that are able to blow away atmosphere and water. In that case, all water should be frozen on the dark side. If water is not enough, settlers will need to divert comets and other Kuiper belt objects, to create oceans. But if that star has no Kuiper belt (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 size, but plants will only grow on the day side. In order to survive, settler will have to transport water. On a dried planet, we will see trains carrying ice and liquid water. It might look like Frank Herbert's Dune, where water is of high price.
Planet with two suns Edit
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 tidal-locked.
On the light side, extra light brought by the other suns will not be so important. Maybe it will heat the air with 0.1 to 0.5 C. On the night side, the extra light will also not influence too much the climate. The main difference is that the little extra light might give plants a chance to live. Plankton in the oceans can use that light to survive until currents will take them back on the light hemisphere. On ground, some plants can survive and keep some ecosystems alive.
Tides from the extra suns can be enough to bring month 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.
Tortured planet Edit
If a planet is tidal locked but has an elliptical orbit, it will face massive volcanism. Just imagine a Io located close to a red dwarf. If somehow settlers manage to terraform it, by changing atmosphere and bringing oceans, there will be massive problems. Earthquakes are expected to happen and be very violent. Planet's crust can create fissures. Volcanoes can blow lava over large surfaces. Gasses can erupt from the interior and from the volcanos, changing atmospheric composition. Still, this harsh environment can become a home for some people.
History has proven that in harsh climate people can become better warriors. It was proven first by the Vikings, then by the Mongols. Tortured volcanic planets are the best candidates for future warrior races.
There might be also other types of tidal-locked planets. Only future settlers will know.