Planets orbiting their hosting stars between 0.6 and 0.3 AU are expected to rotate slower, but still not to be tidal locked. In Solar System, this is the case of Mercury and Venus. For K - type stars, they are likely to be found in the classic Habitable Zone, while around M - type stars they would be in the extended habitable zone, where they would request greenhouse gasses. Around G - type stars, they would still exist.
In comfort zone Edit
Comfort zone around a star is the best orbit from a habitable zone, where an Earth-like planet will experience temperatures just like we have here on Earth.
A model of a low spinning planet is shown on this video. Days and nights are far longer.
If we imagine Earth spinning once every 20 days, we get very hot days and cold nights. During daytime, temperatures will reach Sahara values in Scandinavia, while in the long nights, it will freeze even in Florida. If the rotation period exceeds 100 days, temperatures will reach even greater boundaries. In daytime, it might be so hot that water will reach boiling temperatures. Nights will bring Siberian cold.
Winds will be strong. High temperature variations means that air pressure will change, feeding strong winds.
Vegetation will have to go over massive challenges. Most adapted to this, are plants and animals found in tundra, but even they will hardly survive during hot days. Plants need to be adapted. Closer to the poles, during daytime temperatures will not climb to such high values, so that plants and animals will survive, but they will need to hibernate over the night. Closer to equator, only genetically modified plants and extremophiles are able to survive.
If the day cycle is long enough, animals will most likely migrate. There might be two types of migration. First, some species will move in the morning to the equator, at noon closer to poles and in the evening they will return to equator, where they will hibernate during the night. Second, some animals might migrate together with the sun, staying only in morning or only in evening. Since Earth equator is 40 000 km, migrating along it would require to travel every day 109 km in order to circle the Earth in 365 days. A migration closer to the poles or on a smaller planet would be more easy. However, if the day cycle is short, the only solution for animals will be to use morning and evenings to feed, at noon to stay in cooler places (like caves) and to hibernate in night.
If day cycle is shorter (for example 20 days), weather pattern will be like this: The extreme heat of noon will make atmosphere to accumulate water. Evening sky will be covered with clouds. In night, it will start raining and even snowing. In morning, temperatures are low. If the ground is covered with snow, it will take more time to heat. Morning will have a blue sky and it will remain blue and cloudless until noon, because temperature is rising and the atmosphere can retain more water.
Closer to the sun Edit
On a low-spinning planet, closer to its sun, temperatures will be higher. That planet is in danger of a runaway greenhouse effect. What can be done in that case? Some high-tech engineering is needed. Some solutions (mostly theoretical) exist. The goal is to create a shield that will reflect part of visible light but allow infrared waves to escape the planet in daytime. During the long night, that shield will do the opposite. It will reflect infrared light back to the planet, to keep it warm. To do this, there are a few solutions:
- Build a planetary shield (impossible with current technology)
- Use some special gasses able to do this (mirror greenhouse gasses, not studied in present)
- Create an army of balloon microbots (possible, high cost).
If the night is not too long (below 20 Earth days), plants and animals will survive without hibernation. However, if the night lasts for long (let's say 100 Earth days), plants will exhaust their resources. Without light, plants try to grow as high as possible, but this also is consuming their energy. Genetically modified plants should be needed if we don't want temperature to drop below freezing in night time.
Outer planets Edit
Main article - Outer Planet
a planet outside the classical habitable zone could be still terraformed with greenhouse gasses. The planet receives less solar light to heat, but also radiates less heat because of the greenhouse gas layer. At the orbit of Jupiter, a planet will receive from the sun only 1% of the energy received on Earth. At the orbit of Neptune, the amount of energy is only 0.1%. Also, in the asteroid belt, solar energy is low, around 10%. On a planet receiving 10% of solar energy, with a similar day length with Earth, a day-night cycle will produce temperature variations of only 10% of what we see on Earth. If the day is 10 times longer, the maximum temperature variation will be 10% X 10 = 100% of what we see on Earth. However, because of air currents, the difference will be lowered. If we exclude air currents, we get these maximum temperature variations, assuming a day-night variation of 10 degrees for Earth:
- 10% of Earth light
- 1 degree C for one Earth day cycle
- 10 degrees for 10 Earth days cycle
- 100 degrees for 100 Earth days (in fact not more then 30 C, because of air circulation)
- 1% of Earth light
- 0.1 degrees C for one Earth day cycle
- 1 degree for 10 Earth days cycle
- 10 degrees for 100 Earth days cycle
- 0.1% of Earth light
- 0.01 degrees C for one Earth day cycle
- 0.1 degrees for 10 Earth day cycles
- 1 degree for 100 Earth day cycles.
For a hypothetical planet placed in the asteroid belt, it has to spin 10 times slower then Earth to have significant day-night temperature changes. If it spins slower, this will affect weather. Water evaporated in daytime will rain or snow in night.
For a hypothetical planet placed behind Jupiter, there will be another problem. Plants don't have enough light there. They are able to survive as far as Neptune, but if the night is too long, they will die exhausted. Temperature never drops to freezing point, so plants and animals will not hibernate.
As proposed in other pages, settlers might try to increase rotation speed. They might try to hit the planet with comets or asteroids or use other high-tech technologies. However, they might not be able to use any of these and will have to live with what they have.
Humans find hard but not impossible to adapt to long polar days. however, there are people that have lived all their life in polar regions.
For a planet with a higher rotation speed (5 to 50 days), settlers will have to live with it. Agriculture will require special buildings, like a greenhouse dome for night, with cooler devices for day. Houses will have adequate heater and cooler equipment or will be built underground, where temperatures are more stable. entire cities could be built below surface. Also, since rains are expected to be more often during night, storage facilities (dams) will be needed, to keep water for the hot day, to irrigate and cool the crops.
For a planet with a lower rotation period (over 100 days), settlers will have many problems with agriculture. If possible, they would plant seeds in morning, to harvest in evening. But if the noon heat is too high, they will have to use cultures that can survive the night. Planting seeds in evening and harvesting them in morning would look somehow similar to how we cultivate grains today. Grain crops in temperate regions are planted in autumn and harvested at the beginning of spring. If these solutions are not feasible, settlers will have no options that to cultivate plants into a dome, where they can control light and temperature. they will have to illuminate and heat them in night and to cool the air at noon. Also, they will need water for irrigation. Settlers might find more easy to migrate, running away from extreme heat or cold. There could be moving cities, slowly moving with the twilight zone. Also, we could find cities that are populated in morning and evening, but abandoned at noon and night.
In case of an outer planet, if the rotation time is 5 to 30 days, nothing will happen. However, if rotation time is above 100 days, settlers will have to grow crops that have a lifetime equal to day length. They will be safe from extreme temperature changes.
A low-spinning planet will force settlers to work together to survive and progress. Only massive engineering projects (dams and irrigation schemes, transport networks able to move entire cities) could make that planet suitable for advanced human life.