In my world, daytime lasts for 9 years. I am trying to make this as realistic as possible.
My own theories are that they would migrate with their natural half and that plants would evolve ways of hibernating knowing when night is coming since sunset and sunrise would last around 239 days.
not enough time for a full answer, but a 9 year night is hard to survive, since sun is the source of almost all energy on a planet. I think they life would migrate so that it was always day for them by slowly traveling with the sunlight. Things that can't move (plants) would only bloom once every 9 years. Very little would try to live during the 'night'.
Since migration is simple though, it's easy to have a never-ending day. After that it's pretty easy to look at local life to see how it may adapt. There would likely be a slighter focus on efficient movement (for instance, more upright walkers like humans, who are quite efficient at long distance travel). Likewise the concept of 'territories' may change some with the need to migrate. I imagine many short lived creatures that establish a territory but will die within the 9 years so don't need to worry about migrating. All longer lived creatures will have a tendency to slowly shift their territories to keep up with a slow-pace migration.
You would have creatures evolved to the twilight, the areas where sun light is just starting to hit part of the planet, or just starting to leave as well.
Of course migration only works if you can migrate. Thus I would imagine flying and aquatic creatures as the most common. Land based critters would run into physical limitations preventing them from traveling across the entire glob.
Of course this is only part of the answer. Not all creatures will migrate, and I can think of other adaptations. I don't have time for them right now though, just wanted to toss out the most interesting/common.
Edit:
To answer the question as to rather life would live during night time, yes it would. That's part of the answer I didn't have time to elaborate on.
Life always finds a way to live in the harshest of environments on earth. If there is an area without life something will adapt to live there; so long as some sort of energy source will be found. I could think up many examples of what could, but didn't want to elaborate on all of them. I think migrating species will be the most prominent creatures, by a noticeable margin, and the ones most like mammals of earth and thus most interesting to human readers, but they will not be the only types.
Look at deep see fish, that live in conditions of never-stopping night. Things could live due to heat vents and other geo-thermal energy. Some creatures will be scavengers that live by preying on things that die after night falls. Some equivlents of giant tress will simply loose it's leaves for 9 years while living off of stored energy, in fact some animals will likely evolve some extreme hibernation tactics to allow this as well (there are frogs that can live in a sort of frozen suspended animation for an eternity; and after 9 years you better believe it will be below freezing). If creatures live via the above methods in the dark then creatures will adapt to prey on the above creatures as their food source.
However, all of the above (beyond geothermal energy) effective rely on consuming energy sources generated during the 9 years of daylight, finding ways to store up energy during your 9 years of 'summer' for 9 years of 'winter' effectively. In fact maybe some version of squirrels that can store energy sources to feed them for 9 year nights by stockpiling for 9 year days could exist, though it seems less likely since that strategy only works if the squirrels can manage to keep other creatures from raiding their food supply for 9 years. A species like that would have to have a very good strategy for protecting the resources they stockpiled.
However, living during the night requires some extreme adaptations, which are different then the ones for days. I could, in fact, imagine creatures that migrate with night the same wah some migrate with day, to keep a never-changing night. Imagine a scavenger species that lives in night, and travels right at the 'edge' of night time, looking for creatures that died recently when night fell and they couldn't find food.
Creatures that live in one location for night and day seem less likely, just because of how hard it is to evolve adaptations that work in both enviroments. The most common of such species would be the 'hibernate' type, that simply go dormant during night time and start living again during day. Though, as said all adaptations will exist, some species will find a way to live through night and day cycles somehow, life is resourceful, they will just be quite uncommon.
Oh, I should mention that another option to hibernation that will be very common is 9 year life spans. A creature that lives only while it's day and dies at night, but not before planting the seeds for the next generation. Many species have adopted this strategy, most obviously in the desert where some species only live during rainy seasons and then die after leaving behind seeds/eggs for the next generation. Obviously plants will mostly always do this, but eggs capable of surviving for 9 years are possible too, though any species with this strategy would have to be HEAVILY R select species, which means they lay lots and lots of eggs expecting most to die, and aren't very intelligent.
Which, of course, means one of the most common adaptations for creatures that live during the night may be finding the eggs and seeds left for the next generation to eat. A treasure trove of stored energy left behind by the last day. Imagine a massive evolutionary arms race between better protecting your seeds/eggs so they survive for 9 years, and predators finding better ways to find these well protected eggs.
Still, migration or dying after 9 years are going to be by far the most common strategies. Hibernation (including trees that loose their leaves and similar) being the next most common strategy. Creatures living through both day and night will be quite uncommmon, though some will show up.
And now I wasted even more time on this answer; good going. There are still questions about the world itself, in particular how much energy it gets from the sun and how it regulates it, but also it's gravity, atmosphere, etc etc, which is quite relevant to creatures adaptation. For the most part I assume a 'mostly earth like' world in this answer, but I think that very interesting weather patterns would arrive from one part of the world receiving all the energy while the other doesn't; which would in turn have a strong effect on adaptions. However, I'm afraid I really don't have time to go into them.
I doubt that life could evolve on such a planet, though it might be able to survive if it was put there.
First of all, let's establish some basics - in order to have an orbital period of around 2.4 Earth years (~21/8.9) the planet needs to be about 1.8 times as far from the its sun as the Earth is from ours. In our solar system, that would be about halfway between Mars (1.5 AU) and the inner-most part of the asteroid belt (2.2 AU).
With our sun the planet would probably be too cold, but if you say its sun is about 33% brighter than ours, the planet would be getting about the same amount of energy as Earth currently does.
(Side note - I think an almost-tidally-locked planet that far away from its sun is stretching plausibility a little. The process happens fastest to planets that are much closer to their suns)
So, if the planet is given about as much energy as the Earth gets, what does it look like? An oven on one side and a freezer on the other. With constant sunlight for almost 10 Earth years, temperatures are going to get very high on the day side. There will still be some liquid water but only because the air is so humid that it can't hold any more water. So any pools would have water temperatures above 100 degrees Celsius. There's no way for anything to reduce it's body temperature - everything is uniformly hot.
The night side isn't much better. Without any sunlight for 10 Earth years, it's going to be frozen solid, so any oceans will end up covered in a thick sheet of ice. However, this will insulate the water below it, preventing the ocean from freezing solid. This makes the oceans potentially survivable, though the lack of sunlight prevents a good food chain from being established. Anything on land is going to die - all water is going to be frozen solid, and any plants would shatter as the water inside them froze. Hibernation isn't a viable for animals - body temperatures need to stay above freezing, and with how cold it would be they would still be spending a lot of energy simply staying warm. There's no way they would be able to store up enough energy ahead of time to survive.
So what about the dawn and dusk areas? Here, the temperatures are not so extreme, so you might think that it would be a good place to live. Think about it though - on one side you've got a large mass of cold, dry air, and on the other side you've got a large mass of hot, moist air. That's a recipe for massive storms. Anything living here would be subject to constant thunderstorms and tornados. Unfortunately, that means that there's not enough sunlight for plants (even if they could avoid being torn out by the high wind-speeds).
This planet is going to be very hostile to life. There's no way that life could evolve there - it would not have billions of years of evolution behind it to help it survive extreme conditions. Life would not have enough energy on the night side. Everything gets cooked on the day side. The dawn and dusk areas are perpetually storms, preventing anything from getting established.
What if we turn down the sun a little?
This could make the day side of the planet habitable - if the sun is providing just enough energy to keep the day side at a reasonable temperature, plants and animals could live there. Animals would always be gradually moving toward dawn. Plants would follow pretty much the same strategy as they do here - use wind and animals to spread their seeds, especially toward dawn where no plants are living yet (as it is just becoming newly habitable again).
The night side is still (and even more so) uninhabitable. The temperatures are just too cold for anything to survive.
The dawn and dusk area as still stormy. With temperatures being more moderate on the day side I believe the storms won't be as intense, but I'm not sure how different they would be.
How would life evolve there? This is the hardest part. Ocean currents would have to be just right for millions and millions of years. As life was getting started and evolving, it would not have any capability of migrating on its own. It would have to rely on ocean currents to keep it in on the day side. However there's going to be warm and cold water mixing, just as there's warm and cold air mixing. This makes the ocean about as dangerous as the sky. It's going to cause ocean currents that pull warm water into the night side, and push cold water into the day side. Primitive life would have to stay away from both the dawn and the dusk sides if it hoped to survive and, as I said before, it has no way to do that on its own.
So again, it is very unlikely that such a planet would be capable of developing life, but developed life could be put there and survive.
Without any other forms of heat, the night side of your planet will be extremely cold. With a night of around 4 local years (just over 9 earth years), there will be plenty of time for things to cool.
If the ambient temperature at the coolest time of night (between 3-4 years after the end of twilight) dips below the freezing point of any of your atmospheric gases, you'll end up with some impressive winds.
To start with an earth based example, if the temperature in a locale dropped below -57C (-70F) then all the Carbon Dioxide would freeze out, creating a small local decrease in pressure, causing inbound winds. With CO2 forming less than 0.04% of the air, the effect would be low.
But, what if this happened with Nitrogen (temperature below -196C, -321F)? Your local partial pressure would be very low, with 78% of the atmosphere falling to the ground as liquid or as solid.
So, from an ecological standpoint your environment will include temperature extremes between the depth of night and the height of day. Both sunrise and sunset will be characterised by violent storms with winds blowing across the day-night terminator at high speeds. Mornings will be extremely cold as it will take significant time for the sun to melt all the gases back into the atmosphere.
[The effect reminds me of the world "Trenco" in the Lensman universe, though the technobabble used to justify it there is pretty out there.]
I love this questions, it really captures the imagination! What a great world.
Mushrooms don't photosynthesize. There could be giant fungal forests!
To evolve non hibernating land mammals that don't fly (ie humans), you would need a land mass that includes every longitude. The easiest would be a land mass that just includes one of the poles - it could go all the way around the equator, but that seems less likely.
Rather than being constantly on the move, I think people would stay for years in cities they built the day before. They would migrate throughout different times of the day. Communities could send their members back and forward in different times of the day, depending on their needs. (Need to start setting up the next city - send a group forward towards dawn! Need more food that is harvested in dusk - send them back!)
I particularly love the area just after the sun has set. At first I thought that people would not really spend much time in dusk, but the world is still fairly warm, so I think that many people would still be harvesting all the seeds that were planted in the day.
Did you know there are three different types of dusk?
Civillian dusk (The sun is 6 degrees below the horizon): Civillian dusk is the point at which we need artificial light to read. Objects are still visible. Civillian twilight occurs for 6 degrees ~ 127 of our days.
Nautical dusk (12 degrees below the horizon): objects and the horizon are no longer visible. I think this woul be the time for the serious predators.
Astronomical dusk (18 degrees below the horizon): The sun no longer interferes with astronomical observations - important for science!
If you used an earth sized world you could use your clock for reference with each hour of your time being 137 days(ish) on the alien world. So with that in mind here are some ideas of what it would be like and what would live there. I almost always start with the environment fleshed out before building an alien life form.
With an environment like that I would go with pretty extreme weather between the sides. The continuous light and heat would cause intense storms as the water vapor carrying all that heat would rush up to be cooled in the upper atmosphere and condense back into water. It is possible for the day light side to be covered in constant clouds as the vapor heats and cools in the upper atmosphere and never reaches the surface. It could be a churning storm of lightning and thunder without end. In some areas where there is no water you would have intense heat and wind
Along the boundary there would be intense storms and flooding as all the water comes back out of the upper atmosphere. Further in you would have snow and ice hundreds of stories thick from the constant moisture. As the sun reaches these frozen zones it would begin to flood back into the low lands. The night side would be frozen glaciers hundreds of stories thick. Every dusk would build them up and every dawn they would shatter, melt and flood.
Under this kind of constant assault flood plains would cover the world. Large features would be shattered and shoved along by the flow of ice. On the night side as the ice would freeze and expand being pushed up into towering mountains of ice along raised features.
The rocks and debris of mountains crushed under the ice would cause canyons and fissures to be cut through the landscape. The grand canyons and massive cave systems would be carved world wide.
Now out of this idea you could go with life having to be very hearty. thermal vents and hot springs would have to be the source of many life forms but the flooding would be washing them away. Some may evolve within the clouds boiling along the hot and cold zones of the upper atmosphere.
The cold night mean they would have to drop back to the ground and survive the freezing nights. So life may be more virus like or able to die and come back since nine years of night is a bit much to store food for.
The native life could be a mix of various floaters and fliers that evolved in the upper atmosphere of this world. Made of carbon or other elements of the atmosphere they could forever live in the sky following the clouds.
Other life forms are born from seed like objects on the ground. They hatch when the dawn comes, ready to feed and grow and finally reproduce and die when the snows come some 3 years later.
Yet other life forms simply move with the sun. Their life cycle short enough that they can be born and migrate as the sun moves eating the seeds of the other aliens that haven't warmed enough to hatch yet or preying on other dawn followers.
Humans colonizing this world would need to live on crawling cities. At 3285 days(earth time) for an entire day cycle the city would have to travel at about 3mph to stay in a sweet spot, weather wise.
A city couldn't be made to sit in one spot since the flooding could leave it crushed at the bottom of the ocean or crushed by growing glaciers at night.
NOTE - Having come up with this, I've decided to work it up as a Question in its own right. I'm not sure whether to duplicate what's here or just leave a stub - thoughts welcome.
Let's not be too Earth-centric about this. There is nothing magical about 24 hours. We just think there is because we have evolved to fit in with days of that length. I suggest that whatever life form evolves on a planet will have its metabolism in sync with the local day length. Thus, in relation to Earth, there there could be 'slow' planets and 'fast planets'.
Climate
It is entirely possible that the world's atmosphere protects it from extremes of heat - and from rapid changes in temperature. I can think of several ways this could happen. For simplicity let's posit a planet blanketed in clouds that filter out a lot of the infrared. Some wavelengths of electromagnetic radiation would get through easily and these would be called 'light' by any intelligent inhabitants that had evolved. I fact they would be able to 'see' their sun just as we see ours. It just wouldn't frazzle them.
Life forms
I suspect that the life-forms that would evolve on your planet would at first be similar to slime moulds. They would move and process information extremely slowly.
Given time, it is quite possible that they would evolve intelligence and the ability to 'walk' - they would just perform these actions incredibly slowly by our standards.
For intelligent beings, subjectively their lives would be occurring just as fast as ours do because everything around them would be slow to match.
For example 'tornadoes' could occur in the form of rotational air movements but these could be moving thousands of time slower than on Earth.
Problems
As long as the planet and its life forms were left undisturbed they would experience no problems that we don't.
Difficulties would occur only in the event of an invasion from a faster planet. The aliens would appear to be zipping around at incredible speeds or might not be seen at all. They could make changes at a rate that would be incomprehensible to the natives. Furthermore, the invaders might not even realise that the native species was sentient - they could just see them as plant life.
Conclusion
Life on such a world need be no different in essence to our own. Not as far as the locals are concerned anyway.
Speculation
Maybe we on Earth are really very slow - after all we think that sound travels fast - that's purely a subjective perception. Perhaps we are being observed by a civilisation that is not only advanced but much, much faster in everything they do.
