My planet is a frozen world. Roughly the size of earth and in a perpetual ice age. The planet has very cold temperatures at night and during the day its surface becomes very bright. The intense solar radiation means that every night its beautiful landscape is illuminated by auroras.

There is some geothermal activity, mostly geysers and hot springs where some wildlife finds refuge. The planet has a single ocean with its surface entirely covered by moss with colossal root pillars descending to gather minerals. The green sea houses two ecosystems one on top and one underneath.

The snowy surface of this planet has some species of plant. The most common being the "shy moss" that turns white like the snow around it whenever it is disturbed. The other largest plant is the "bunker bud" a house sized plant that opens during the day and closes into an armored bud at nightfall. Its seeds are like hot air balloons that float far away and then fall into a new location. The only creature powerful enough to eat it is the "branchiosaurus". Named due to its branch-like spikes with retractable leaves, this peaceful giant has jaws powerful enough to pierce the plants armor.

Because of the planets conditions the creatures on the planet (including a sapient race) have some sort of photosynthesis that makes their bodies dark in color. They are nocturnal creatures that collect sunlight during the day and become active at night. This allows them to survive for a very long time without food.

I've got all of this well figured out, the ecosystem the environment even the culture of its inhabitants. The problem I'm having is with different ways to resist cold temperatures. I figured that fur inside of a folded photosynthetic membrane, insulating fat and warm blood for my creatures might be enough. But I also thought if increasing their salinity could lower the freezing point of water and give them some extra resistance. What do you guys think? Is there anything I could add? A name for the planet would also be nice.

  • $\begingroup$ Welcome to WorldBuilding.SE! The last part about a name for your planet is sadly off-topic, as "what to name something" is inherently subjective. The rest of your question looks fine to me, though, you've clearly put a lot of thought into this already. Only thing I can suggest is to quantify "very cold" - how cold exactly are we talking? Adaptations for -5C will be very different to adaptations for, say, -100C. $\endgroup$ – F1Krazy Oct 17 '19 at 9:10
  • $\begingroup$ Thanks for pointing that out. Although I am a little sad that the name part is off-topic. I was simply hoping that different people would give different name suggestions, as the name generator did not yield any good names. As for quantifying the exact temperature that part remains very difficult, as different times of the year and weather conditions can change this so I did not bother specifying it. Though on average -50C looks like a reasonable temperature. $\endgroup$ – user69494 Oct 17 '19 at 9:26

Aside from increasing the salinity, you may play with some additional concepts.

Even on earth there exists something called Antifreeze proteins that allow some fish to exist below the freezing temperature of Earth's ocean (about –1.9 °C) without increasing salinity of their bodies.

In fact, the salinity balancing in vertebrates is a rater complex subject. You sort of have to get it "just right". But this is specific to life that evolved on Earth and does not have to carry on to evolution on fictional planets :)

You can also make your planet's life depend on some chemical other than oxygen, that in itself allows for life in a frozen world.

If the life depends on oxygen, the animals may use something other than hemoglobin to carry the oxygen in the blood, for example on Earth hemocyanin lets some creatures survive sub-zero temperatures.

About your fur and photosynthetic membrane ideas. This seems unnecessarily complicated to me (although I may not understand it correctly, English is not my native language). Why not just have photosynthetic fur? As in, the fur itself is the actual organ responsible for photosynthesis.

Also, if the animals can do photosynthesis, and depend on the oxygen, and one of the products of the photosynthesis is oxygen (assuming it works in the same fashion as it does on earth) then does it mean that they won't need to breath at all during the day? Or they will breath in CO2? Why not absorb CO2 through the fur, directly where it is needed? Do they need lungs at all? I mean, plants don't have lungs, they get the CO2 (and O2 at night due to cellular respiration) using their entire surface.

If you go with the photosynthetic fur idea, then the "working" surface of the fur will be enormous, so what will they need lungs for? But hten, what about speech/noises if there is no lungs? Sign language for your sentient species? Some sort of visual/light/color signaling? Other noise-making organ? Telepathy?

Another factor in favor of internal lungs instead of "external" is that the blood (or other fluids) that carry nutrients around will be more exposed to the temperature. Although not necessarily, maybe there can be some isolating layer and the blood won't be directly exposed to the cold.

Also, there exist some animals that can survive being completely or partially frozen. Maybe you can find some inspiration there?

  • $\begingroup$ Thanks for showing me the antifreeze proteins, I'll dig into that! And to answer your questions there is no photosynthetic fur because fur is composed of chitin and not living cells, which is why the photosynthetic layer needs to be on the skin. Yes they do have lungs, as they are nocturnal they don't get sunlight during the night so they do need to breathe oxygen, although they recycle their carbon dioxide during the day. Last about their language, I did not explain it here but they communicate via electromagnetism much like radio waves. (Don't worry english isn't my native language either) $\endgroup$ – user69494 Oct 17 '19 at 13:43

enter image description here Source At roughly 29-30% salinity, the lowest freezing point is -21.1C. This is called a eutectic point: The point at which a certain ratio of two different materials gives a freezing point which is lower than freezing point of each material on its own. Just check that this freezing point matches your climate.

  • $\begingroup$ Thank you. That's very good to know. However this raises the question of how high can the salt concentration be in a living being. $\endgroup$ – user69494 Oct 17 '19 at 10:46

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