I'm drawing from Norse mythology on light and dark elves to write about an alien race and planet with one humanoid 'tribe' residing on the sunlit side and the other on the dark side. I mention blood temps because I'm playing around with having the sunlit humanoids cold-blooded.

I'm finding the light side easier to contemplate as I know how sun=life and in what ways already. As for dark/I assume cold environments, I have no clue what that could possibly look like. Plant and or animal life, as well. Any and all advice greatly appreciated.

  • $\begingroup$ I fear you should just make some stuff up. Really, nothing much will grow or evolve on a permanently dark side of a planet. Not that it's really impossible, but live as we know it wouldn't work there. $\endgroup$
    – Nobody
    Commented Sep 11, 2016 at 22:52
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    $\begingroup$ The sunlight side would be a parched wasteland, and the dark side would be frozen wasteland. You might get some life in temperate/twilight zone, but it would be difficult and sparse. Just hand wave it and make them normal humanoids. $\endgroup$ Commented Sep 11, 2016 at 23:16
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    $\begingroup$ there could be life that uses geothermal energy on the dark side.... $\endgroup$
    – user24999
    Commented Sep 11, 2016 at 23:41
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    $\begingroup$ When you mention "light side" and "dark side" of a planet, are we to take that to mean that your planet is tidally locked with its star? As already pointed out, tidal locking results in huge extremes on both sides, especially when you add things like an atmosphere and at all habitable temperature ranges anywhere on the planet to the mix... $\endgroup$
    – user
    Commented Sep 12, 2016 at 20:13
  • $\begingroup$ It doesn't appear that anyone has addressed the warm/cold-blooded aspect. Why would organisms living on the dark side mutate in a way to be warm-blooded? Organisms that are warm-blooded are so due to their metabolism. A snake, for example, metabolizes its food slower than a human. But even cold-blooded organisms need to get into some shade after being exposed to sunlight. $\endgroup$ Commented May 17, 2017 at 1:51

4 Answers 4


This is so much more than most would realise at first glance.

First let's focus on the planet itself. As Xavon pointed out, there would be a scorched wasteland on the 'bright' side, and a frozen wasteland on the 'dark' side. Look up some information on Ryloth (from Star Wars Expanded universe). What would this mean for your planet? Well, as already pointed out, the most logical place for life to start is in the regions between the two extremes.

Note, life would 'START' in the twilight band, it wouldn't be only there. So long as there is a starting platform, extremophiles will migrate and given enough time (millions and millions of years, I'm sure everyone knows the evolutionary shtick) could evolve into higher lifeforms.

Second, local weather would be chaotic because of these two extremes. The hot air from the bright side would expand, and try to flood into the cooler dark side (air pressures trying to find an equilibrium). But the energy wouldn't be lost, and cold air would eventually try to trickle back to the warmer side. This would mean there aren't 'seasons' as we know them. Everything would be considerably more chaotic, changing by the day. Any life that forms would have to be hardy. Plant life, microbial life, wild life, your sapient life -- everyone needs to be tough as nails to survive on a planet like that.

In all likelihood, just as in the Ryloth example, cities would form underground. I mean, if the outside world is so chaotic? They'd need a stable place to call home. It would no doubt lead to highly social groups, seeing as they need to depend on each other to survive, and to raise the young if something happens to the parents. There would most likely be fungi and meats to feast on (their staple food, seeing as both could grow naturally in caves where they call home), and there would likely be hunting parties that go out into that big, bad, chaotic world to see what they can see.

There are also a lot of other factors that would/could come into play, like the culture revolving around what scant food there is (as is the case in my example) which leads to fat people being considered rich, leaders (and this is a real world example, if you look at certain African countries, India, the Middle East, fat doesn't have the same stigma as in the West).

EDIT: You won't find cold-blooded beings in a world (or region, if you want to get specific) without sunlight. Seeing as they cannot regulate their own body temperatures, they cannot hope to survive without a source of heat. Expect warm-blooded creatures, and furthermore you can even expect hairy/furry creatures with excellent night vision, excellent sense of smell and hearing, and lots of excess fat.

Why? Fur helps keep warmth in, losing less energy to the frigid air around them. Fat retains heat as well. With no light, they need other means of studying the world around them--hence the other senses will be heightened. I suspect no one answered your original question, because you didn't google life here on earth, or the science behind cold-blooded animals. Cold-blooded animals need sunlight, need warmth, need a means to regulate their temperature (as I already stated). This necessitates a heat source, by definition.


I recently read a great article about something similar to this.


Here is a bit from the article that could answer your question: Astronomers think Proxima b is tidally locked like the Moon, where one side of the world always faces Earth. But instead of always facing the Earth, one side of Proxima b always faces its star: awash in permanent daylight, the other side trapped in an endless cold night.

If Proxima b does have an atmosphere, though, says Loeb, it'd not only circulate warmth from the day side to the night side, but also prevent the planet's water from boiling off into space.

Hope this helps!

  • $\begingroup$ It will probably help, but in no way answers the question about cold / hot blood. $\endgroup$
    – Mołot
    Commented Sep 12, 2016 at 9:11

The problem with a tidal locking situation may be less about the biology of the race, but rather concerning the astronomical data of the world itself. To create the races you describe, we must first position the world in such a way that it sustains life.

This is where we find the issue. If the sun is too close to the planet, one side will constantly be scorched. If the sun is too far, the dark side will be far too cold, (keeping in mind that it would already be incredibly cold as it is.)

If we distance the planet from the sun a ways, we may potentially heat up the far side by increasing the atmosphere to implement the greenhouse effect. This solution, however, would again heat the bright side to life-killing heights, and again there is little hope of life.

Any life would have to live in the in-between zone, as outlined by the other posts. There could theoretically be two similar species that separated, but it is unlikely that they would live on the extremes of the world, but rather within a few hundred miles of each other, in a narrow band around the edge.

I do potentially have an answer to how this planet might function, however. If we distance the planet from its sun, and then provide heat not through sunlight but rather through a very thin crust over an extremely hot mantle, we might be able to sustain life with geothermal energy. In the case that there is sufficient heat on both sides, it might be possible for the whole planet to contain life.

On the light side, we have photosynthetic flora, light-loving reptiles with cold blood as you described, and other bright and happy things that we archetype as "good guy stuff", like light elves.

On the dark side, we find fungi, warm blooded creatures that are hardy enough to survive the potentially still much lower temperatures, and not much color or other enjoyable features to speak of.

And this, finally, is where we must decide how these "elves" could exist on both sides. On a planet with any real extremes, it would be quite difficult to explain how a cold-blooded creature would have any reason to exist on the dark side of a planet, or why a warm-blooded one would want to live on the light side.

I propose that the elves are one uniform race living on a planet that is life-sustaining as I described equally well on both sides, and that the only reason they split is over moral or religious issues. Any heavy adaptation would occur over the course of innumerable years, especially because to look anything alike, this race must have a common ancestor which was neither light nor dark elf, bur rather some sort of neutral elf. One race's physiology couldn't be wildly different from the other, such as the difference between warm and cold-blooded creatures, because doing so would create two kinds of animal too different to both be called "elves".


Such radically different sub-species are highly unlikely Since your elves most likely evolved in the temperate zone between the two extremes, divergent evolution should produce biologies no more different than, say the Inuit and the Polynesians. The technologies they develop will be the more distinguishing feature, not their biology.

You might posit two completely different species evolving on the light and dark sides, but then you're into some highly speculative corner-case stuff with respect to evolution, and you can (and should) make up whatever you want. As it is, extreme environments tend to produce simple, highly-efficient, narrowly adapted forms of life. Us humans only moved into our extreme environments after evolving our highly-adaptive behaviour in gentler climes.


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