What skin, hair and eye colors would people living under different stars have?

Question

My setting includes several natural and terraformed earth-like worlds orbiting various main sequence stars, from M class dwarfs to A class giants. Humanity has independently developed from stone age to space age across each of these worlds, and though they share common ancestry and little genetic deviation from Earth humans, they have had ample time to physically adapt to their “new” environments.

My current model for physical traits across different stellar spectra is that low-mass, dim stars have pale-skinned people, while high-mass stars brighter than Sol favor dark skin. Hair and eye color is generally black or brown, although I describe many of my pale-skinned characters as having bright colored hair and eyes, in accordance to most such real-life humans. This is a pretty common interpretation in science fiction, and on the surface it makes sense.

However, after doing some research, I found that since a dimmer star’s habitable zone is closer to the star, it may receive more radiation than a brighter star, while at the same time providing less visible light. In the same way, a star brighter than Sol may emit less radiation at habitable distances. Does this throw off my current assumptions of stellar spectra-based skin tones? How realistic is all of it? Does hair and eye color factor into this?

Answer 1

Why Do Humans Vary?

First, let's ask why these changes happened in humans in the first place. But remember, these changes took place over thousands of years.

Skin

We don't know for sure why human skin color varies the way it does, but leading theories have to do with UV exposure. Lighter skin colors did not encounter much UV, so developed their skin color to maximize Vitamin D creation (a somewhat unique feature of humans exposed to sunlight). Darker skin colors encountered a lot of UV, so their skin color is to maximize protection against excess UV levels.

So for skin features, decide what your UV exposure is. You're on the right track talking about your star, but also consider where on the planet your people might be. The poles and the equator might offer very different experiences.

Eyes

Our best evidence right now is that lighter color eyes are connected to skin color, and did not evolve on their own. This is a great example of how one gene rarely does just one thing. Eye color may not be "optimized" at all, but connected to skin color. Decide skin color and why it is the way it is, then connect eye color to that.

Hair

Lighter colored hair has evolved multiple times among humans and in multiple geographical locations that can't possibly be connected. Our best evidence right now is that lighter hair colors are purely ornamental, designed to give a competitive advantage in mate selection. Apparently, whenever humans have limited mating opportunities, lighter hair may evolve. Since you're in a high tech scenario, this is unlikely to be a concern, but if you are imagining some massive disaster in the past, sexual selection could do some interesting things to hair color.

Answer 2

Local Fauna Will See Differently Than We Do

While Michael already pointed out the UV theory about skin color diversity, another has to do with natural camouflage. Most animals on Earth develop natural camouflage to either hide from predators or better sneak up on prey, and there is no reason to think that humans are any different. Light skin evolved in places that would have been normally frozen over in the ice age, and dark skin in places where the world remained unfrozen.

So for skin color to experience any major pressure to change over time, your humans will need to undergo a long period of predation from local fauna where natural camo is again important. Otherwise, inner breading will just lead to a homogenized medium skin tone over time.

However, sun color may still be important in ways you do not expect. On Earth, Yellow is pretty much central in the visible light spectrum to maximize our ability to use sunlight to see, but the local fauna under a red or blue star may have a very different visible light range than we do. Overtime your humans could evolve to see deeper into the IR or UV spectrum and our skin color could also become specialized for that as well. This being the case, humans could very well start treating colors outside of the visible range of predators as either anything or nothing goes.

A classic example of this on Earth of anything goes are blackbirds. In our visible spectrum, there are many species of birds that appear black or gray, bur in a bird's visible spectrum, they are brightly colored like a parrot. Since most predators can't see their bright coloration, UV colorization became a perfect secondary trait for mate attration.

Or for an example of nothing goes... consider transparent cave dwelling fish that have evolved to not need pigments at all.

A human population given enough time to evolve under a foreign sun could go either way when it next comes time to be around a good old fashioned yellow suner.

Answer 3

What is the only known species they all humans consider to be more less "people"?

Homo sapiens.

Where do members of the species Homo sapiens live?

On the planet Earth, sometimes called Terra or other names.

What star does the planet Earth, where members of the species Homo sapiens live, orbit?

The star called The Sun, or Sol, which is a G2V, or posibly a G0V, class main sequence star.

So what is the color of Homo sapiens skin, and what is the color of Homo sapiens hair, and what is the color of Homo sapiens eyes?

There is no one single color of Homo sapiens skin, Homo sapiens hair, or Homo sapiens eyes. Instead there are different possible ranges of colors, shades, and hues of Homo sapiens skin, Homo sapiens hair, and Homo sapiens eyes.

Clearly each and every actual and existing color, shade, and hue of Homo sapiens skin, Homo sapiens hair, and Homo sapiens eyes is a possible one in the environment where Homo sapiens live.

Other colors, shades, and hues of skin, hair, and eyes might or might not be possible in this environoment, but are not produced by specimens of Homo sapiens at this moment so we can't tell whether Homo sapiens specimens with those colors could survive in the present environment. Possibly they could not survive in this environment. Possivly there is simply no biological method at the present for members of the species Homo sapiens to produce those colors.

If planets orbiting stars of different spectral types are colonized during the future, there could be selection processes favoring various skin colors.

If a planet orbiting a star with a hotter surface temperature that produces a higher percentage of ultra violent ultrviolet radiation is colonisted by nudists, they should have a higher incidence of skin cancer and other ultraviolet induced conditions that people living on Earth. Over the generations, centuries, milllennia, tens of millenia, etc. the highter average survival rate of people with darker skin might make the population have a darker average skin color than the original colonists had.

And if a planet orbiting a star with a lower surface temperature that produces a lower percentage of ultra violent ultrviolet radiation is colonisted by nudists, they should have a lower incidence of skin cancer and other ultraviolet induced conditions that people living on Earth. Over the generations, centuries, milllennia, tens of millenia, etc. the higher average survival rate of people with darker skin would be less than on Earth. So the poplulatin would tend to retain it oribinal proportion of lighter skin tones much longer than on the high ultraviolet planet, and even much longer than a nudist colony on Earth.

And what if the colonists are not nudists, but wear clothing?

What if the colonists wear sunblock when outside, as well as broad brimmed hats and other sun protective clothing?

What if the colonists get so used to living indoors durinb the voyage that they continue to live indoors on the planet, each of their settlements consisting of a single multistoried building containing an anetire village, town, or city, and thus they can all be inside opague walls and ceilings and ultraviolet stopping windows?

What if the planet has a strong ozone layer and other atmospheric gases and airborne particles which absorb or reflect ulta violet light?

It is possible that even if the star emits a higher percentage of ultraviolet light than the Sun, the surface of the planet might possibly receive a lower percentage of ultraviolet light than the surfece of the Earth does, due to increased atmospheric blocking of ultrviolet light.

And on the other hand, even if the star emits a lower percentage of ultraviolet light than the Sun, the surface of the planet might possibly receive a higher percentage of ultraviolet light than the surfece of the Earth does, due to decreased atmospheric blocking of ultrviolet light.

I also note that a star with a specific luminosity will have an oribital distance for a planet where a palnet would receive exactly as much radiation from that star as Earth receives from the Sun. I call that the Earth Equivalent Distance or EED.

The answer by user177107 to this question:

https://astronomy.stackexchange.com/questions/40746/how-would-the-characteristics-of-a-habitable-planet-change-with-stars-of-differe/40751#40751

Has a table describing the properties of various types of stars. Including what I call their EED.

So if a real, hypothetical, or fictional planet orbits a star in the EED of that star the upper atmosphere of the planet will receive exactly the same total radiaiton as Earth gets from the Sun. But both the distribution of that radiation among various frequencies and the reflective and absorbing properties of every layer of the planet's atmosphere will determine how much radiation reaches the ground and the distribution of that radiation among varius frequencies.

As a general rule, more ultraviolet radiation will reach the surface of planet in the EED of a hotter star than a planet in the EED of a cooler star, if the two planets have identical atmospheres.

What happens if a planet orbits its star closer to the star than the star's EED?

The planet will receive more total radiation from its star than Earth gets from the Sun.

What happens if a planet orbits its star farther from the star than the star's EED?

The planet will receive less total radiation from its star than Earth gets from the Sun.

A planet too much closer to the star than the star's EED will get too much radiation and will be too hot for humans, and a planet even closer to its star will be too hot for any type of liquid water using life.

A planet too much farther from the star than the star's EED will get too little radiation and will be cold for humans, and a planet even farther from its star will be too cold for any type of liquid water using life.

The inner and outer limits to where planets can have the right temperatures for liquid water using life forms define its circumstellar habitable zone.

Various estimates for the inner and outer edges of the Sun's circumstellar habitable zone can be seen in the table at:

circumstellar habitable zone

And they vary a lot.

A writer who plans to have only one habitable planet in the circumstellar habitable zone of a star can put it within one percent or so of the star's EED and not worry about later discoveries putting it outside the habitable zone. But if a writer wants to have two, three, or four habitable planets in a star's circumstellar habitable zone they will have to make the habitable zone a wide one, and will have to worry whether later discoveries will prove that a habitable zone can't get that wide or hold that many planetary orbits.

Anyway, it seems to me that there is a rather small probability that the skin, hair, and eye colors of human colonists on planets of other stars will change over ages of time due to the spectral types of the stars of those planets.

Of course people with different skin, hair, and eye colors might have preferences about the types of stars they would desire to settle on planets of.

I note that when people move from indoors with artificial lighting to the outside, or between indoor regions with different types of artifical lighting producing different wavelengths of light, their apparent skin, hair, and eye colors change slightly.

So even if the skin, hair, and eyes colors of colonists on a distant planet are not likely to slowly change due to the different wavelengths emitted by the star of that planet, the difference strengths of different wavelegenths to reflect will make their skin, hair, and eye color look slightly different in the light of those stars than on Earth.

And perhaps I should point out that there are restrictions on the classes of stars which can have planets that are naturally habitable for humans - and all other lifeforms which require oxygen rich atmospheres to survive.

Such restrictions became known to some scientists at least as early as the 1950s, and some science fiction writers soon learned of them.

For example, Robert A. Heinlein mentioned such restrictions in his juvenile science fiction novels Starman Jones (1953) and Time for the Stars (1956).

As far as I know, there is only one scientfic study of what is necessary for a planet to be habitable for humans specifically, instead of for liquid water using lifeforms in general. You may note that there are many lifeforms on Earth surviving and thriving in conditions where unprotected humans would swiftly die if teleported to without warning. A planet habitable for some forms of Earth type life is not necessarily habitable for humans.

This single scienftic study that I know of about what is necessary for a planet to be habitable for humans is Habitable Planets for Man, Stephen H. Dole, 1964.

https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf

And it explains the limitations on the spectral and luminosity classes of stars which can have planets habitable for humans, planets which humans would be able to settle without extensive terraforming.

So you will see there are limits to how blue or how red the light of a star can be if it has a planet which humans can colonize.