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F main sequence stars are hotter and brighter than the Sun and therefore they emit more UV radiation than sun-like stars. Many scientists think that complex life could develop in planets orbiting such stars.

Lets say an habitable planet is orbiting a F9V or a F8V star. This planet could have a thicker ozone layer or other substance to block the high UV radiation, or land based lifeforms would develop new kinds of pigments that would protect them from UV radiation.

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    $\begingroup$ VTC:NDC. A really good question about exoplanet sky color will help you with your thoughts, but keep in mind that "colorful" only has meaning from a human perspective. We can't see UV, so its effect on plants would be irrelevant. Further, color would be based on the nature of life (e.g., do they use photosynthesis?) and the nature of the planet, and that means you haven't provided nearly enough information for an answer. Worst of all - this feels way too much like you're trying to start a discussion, not solve a specific problem. $\endgroup$
    – JBH
    Feb 12, 2022 at 6:01
  • $\begingroup$ There is no question mark in this question. $\endgroup$
    – Daron
    Feb 12, 2022 at 15:20

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According to wikipedia, an F8V is ~95% brighter and ~7% hotter than our Sun. The emission spectrum of that star would peak at ~470nm, unambiguously bluish, and 15% of its emitted light would be in the UV range (10-400nm), vs 10% for the Sun.

To get the same solar flux as Earth, the orbital radius of your planet would need to be more like 1.4AU. You probably wouldn't want to be any closer than that, as you're pushing habitable zone limits somewhat. The parent star would appear slightly smaller than the sun, but not by much... an angular diameter of 28 minutes of arc vs 32 for the Sun seen from Earth.

You've two problems. Firstly, the star will age faster than the sun... the light that burns twice as bright, etc etc. Its lifetime will be more like ~6.6 billion years, vs the Sun's ~10 billion years. That's still more than the age of the Earth, so it isn't too unreasonable to assume that life will form before the star swells up and eats all its children.

Secondly, that UV flux is bad news. Not just for life on the planet's surface, but for the lifetime of the atmosphere too... UV causes photodissociation of molecular gasses in the atmosphere to produce lightweight monatomic gasses which will be much more able to escape the planet's gravity, with the end result that the atmosphere will blow away faster and the planet will end up more like Mars than Earth. You may need a larger world to hang on to the atmosphere, and that in turn will have knock-on effects like becoming a watery super-earth during its formation.

On the surface, UV-A and UV-B flux will depend on the nature of the atmosphere, but with an Earth-like atmosphere it'll be about 40% stronger than on Earth. The ocean provides good protection against UV, but it also attenuates long wavelength red light quite effectively:

Absorption coefficient of visible and near-visible light in water

(source: Underwater image enhancement by wavelet based fusion. Bigger absorption coefficients mean the light travels shorter distances in water.)

Things forming in deeper water to take advantage of the attenuation of UV will also get less red light. Terrestrial photosynthesizers, not being exposed to the same level of UV, would be fine in shallower water or at the surface and so there's more red light to take advantage of. If surface conditions do push photosynthesizers deeper, you'll end up with pigments like you'd find in deeper waters on Earth.

Absorption spectra of terrestrial photosynthetic pigments

(original source at plantphys.info no longer exists)

You'll notice that the compounds that tend to absorb bluer light alone are orange (carotene) or yellow (xanthophylls) or red (lycopene) colored, vs the green color of terrestrial plants... consider brown algae species on Earth which are rich in chorophyll-C (not shown on the chart above) and fucoxanthan (a yellowy-brown xanthophyll).

Now, at the surface the energy levels in the visible light spectrum are quite similar in both the blue and yellow star... the blue star produces about 7% more visible light at the same total irradiance, but the relative proportions of the colors are pretty close. This means that terrestrial photo-pigments would work just fine, assuming the plants were suitably protected against the greatly increased levels of UV.

new kinds of pigments that would protect them from UV radiation.

Terrestrial plants have evolved a range of sunscreen chemicals, and commercial sunscreens make use of cinnamate derivatives, where cinnamate is itself a chemical found in various plants.

Its hard to get a handle on what color those sunscreens are in bulk form, but they appear to be often brown or black (like melanin found in various things, including humans) and sometimes white. There's some interesting hints that they may even be fluorescent... sinapates can be, and corals produce fluorescent proteins. Human eyes may not be able to spot this effect without assistance, though I wonder if leaves of vegetation would seem to "pop" at twilight in the manner of hi-vis fabrics. It is hard to say.

These things taken together seem like you might end up with a more diverse range of colors than on Earth... perhaps more yellow-orange-red ones and fewer green ones, with colors of surface and especially high-altitude vegetation tending towards pale colors and browns due to the additional UV-protective chemicals required.


Now, there's a big problem here with asking about colors... the chances are that things which evolve eyes that work on the surface of a world with a high UV flux will see things quite differently to us, and their eyes will have photopigments that are sensitive to some or more ranges of UV and are much less likely to have things like UV-absorbing lenses or corneas or other protective covers. As such, the colors they produce and react to may well have no analog on Earth, and no human words. What might look like a fairly muted palette of dark oranges, browns and blacks to us might be a much more vivid and interesting sight to the locals.

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  • $\begingroup$ How much of an influence will the extra 50% UV radiation have on the atmospheric lifetime? The Star's lifetime going from 10 billion to 6 billion years doesn't sound like a deal breaker for complex life. $\endgroup$
    – Daron
    Feb 12, 2022 at 15:22
  • $\begingroup$ @Daron I'm not sure, unfortunately. Atmospheric escape is a really difficult thing to model effectively, even without considering differing ionizing radiation fluxes rather than merely thermal ones. It is a big red flag, though it could be quietly ignored in a non-rock-hard scifi setting. $\endgroup$ Feb 12, 2022 at 15:28
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It seems rubbish and nonsense but I believe that animals and plants in this planet could develop metallic colored and brightly exotic looking pigment much like the jewel bettle has a bright metallic looking body. I think that since sunlight in planets orbiting F stars would look stronger than sunlight on Earth the lifeforms in this alien planet would have a more colorful skin as well. I do not believe black pigments would evolve as much.

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    $\begingroup$ Sunlight would only look stronger if the planet were the same distance from the star as Earth is from Sol and had the same atmosphere and magnetosphere as Earth. Such a planet might not be within the goldilocks zone for an F-type star. $\endgroup$
    – JBH
    Feb 12, 2022 at 6:05
  • $\begingroup$ @JoinJBHonCodidact interestingly, for the same total irradiance, an F8V star has about 8% more energy in the visible light spectrum than the G2V star, at the expense of the long-wavelength red-to-far-IR spectrum. So brighter, but it might feel slightly cooler (depending on how reflective you are). $\endgroup$ Feb 12, 2022 at 14:54

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