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So, let's say there's a white dwarf star that has recently been born (by recently, let's say about 3 million years ago) Now around that time, a rogue planet, about the same size and mass as earth, came into the solar system and was pulled by the star's gravity, forcing it to orbit the star in it's habitable zone. Now, this rogue planet had frozen water all across its surface when it first came, but after it entered the habitable zone of the star its ice started to melt and eventually oceans were formed. Now, let's say after billions of years, life starts to develop and eventually it finds it's way towards the land. Now, if plants evolved on that planet, what colour would they be?

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  • $\begingroup$ The issue with white dwarf planets is that, while they are in the fairly stable habitable zone of the stellar remnant, they are subject to extreme tidal forces. This means either that they are very, very vulcanic and tectonically unstable like the Jovian moon Io at best or a lava planet with no solid surface. This is of cause given that they are still a planet and haven't been ripped apart. The stars Roche Limit is often within the habitable zone and this means the planet is about to be turned into a glowing ring system. $\endgroup$ – TheDyingOfLight Sep 13 at 12:00
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    $\begingroup$ Making matters worse the fact that your planet has been captured means that its orbit will extremely eccentric and probably highly inclined. The eccentricity makes the tidal effects even worse and introduces extreme "seasons" as the intensity of light scales with the inverse square of the distance. This means that you planet will only receive 1/4th of the light it receives on its periapis when it is on his apoapsis, given it is two times further out(which is not unreasonable). This obviously bad for habitability. $\endgroup$ – TheDyingOfLight Sep 13 at 12:13
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    $\begingroup$ @TheDyingOfLight the definition of "habitable zone" will be a bit fuzzy, precisely because of that tidal heating. The outer edges of the HZ will be a warmer than the equivalent locations on main sequence stars, so there's a broader band where liquid water can exist.. $\endgroup$ – Starfish Prime Sep 13 at 12:13
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    $\begingroup$ @StarfishPrime Extremophiles are not per se prohibiting complex lifeforms. After all, Earths current environment is ruled by extremophiles adapted to the extremely reactive gas oxygen. $\endgroup$ – TheDyingOfLight Sep 13 at 13:04
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    $\begingroup$ Earth has red, brown, green and cyan algae. It so happened that the ancestors of our land plants were among the green algae. They could have been among the Rhodophytes, of the Phaeophytes, or the Cyanophytes. There is no way to tell how the dice would have rolled on your fictitious planet. $\endgroup$ – AlexP Sep 13 at 16:40
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Interestingly enough, probably green.

Plants don't need a whole lot of energy to function. If we ignore factors like the refraction of light in your planet's atmosphere, the radiation hitting your planet's surface will be far shorter in wavelength than that hitting Earth's. However, as found here, photosynthesis is all about capturing those higher-frequency electrons that are in the sweet spot of high energy, not destructive. It's about finding photon wavelengths that are energetic enough to trigger photosynthesis and not cellular disintegration.

You could say 'hey, my plants are extra-radiation resistant', but realistically your plants are only going to want those same red and blue photons. They may reflect some black body (which isn't in the visible spectrum)- outside of that, plants on Earth have no use for the more powerful UV rays that penetrate our atmosphere. Why would plants on another planet?

I'm totally open, however, to the notion of deep blue or black plants that use very high-energy photons in smaller amounts. Plants here need a balance of lower energy red and higher energy blue, meaning they reflect the super-average green. If your white dwarf is mostly emitting in <500nm wavelengths, then it makes total sense for your plants to move up the spectrum and try to get those UV and yellow photons, thus meaning they reflect the (now) super-average blue spectrum.

Hope it helps!

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