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The planet in question is part of a Binary Planet pair, both of which are tidally locked to each other at a fairly close distance (~129,000 km) from each other, meaning that the "moon" is very large in the sky, and therefore very bright. Day-Night cycle lasts ~80 hours.

Using a variety of measurements of the "moon" I was able to determine that its brightness at night maxes out at an Apparent Magnitude of -19.91, which for reference is about 840x brighter than the full moon. What this means is that "night time" on this part of the planet is essentially just a slightly dimmer, slightly colder daytime. (By mere coincidence, I also found that the brightness of the moon is almost exactly the same as NASA's Pluto Time, which is basically sunset during the summer)

Since the binary planets are tidally locked, the 'moon' would appear half full at sunset and sunrise, and would be full at midnight. I assume the brightness before midnight would be a little dimmer, but still significantly brighter than Earth's night. Also, this only applies to the half of the planet that is always facing the "moon", the other side would have no visible moon, and would presumably be nearly pitch-black.

With this in mind: How would plants on this part of the world evolve? And how would they compare to the other side of the world whose night is pitch black?

(Assume that the plants on this world function similar to how they do on Earth)

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  • $\begingroup$ Not sure if this would be relevant to the answer, but orbital plane of the two planets is close to 0 Degrees, and there is an eclipse covering the entire planet when the "moon" and planet are lined up with the sun. This would last around 2 hours. $\endgroup$
    – Foosic17
    Jul 20, 2023 at 1:08

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With a few exceptions, plants native to Earth do not seem to be negatively affected by continuous illumination. Notable exceptions are the Solanaceae and belladonna plants (tomatoes, potatoes and eggplants), which can be sickened if they aren't given a break to rest.

I therefore would postulate that plants evolving under said moon would likely flourish, and be even better adapted to it than our earth plants are.

Citation: https://www.researchgate.net/post/What-happens-to-a-plant-that-is-kept-under-constant-light/528dc5f6d2fd6404088b46b5/citation/download

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