Imagine a red dwarf star orbited by a considerably Earth-sized planet in the star's habitable zone. Assuming the planet has a sufficiently powerful magnetic field to protect its surface from the star's radiation, and life thrives on the world, what would be the most efficient pigment for plants on that world to evolve?

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    $\begingroup$ Does the planet in question has an earth-like atmosphere? $\endgroup$ – PipperChip Mar 28 '15 at 20:37
  • $\begingroup$ I assume it (for the sake of simplicity) to be earth-like. Though earth never have a stable atmosphere in geologic time scale (compare pre-cambrian era and now) $\endgroup$ – Hendrik Lie Mar 29 '15 at 6:27

Assuming that other variables are Earth-like, I would say: Black.

The colour of the plants we see is not the colour of the light used for photosyntesis. The light used for photosyntheses is absorbed (*), the pigment reflects the light that is useless for the plant.

In a red dwarf, I would expect the light arriving to have a very weak green component, so almost no light would be reflected by the leafs.

Additionally, I would expect a planet orbiting a red dwarf to be "cooler" than Earth, so a black leaf would help keeping the plant warm by absorbing most of the energy from the light.

Of course, evolution sometimes takes some curious paths, so other solutions would be possible. Maybe the plant wants to show more bright colours to attract insects to polenize them, or maybe black makes them too easy to be noticed by animals feeding on them, forcing variations from the initial solution.

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    $\begingroup$ Good points. Just a note here that not all leaves are green: Japanese maples are often scarlet or, sometimes, purple. $\endgroup$ – Isaac Kotlicky Mar 29 '15 at 4:32
  • $\begingroup$ I guess animals at the planet would evolve on that kind of light exposure either, so they wouldn't perceive visible light spectrum like us. Perhaps at the planet, its inhabitant perceive heat (like some species of snakes) or other "below blue" spectrum :/ $\endgroup$ – Hendrik Lie Mar 29 '15 at 6:30

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