For plants on alien worlds, it isn't easy being green

The greenery on other planets may not be green. Astrobiologists say plants on Earth-sized planets orbiting stars somewhat brighter than the Sun may look yellow or orange, while those on planets orbiting stars much fainter than the Sun might look black.
Vegetation colour matters to astrobiologists because they want to know what to look for as a sign of life on planets outside the solar system. Terrestrial photosynthesis depends mostly on red light, the most abundant wavelength reaching the Earth's surface, and blue light, the most energetic. Plants also absorb green light, but not as strongly, so leaves look green to the eye.
Extraterrestrial plants will look different because they have evolved their own pigments based on the colours of light reaching their surfaces, says Nancy Kiang of the NASA Goddard Institute for Space Sciences in New York, US.
To determine the best colours for photosynthesis on other planets, Kiang worked with NASA's Virtual Planetary Laboratory at Caltech to determine the light reaching the surfaces of Earth-sized worlds orbiting their host stars at distances where liquid water – and therefore life – could exist. The results depended on the star's brightness and the planet's atmosphere.
Autumn colours
Brighter stars emit more blue and ultraviolet light than the Sun. An oxygen atmosphere would form ozone that blocks the ultraviolet but transmits more blue light to the ground than on the Earth. In response, life would evolve a type of photosynthesis that strongly absorbs blue light, and probably green as well. Kiang says yellow, orange, and red would likely be reflected, so the foliage would wear bright autumn colours all year round.
A star slightly dimmer than the Sun would deliver a solar-like spectrum to the surface of a terrestrial planet, so its foliage would look much like the Earth's.
But plants would be different on planets orbiting small M-type stars, or red dwarfs, which are between 10% and 50% the mass of the Sun. Red dwarfs, which comprise 85% of the galaxy's stars, emit strongly at invisible infrared wavelengths but produce little blue light.
"They'll definitely be absorbing in the infrared," unlike terrestrial plants, Kiang told New Scientist. Because they would benefit by absorbing visible light, she says they might look black, although she admits that any colour might be possible. Whatever their colour, the plants would likely look dark to humans because little visible light would reach the ground.
Floating and sinking
Photosynthesis might not draw enough energy from infrared light to produce the oxygen needed to block dangerous ultraviolet light from the dwarfs.
But if there were at least 9 metres of water on the planet, mats of algae would be protected from the planet-scalding ultraviolet flares produced by young red dwarf stars, says Victoria Meadows of Caltech, principal investigator at the Virtual Planetary Laboratory.
She envisions a bizarre world where microbial mats float near the surface for efficient photosynthesis when the star is calm, then sink to a safe depth when a flare hits.
Life could spread further when the stars pass their flare stage, she told New Scientist: "M stars don't produce a lot of ultraviolet once they quiet down, so you don't need an oxygen layer to shield [life] from the ultraviolet."
Journal reference: Astrobiology