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

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Date posted: 11/04/2007

Work done by Nancy Kiang of the NASA Goddard Institute for Space Sciences in New York may have a bearing on the environment within realistic virtual worlds.

Kiang worked with NASA's Virtual Planetary Laboratory at Caltech to determine the best plant leaf colours for photosynthesis on worlds which have differing coloured suns.

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 reflect a good deal of it back, so leaves usually look green to the eye.

Because plants on planets with stars of differing colour and size, will be dealing with different radiation bands, their colouring would be radically different.

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 said. 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 for standard human vision, although not for creatures who see in infrared ? who might see any colour.

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.

See the full Story via external site: space.newscientist.com

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