Plants under Alien Suns

The Sun (spectral type G2) radiates light in a particular distribution of colors, emitting more of some colors than others. Gases in Earth's atmosphere subsequently filter that sunlight, absorbing some colors (wavelengths), and so more red light photons reach Earth's surface than blue or green ones. Not surprisingly then, photosynthetic life on Earth's land surfaces such as plants (which includes multicellular organisms from grass to trees) tends to depend mostly on red light, because it is the most abundant wavelength reaching the surface, and on blue light, because it is the most energetic. Earth plants also absorb green light, but not as strongly, so leaves look green to the eye, having adapted to the conditions most commonly found around our Sun and on Earth's planetary surface. As most stars do not have the same distribution of light in color wavelengths as our Sun, however, some researchers hypothesize that photosynthetic life on extrasolar planets will not necessarily have the same colors as on Earth.

Nigel Sharp, Kitts Peak National Observatory/NOAO/NSF
(Spectrum of the Sun from 4,000 to 7,000 angstroms -- larger image)
© Association of Universities for Research in Astronomy, Inc. (AURA). All Rights Reserved.

Extraterrestrial photosynthetic plant-type life may look quite look different in color because they have evolved their own pigments based on the colors of light reaching their surfaces. Nancy Kiang of NASA's Goddard Institute for Space Sciences has modelled the light reaching the surfaces of Earth-sized worlds orbiting their host stars at distances hospitable to Earth-type life, where liquid water could exist on a planetary surface, where depending on the star's brightness (and color) and the planet's atmosphere. Kiang found that "plants" on Earth-like planets orbiting stars somewhat brighter and bluer than the Sun might look yellow or orange, and even look bluish by reflecting a dangerous overabundance of more energetic blue light. On the other hand, plants on planets orbiting stars much fainter and redder than the Sun might look black. Hence, astrobiologists seeking signs of life on planets outside the Solar System may want to look for colors reflected by planetary vegetation that is colored differently than the green wavelengths found on Earth (NASA/GSFC press release; Spitzer news release; Nancy Y. Klang, Scientific American, April 2008; Astrobiology; Kiang et al, 2007a; and Kiang et al, 2007b).

GSFC, NASA





Larger illustration.





On Earth, photosynthetic
plants on land tends to
use relatively abundant
red and more energetic
blue light (more).

Autumnal to bluish colors. Main sequence stars brighter than the Sun (spectral types F and A and the very short-lived B and O) emit more blue and ultraviolet light than the Sun. Given sufficient time for Earth-type photosynthetic life to evolve (e.g., hundreds of millions to billions of years), planets around such stars could develop an oxygen atmosphere with a layer of ozone that blocks more energetic but potentially harmful ultraviolet but transmits more blue light to the ground than on the Earth. In response, life could evolve a type of photosynthesis that strongly absorbs blue light, and probably green as well. In contrast, yellow, orange, and red wavelengths of light would likely be reflected by such plants, so the foliage would have the bright colors found during autumn in Earth's deciduous forests all year round. On the other hand, some plants may reflect some blue light due to its overabundance and potential to "burn" photosynthetic organisms (e.g., like sunburn from ultraviolet exposure on Earth).

GSFC, NASA

Larger image.

Main sequence stars radiate
more or less red or blue light
than the Sun depending on
their spectral type. which
photosynthetic life must
adapt to (more).

Darker schemes. A main sequence star that is dimmer and redder than the Sun (spectral type K and M -- red dwarfs) could have plants that absorb more red and infrared wavelengths. Red dwarf stars, which only have some 10 to 50 percent of the Sun's mass but comprise perhaps 85 percent our Milky Way galaxy's stars, radiate most strongly at invisible infrared wavelengths and produce little blue light. If absorb visible light, such plants might look black but any color might be possible. Whatever their color, however, such plants would likely look dark to humans because little visible light would reaches the ground.

Tim Pyle, SSC, CalTech,
JPL, NASA (Original photo
courtesy of PDPhoto.org)





Larger image.





Plants would appear
darker under much
dimmer, redder stars
that emit more
infrared than visual
wavelengths of light
but the color could
vary widely (more).