Color of Life

Image credit: Caltech/Doug Cummings

Scientists, including biometerologist Nancy Kiang of NASA's Goddard Institute for Space Studies, have been speculating to the color of alien life in response to the type of star, or even atmosphere, of their planet.

Plant like life would probably be fairly common since starlight is a very useful, and fairly constant source of energy for life to take advantage of. On the Earth, for instance, phytoplankton (microbial plants) are extremely abundant and "provide the basis for most of the marine food chain, half the oxygen in our atmosphere and ultimately much of the life on Earth" ("Breakthrough").

Studying plants on Earth, the scientists discovered something: at first plants seem rather inefficient because they reflect light at its highest energy output - green light. The sun light energy that hits the surface of our planet actually peaks in the green band. However, photosynthesis uses particles (photons) of the light rather than just the energy. The photons peak in the red range. This is because red light penetrates through the atmosphere easier than blue light which gets scattered mainly by atmospheric ozone (which is why the sky looks blue, and the sun appears to be red when it is setting - the light from the setting sun has to travel through more atmosphere than the noon time sun, and the only light that makes it through with the least scattering is red).

But even though blue light is scattered somewhat by our atmosphere, enough still reaches the ground that plants can find it useful; while photosynthesis relies on photons, more energetic photons tend to be more efficient - blue photons are far more energetic than red photons. Though there is a limit as to how much energy a plant can take in. For most Earth plants, concentrating on the peak in the red  range is enough.

By the way, while the sun puts out more light energy in the green band, it looks yellow to us on the surface because, as mentioned above, some of the blue is being scattered by the atmosphere. From space, the sun looks white, but that is because of how our eyes work - when flooded by the entire spectrum, especially from a bright source, our eyes will perceive the source to be white, even if it is not fully white.

Image credit: NASA/Caltech/T. Pyle (SSC)

Using this information about photosynthesis, Nancy Kiang and her fellow scientists speculated what color life would prefer in alien environments. F-type stars, for instance, are hot blue stars that give off more blue photons than photons of other colors, and definitely far more than the sun. On a planet circling such a star, any plant like organisms that finds the chemistry of photosynthesis to be as useful as do Earth plants (*), then such plant like organisms may want to concentrate on absorbing blue particles. They would probably reflect red and orange, since those wavelengths are of little use (not efficient to use them).

Around cooler, and dimmer, red M-type stars, the light may be so little that plant life will need all the particles they can get, and thus they would reflect little to no light back (black plants - a goth planet!). Even if the plant life used chlorophyll that absorbed mostly in the infrared range (scientists have discovered two types of chlorophyll on Earth that absorb in the infrared range), such plants may want to absorb as much heat as possible. Or for a planet or habitable moon circling a gas giant far from the central sun, with a thick atmosphere (reflecting even more of the blue wavelength that our atmosphere), plants on such a planet may need to use all available light as well. (Note: Apparently solitary - not binary - red dwarf M-type stars are the most common in our galaxy).

What would this mean for sentient cultures? Just that each section of their spectrum could easily have rather different cultural significances or cultural or theological metaphors. Think of what yellow means to us: warmth, light, day, energy - it is a positive color. Red is connected to blood, and often means life, and from spilled blood, sacrifice or death. Blue is a color of coolness, water, and sky. And of course green is for food, sustenance, fertility, serenity, and life.

Around different stars, these colors could easily take on other meanings. Around a hot blue star, blue may not be a color of coolness. The sky may very well be blue, with a brighter blue spot for the sun - which could be interesting. Think about what if our sky was yellow? Our yellow sun would be this bright part of the yellow sky, a bright spot that moved. Maybe we wouldn't be able to tell exactly the boundaries of the sun and so not, at first, recognize it as a self contained body circling the Earth, but instead just a brightness that moves across the sky. So too, possibly, for some planets circling a blue star.

On a planet with black plants, black could come to represent, to the primitive sentient mind, life. And if black was food, sustenance, fertility, and thus life - then what of the black night sky?

And what of a planet that had both blue water and blue plants? The color blue could take on such a huge significance. Maybe even some of the animal life would have blue pigmentation (to blend in with the vegetation, for instance). However, Ms. Kiang feels that totally blue is the least likely color for plants, since blue light has very high energy photons.

Speaking of red stars, class M stars tend to flare more than sun, and more strongly. This can cause problems for life as the flare floods the planets with strong radiation. However, life is tenacious, "life always finds a way," and not only are there small life forms on Earth that can survive in outer space, but water is a good shield - life forms 9 to 10 meters below the surface would be protected from the flares while still getting enough life giving photons.

* Because of the universality of the laws of physics and chemistry, it is conceivable that there are universal laws of biology, which are based on physics and chemistry. Not all biologies may discover photosynthesis, as there are many chemical and physical variables within those universal laws, variables that may vary enough that some biologies may not "discover" or even need photosynthesis, or may find alternative versions of photosynthesis that are not needed or were not "discovered" by the biology of our planet. However, chlorophyll is a remarkable molecule; it is a very useful source of energy production for life and so seems highly likely to be popular among life in the universe (though again, that does not rule out exceptions).



References [updated]:
Chen, Min, et. al. "A Red-Shifted Chlorophyll." Science Magazine. 19 August 2010. Web. 21 August 2010. <http://www.sciencemag.org/cgi/content/abstract/science.1191127>

Berman, Bob. "Sky Lights." Discover Magazine. 23 Feb. 2007. Web. 22 Nov. 2007. <http://discovermagazine.com/2003/jun/featsky>.

"Breakthrough Method System for Understanding Ocean Plant Life." Earth Observation News. 1 Mar. 2005. Web. 22 Nov. 2007. <http://news.eoportal.org/research/050301_unicalifornia.html>.

"Extraterrestrial Landscaping." Discover. July 2007. 15. Print.

Kiang, Nancy. "La Couleur des Plantes Extraterrestres." Astrobiologie. Pour la Science. June 2009. Web. 30 July 2009. [Article is in French].

Lada, Charles J. "Stellar Multiplicity and the IMF: Most Stars Are Single." The Astrophysical Journal Letters. 640, L63-L66. Print. Also found at <http://www.cfa.harvard.edu/~clada/pubs_html/binaries.html> (as of 22 November 2007) and reprinted in part at <http://www.sciencedaily.com/releases/2006/02/060206233911.htm> (as of 22 November 2007).

Meadows, Vikki. "Colors of Alien Plants." Astrobiology Magazine. 1 Oct. 2007. Web. 22 Nov. 2007. <http://www.astrobio.net/news/article2477.html>.