 are feeble, so plants on an orbiting Earth-like world might need to be black to absorb all the available light (first panel). Young M stars fry planetary surfaces with ultraviolet flares, so any organisms must be aquatic (second). Our sun is type G (third). Around F stars, plants might get too much light and need to reflect much of it (fourth).)
RED EARTH, GREEN EARTH, BLUE EARTH: Type M stars (red dwarfs) are feeble, so plants on an orbiting Earth-like world might need to be black to absorb all the available light (first panel). Young M stars fry planetary surfaces with ultraviolet flares, so any organisms must be aquatic (second). Our sun is type G (third). Around F stars, plants might get too much light and need to reflect much of it (fourth).
Image:
In Brief
- What color will alien plants be? The question matters scientifically because the surface color of a planet can reveal whether anything lives there—specifically, whether organisms collect energy from the parent star by the process of photosynthesis.
- Photosynthesis is adapted to the spectrum of light that reaches organisms. This spectrum is the result of the parent star’s radiation spectrum, combined with the filtering effects of the planet’s atmosphere and, for aquatic creatures, of liquid water.
- Light of any color from deep violet through the near-infrared could power photosynthesis. Around stars hotter and bluer than our sun, plants would tend to absorb blue light and could look green to yellow to red. Around cooler stars such as red dwarfs, planets receive less visible light, so plants might try to absorb as much of it as possible, making them look black.
More In This Article
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Sidebar
Biosignatures For Developing Life
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Sidebar
Timeline of Photosynthesis on Earth
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Infographic
Filtering Starlight
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Photo Album
The Shocking Colors of Alien Plants [Slideshow]
The prospect of finding extraterrestrial life is no longer the domain of science fiction or UFO hunters. Rather than waiting for aliens to come to us, we are looking for them. We may not find technologically advanced civilizations, but we can look for the physical and chemical signs of fundamental life processes: “biosignatures.” Beyond the solar system, astronomers have discovered more than 200 worlds orbiting other stars, so-called extrasolar planets. Although we have not been able to tell whether these planets harbor life, it is only a matter of time now. Last July astronomers confirmed the presence of water vapor on an extrasolar planet by observing the passage of starlight through the planet’s atmosphere. The world’s space agencies are now developing telescopes that will search for signs of life on Earth-size planets by observing the planets’ light spectra.
Photosynthesis, in particular, could produce very conspicuous biosignatures. How plausible is it for photosynthesis to arise on another planet? Very. On Earth, the process is so successful that it is the foundation for nearly all life. Although some organisms live off the heat and methane of oceanic hydrothermal vents, the rich ecosystems on the planet’s surface all depend on sunlight.
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12 Comments
Add CommentThanks for a very fascinating article. If plant colors are different, animals and aliens must have a very different aesthetics. Finally I understood why the alien globe that landed near Arkham, MS, had such a strange, undescribable, uneartly color. (See H.P.Lovecraft, "The Colour out of space")
Reply | Report Abuse | Link to thisThis is a great article, and quite timely. I just recently had a discussion on the typical sci-fi alien's blood color and how it could come to be. The short story is that the iron in hemoglobin would be replaced by copper. Oxygenated blood is red due to the iron being oxidized (rust), whereas copper turns green when oxidized. Fun stuff to contemplate!
Reply | Report Abuse | Link to thisIs anyone here thats hip to Larry Nivens "Slaver Sunflowers" A plant that grows a concave mirror, It`s primary purpose to focus dim light on a photosynthetic node an it`s secondary purpose is as a weapon. They grow in vast fields and when 10-20 thousand of them all focus on an enemy it tends to get a bit warm (literaly up in smoke!) First appears in His classic novel, "Ringworld"
Reply | Report Abuse | Link to thisSide pocket Bob
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Edited by Side Pocket at 04/08/2008 1:29 AM
Biosignatures and conditions for life on other planets is also being studied by the top scientists as Harvard in the Origins of Life Initiative.
Reply | Report Abuse | Link to thisThe author is posting this correction: brown algae do not have phycobilin pigments, unlike red algae. Brown algae have some carotenoid pigments that make them brown, and which enable them to live in deeper, dimmer water than green algae, but not so deep as red algae.
Reply | Report Abuse | Link to thisAlso, the article may give readers the misimpression that plants do not absorb green light at all, so, just to clarify:. Plants do absorb and utilize a considerable fraction of green photons, and "photosynthetically active radiation" for plants is generally considered to be the entire visible range (400-700 nm). The text on p. 50 states that "plants have adapted to absorb fewer of [green photons]," and on p. 51 it states that "The array of pigments can also convert cyan, green or yellow to red." Plants accessory light harvesting pigments -- chlorophyll b, carotenoids have some absorbance in the green, although that is not their color of peak absorbance. The overall effect for a whole leaf is a good fraction of green photons being absorbed, but a slightly greater reflectance of green light compared to blue or red, giving plants their green color, and plants have not invested in extra pigments like phycobilins to absorb that extra amount of green light. The article's main purpose was to point out why chlorophyll a has its spectral bias and why the mix of pigments in a photosynthetic organism is a result of adaptation (and even acclimation) to the spectral light environment. For more details and spectra of whole plant leaves (and moss, lichens, algae), one can read more information here and download the original journal article that reviews Earth organisms:
http://www.giss.nasa.gov/research/briefs/kiang_01/
@Side Pocket: That was my first thought when I saw the "shiny" plants under F-type suns. Always have thought that was a clever invention of Nivens.
Reply | Report Abuse | Link to this--
Edited by subterrene at 05/02/2008 12:06 PM
This is a longwinded blog consisting of four points: two critical of the notion of chemical biosignatures; one critical of story-telling in natural history; and a final one of admiring wide-eyed wonder.
Reply | Report Abuse | Link to this1. Is there such a thing as a chemical biosignature?: There is an intrinsic problem with the use of simple molecules as biosignatures this is because we can never say (with only starlight to go by) that a given molecule must be the product of a biological process. What is abundant locally as biological waste product may be produced elsewhere by non-biological processes favoured by local conditions. Methane was once considered a biosignature, but since its detection in the atmospheres of our cold outer worlds is no longer considered such a certain sign of extra-terrestrial flatus. This same form of ambiguity was the issue with the contentious tests for life in earlier Mars lander programs every chemical event in the tests had possible biological and non-biological explanations. These simple molecules are not signatures, their presence in the atmosphere of other worlds is at best an ambiguous sign whose interpretation will be vigorously debated.
2. Life without chemical biosignatures?: It is debatable whether light harvesting organisms must produce any biochemical waste-product for detection. Living organisms store much of their energy in the form of electrochemical gradients across membranes (ions of sodium, potassium, hydrogen, calcium) or within recyclable chemical systems that dont have any wastes (e.g. ATP, NADPH). It is therefore not inconceivable that light harvesting organisms elsewhere could use the energy they collect to drive non-polluting systems equivalent to terrestrial ion pumps or ATP-synthases. Life elsewhere may not be as high-emission as life here on earth, where 21% of our atmosphere consists of a noxious pollutant from a very messy form of photosynthesis invented by the cyanobacteria. [Is it any wonder we dont get any visitors?]
3. Just so stories in natural history: The aspect of the article that is troubling is that it has a strongly teleological bent - i.e. that evolution elsewhere will, in important respects, parallel the course of events here in particular: (a) the discovery of oxygen producing photosynthesis; and (b) that an oxygen rich atmosphere is a necessary precursor to colonization of the land. As the article makes clear, oxygen producing photosyntheses arose more than half a billion years after photosynthesis was first undertaken. Many contemporary light harvesting systems used by archaobacteria and bacteria (other than cyanobacteria) do not produce oxygen as a waste product, and the central process of carbon fixation in vascular plants (the Calvin cycle) uses carbon dioxide but does not produce or require oxygen. It is therefore not so certain that oxygen-producing photosynthesis was a fated inevitable in the development in our ecosystem, and therefore should not be assumed to be so for other worldly ecosystems. It is also often told that without ozone (from an oxygen rich atmosphere) life could not have extended from the seas to the land. This is a nice story, but history is not the same as necessity. It is not impossible that life here, or elsewhere, could colonize the land using a variety of means to reduce UV exposure such as by making use of a UV-blocking cover of earth or by bringing along its own built-in sun-block (mineral rich biomaterial like bone, or pigments like melanin, etc.) a imaginary light-harvesting organism could use these means to shield its reproductive parts (or stem cells) and only expose UV tolerant, or replaceable, light harvesting structures to full light. Even more likely, light-harvesting life elsewhere could bask in full light by making use of selective UV-filters (such as silicates). We know from life here on earth that extremes of single physical parameters (pH, temperature, salinity, aridity, etc.) are not insurmountable barriers to evolution.
4. Uncanny absorption lines: The strongest argument in the article, and the one that made the hairs stand up on the back of my neck, is the idea that in an evolving biosphere the light harvesting organisms will always fine tune their absorption maxima to the available spectrum given the output from their home star and the absorption characteristics of the worldly atmosphere. Dr. Kiang shows us in her article how this is the case for chlorophyll. As noted above, chemical biosignatures are highly ambiguous - in contrast, it would be a coincidence beggaring a non-biological explanation if (when?) it is found that there are significant absorption at two regions in the spectrum of another world that correspond to both the most numerous and the most energetic photons that can penetrate deep into the atmosphere. This is a biosignature writ in starlight for all to see.
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Reply | Report Abuse | Link to thisMOthaaaaaaaaaaa
Reply | Report Abuse | Link to thisComment 2
Reply | Report Abuse | Link to thisI can only imagine what the future holds. Because all of it is only a matter of "Time". Humans have only been a blip of existence on our home planet, compared to the dinosaurs, which had a run of about 300 million years or so. If we last that long, or even half that amount of time, who knows what will be in store for us. Think of how far we have advanced in only the past 100 years. It boggles the mind if you really think about it.
Reply | Report Abuse | Link to thisI've been questioning since my own college Biology almost two decades ago why it seems everyone assumes photosynthetic organisms on other plants would have to be green (in addition to the assumption that the star being used would show up as yellow in the alien sky). Not that I've been following specific research discussions or anything, but it's almost annoying to me with all the Scifi that's out now in movies, books, and videogames, not once have I seen anyone randomly try something like this.
Reply | Report Abuse | Link to thisAvatar, with a forest moon full of blue- and purple-skinned creatures and glow-in-the-dark plants is still a standard-looking Earth-like rainforest during the day. Bah!