Plants under Alien Suns

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Doug Cummings, CalTech,
GSFC, NASA

Larger image.

Plant-like lifeforms under a bluer and
brighter star than the Sun (such as
Procyon A or Sirius A) may reflect
less useful or abundant red and yellow
light, or even an overabundance of
dangerously energetic blue light (more).

Worlds Colored by Photosynthesizing Life

In visible light, the abundance of greenish plant life on Earth's land surfaces can be easily observed from space. Most photosynthetic plants on Earth use chlorophyll which absorbs blue and red light and less green light and so appears green. Although some green color is absorbed by Earth's plants, less is absorbed than the other colors. Although many Earth-type plants were once thought to be not as efficient as they could be because they do not use more green light, some scientists no longer think that this is true.

NASA (Earth Observatory) -- larger and jumbo images;
Asia-Africa (jumbo) and Northern Americas (jumbo); and
cloudless Africa-Eurasia (jumbo) and Southern Americas (jumbo)

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.

Depending on a main sequence star's spectral type, even a planet with Earth's atmospheric composition may be colored differently. In general, larger and more massive, main-sequence ("dwarf") stars have hotter surface temperatures than our Sun, Sol, and so they radiate more photons, particularly towards the more energetic, bluish end of the spectrum. As a result of their greater luminosity, Earth-like planets would orbit farther away from hotter dwarf stars to avoid getting scorched, but their skies would still appear bluish due to Rayleigh scattering of abundant bluish photons. Around smaller, less massive and dimmer dwarf stars, however, planets would have to orbit closer in order to sustain a surface temperature that is warm enough to keep water liquid and so the star would appear larger in the sky. In addition, stars with surface temperatures of 3,300 kelvins or lower (red dwarfs of spectral type M2.5 such as Gliese 581, or redder) would emit so fewer photons towards the bluish wavelengths compared to Sol that the sky would appear whitish down to reddish to Human eyes (more from Earth Science Picture of the Day). If comparatively more bluish or reddish light reaches a planet's surface than on Earth, photosynthetic plant-type life may may not be greenish in color, because such life will have evolved to different pigments in order to optimize their use of available and so color the appearance of the planet's land surfaces accordingly.

© Ivan Gonçalves, EPOD, USRA, NASA / ESD
(used with permission)

Larger and jumbo composite images.

Depending on star type, a planet with Earth's atmosphere
in a habitable zone orbit would have a different colored
sky and apparent size of their "Sun" (more).

Extraterrestrial photosynthetic plant-type life may look quite look different in color because they will 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. By absorbing the entire spectrum of visible light more completely, 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).

Binary and multiple stars. Binary and multi-star systems with both "Sun-like" (typically defined as those of spectral types of late F, G, and early K) and red dwarf (M type) stars are common in the Solar neighborhood. Recent statistics indicate that over a fourth of Sun-like stars and roughly a half of red dwarfs in our Milky Way Galaxy have been found in multi-star systems -- around 44 percent of of spectral types F6 to K3 and possibly declining to one third to one fourth of very dim type M stars that are difficult to observe (Raghavan et al, 2010; Charles J. Lada, 2006; and Duquennoy and Mayor, 1991). Discoveries of Sun-like stars with host exoplanets as well as red dwarf companions have been common, and many appear to be old and stable enough for life to have evolved (RAS new releases of April 16 and April 19, 2011; and University of St. Andrews press release).

Luis Calçada, ESO

Large and jumbo illustrations.

A planet with at least 5.7 Earth-masses
has been found in orbit around Star C
of triple-star system MLO 4 at an orbital
distance of only 0.05 AUs (more info
and video).

The results of simulations comparing differences in light exposure for planets orbiting close versus wide binary star systems were presented on April 19, 2011 at a RAS National Astronomy Meeting by a researcher at the University of St. Andrews. In one case, an Earth-sized planet could orbit in the habitable zone (capable of having liquid water on their planetary surface) around two stars close together. Alternatively, the planet could have a habitable zone orbit around one of two widely separated stars. The reseachers also examined combinations of those two scenarios for multiple star systems -- for example, where two tightly orbiting stars are gravitationally bound to a more distant star. (Royal Astronomical Society news releases of April 16 and April 19, 2011; University of St. Andrews press release; and Alan Boyle, MSNBC, April 19, 2011).

Jack O'Malley-James, U-SA

Larger and jumbo illustrations (source).

Photosynthesizing life would be exposed to varying
different amounts near infrared, visual, and uv radiation
depending on the nature of the host stars that they orbit,
the orbital distance from each star, and whether they
close or wide binaries (more).

The simulations indicated that habitable planets in multi-star systems could host exotic forms of the more familiar plants found on Earth. As indicated in NASA studies announced in 2007, plants evolved under dim red dwarf suns or in more distance habitable orbits around a brighter star may appear black to Human eyes because they would probably need to absorbing more parts of the visible wavelength range to more effectively exploit as much of the available light as possible. Indeed, some in particularly dim environments may also evolve to use energy from infrared or ultraviolet radiation to power photosynthesis.

Jack O'Malley-James, U-SA

Larger illustration (source).

In dim habitats, alien vegetation
would need more photosynthetic
pigments that capture radiation
in a wider range of wavelengths,
which would give them a dark
appearance like many dark plants
and flowers on Earth (more).

In many systems, planets may be subject to stellar flares, particularly in the close habitable zone orbits around dim red dwarfs. Even planets orbiting two bright Sun-like stars at farther habitable orbital distances, however, could be still be subject to harmful ultraviolet (UV) radiation, particularly from stellar flares. As a result, plants that evolved in such systems would need to develop UV-blocking sun-screens to coat their surfaces on in their tissues. Even so, photosynthesising microorganisms may need to move during a sudden flare, especially those lifeforms that developed in aquatic environments subject to frequent flares but become more protected by a thicker layer of water overhead if they can move deeper underwater -- more discussion below). (See also: Royal Astronomical Society news releases of April 16 and April 19, 2011; University of St. Andrews press release; and Alan Boyle, MSNBC, April 19, 2011)

Jack O'Malley-James, U-SA

Larger and jumbo illustrations (source).

Plants on planets orbiting
dim stars or in distant orbits
around brighter stars may
appear dark because they
need to capture more of the
visual and even of the nearer
parts of the infrared and UV
spectrum (more).

Photosynthetic Life under Water

Under red dwarf stars, plant-type life on land may not be possible because photosynthesis might not generate sufficient energy from infrared light to produce the oxygen needed to block dangerous ultraviolet light from such stars at the very close orbital distances needed for a planet to be warmed enough to have liquid water on its surface. Given at least nine meters (roughly 30 feet) of water on the planet, photosynthetic microbes (including mats of algae, cyanobacteria, and other photosynthetic bacteria) and plant-like protoctists (such as floating seaweed or kelp forests attached to the seafloor) could be protected from "planet-scalding" ultraviolet flares produced by young red dwarf stars, according to Victoria Meadows of Caltech, principal investigator at the NASA Astrobiology Institute's Virtual Planetary Laboratory. Microbial mats could float near the water's surface for efficient photosynthesis when a star is calm, then sink to a safe depth when a flare hits. Life could eventually spread farther when such stars evolve pass their flare stage, since spectral-type M stars emit much less ultraviolet radiation once they quiet down. Until an atmospheric ozone-layer develops from oxygen gas released by early photosynthetic bacterial life such as cyanobacteria, Earth-type life may need to stay underwater to stay shield from damaging stellar ultraviolet radiation even under less active stars.

Cyanosite and PSARC

Larger modern and fossilized images.

Mix of cyanobacteria from a microbial mat that includes several filamentous forms.

History of Photosynthetic Life on Earth

(See Nancy Y. Klang, Scientific American, April 2008; and more generally, the History of Life on Earth)

Years 0 - 0.7 Billion: After the formation of the Earth, life may have developed quickly or been "seeded" from Venus or Mars, or other extraterrestrial habitats that became hospitable to the development of life earlier than it did on Earth. While firm fossil evidence of the first photosynthetic bacteria have been found by year 1.2 billion, earlier fossils exhibit signs of photosynthesis. Early photosynthesizers probably had to remain underwater to enjoy shielding from harmful Solar ultraviolet radiation since the ozone layer in the atmosphere did not yet exist -- not just because water is a good solvent for biochemical reactions. These bacteria would have used photosynthetic pigments (i.e., bacteriochlorophylls) to absorb near-infrared rather than visible light in chemical reactions that involved hydrogen, hydrogen sulfide, or iron to produce sulfur or sulfate compounds rather than oxygen; hence, such early photosynthesis did not produce oxygen gas. Today, "aerobic" purple bacteria (a type of proteobacteria) are able to use Solar infrared radiation as well as visible light to produce food for itself.

Cyanosite -- NASA image of Chroococcidiopsis

Dividing Chroococcus sp., a type of cyanobacteria,
photosynthetic microbes that also produce oxygen.
While "primitive," Chroococcidiopsis survives in
extremely dry, cold, and salty environments.

Years 0.7 - 1.9 Billion: Cyanobacteria evolve as the first oxygen producers by absorbing visible light using a mix of pigments (including phycobilins, carotenoids, and several forms of chlorophyll). As proposed by Andrew Goldsworthy in 1987, cyanobacteria and later chloroplast-related protists and plants developed after microbes that used a purple pigment bacteriorhodopsin that absorbs green light dominated the oceans, and so the new photosynthetic cyanobacteria were forced to use the left-over light with chlorophyll that reflects green light, which was too complex to change even after purple-reflecting photosynthetic lifeforms were no longer dominant (Debora MacKenzie, New Scientist, September 10, 2010).

Years 1.9 - 2.6 Billion - Eukaryotic red and brown algae (protoctists) develop photosynthetic organelles known as chlorophlasts through endosymbiosis with prokaryotic cyanobacteria. Like cyanobacteria, red algae photosynthesize with phycobilin pigments (phycoerythrin, and phycocyanin) as well as chlorophyll a and d. Brown algae, however, use certain carotenoids (as well as chlorophyll a and c) which gives them their color and allows them to photosynthesize in deeper water with dimmer light than green algae, but not as deep as red algae (which live down to 200 meters or more in clear waters -- over 600 feet deep).

© Mike Guiry. Courtesy
of the Irish Seaweed
Industry Organisation

Fucus serratus, or
"Serrated Wrack," a
large multi-cellular
protoctist and a
type of brown algae.

Years 2.6 - 2.8 Billion: First multi-cellular lifeforms develop, including those of red and brown algae. Green algae evolve to survive in the strong light of shallow water better than red or brown algae and photosynthesize without phycobilins. Like plants, green algae use two forms of chlorophyll to capture light energy for producing sugars, but unlike plants they are primarily aquatic (e.g., multi-cellular seaweeds).

Courtesy of M.D. Little, L.D. Ritchie, Smithsonian Institute

Palmophyllum umbracola image.

Genetic analysis reveals that two types of "seaweed,"
Verdigellas and Palmphyllum, are ancient forms of
Green Algae that diverged around a billion years
ago from the ancestors of all green "plants" (more).

Years 2.8 - 3.8 Billion; True plants develop from green algae, using: chlorophylls a and b and carotenoids, starch as their primary carbohydrate food reserve in the chloroplasts (not the cytoplasm as in other algal groups), and cellulose for the principal component of cell walls. (More discussion at: Matt Walker, BBC News, November, 18, 2010; Zechman et al, 2010; and Molecular Phylogenetics and Evolution of Marine and Freshwater Algae.)

Courtesy of Eric B. Guinther,
Rubenstein et al, 2010;
New Phytologist

Cryptospores image.

Spores of ancient liverworts
from 473 to 471 million years
ago support the hypothesis
that these primitive plants
are probably the ancestors
of all modern plants (more).

Years 3.8 - 4.1 Billion: First land plants (liverworts then mosses) develop from green algae without stems and roots (vascular structures) to pull water from the soil but are unable to grow tall more than 472 million years ago (Matt Walker, BBC News, October 12, 2010; Rubenstein et al, 2010; and Charles H. Wellman, 2010).

Years 4.1 - 4.6 Billion: Vascular plants (ferns to trees) develop with stems and roots to pull water from the soil and grow taller in competition towards the Sun to gather more Solar radiation for photosynthesis.

Other Information

Try NASA's Astrobiology Institute (NAI).