Many exoplanets likely present friendly conditions for plant growth, even if evolution never reaches complex organisms and animals. However, if mosses, algae and lichens were growing on planets in the distant reaches of the Milky Way, these worlds and the stars they surround could be completely different from our own. The alien plant kingdom is unique and completely different from anything we have ever seen.
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Most exoplanets discovered so far orbit red dwarfs, the most common type of star in the galaxy. They absorb dimmer, redder light than the sun. The conditions around red dwarf stars are not life-threatening. However, life there may have adapted very differently to the redder light.
Most plants on Earth use photosynthesis to convert sunlight and carbon dioxide into energy and oxygen. Plants use chlorophyll as a pigment to convert solar energy into chemical energy. Chlorophyll gives plants their characteristic green color and absorbs sunlight in a wide range from blue-violet to orange-red. This process is precisely tuned for efficient absorption of solar energy by plants.
Astrobiologists have observed an apparent “red edge” in vegetation, indicating that chlorophyll does not absorb many photons at wavelengths longer than 700 nanometers, which are redder. These are the exact wavelengths at which these small red dwarf stars emit most of their light. This appears to be a problem for photosynthetic species.
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The researchers tried to imagine how alien photosynthesis might work, even under unusual conditions. Specifically, they modeled light-harvesting antennae—pigment-protein complexes that all photosynthetic organisms have—that collect photons and channel the light energy to a reaction center that performs the photochemistry needed to convert it into chemical energy.
If scientists are right that plant life can arise in red dwarf systems, then astronomers need to figure out where to point their telescopes to find it. Initially, scientists focus primarily on the habitable zone around each star, as it has the ideal temperature for water to exist on the surface of a planet, not too hot and not too cold. Because water is likely essential for the majority of life forms, it is exciting when astronomers discover a rocky planet in this zone – or in the case of the TRAPPIST-1 system, multiple planets.
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There are five promising worlds that have already been discovered: Kepler-452 b, Kepler-1638 b, Kepler-1544 b, Kepler-62 e , and Kepler-62 f. They are rocky planets in the Milky Way, mostly a little larger than Earth, but not gas giants like the "mini-Neptunes," and they spend a significant portion of their orbits, if not their entire orbits, within the photosynthetic habitable zone of their star.
