Photosynthesis on Exoplanets

Imagine an astronaut stepping out of a spacecraft onto the surface of an extrasolar planet that is capable of sustaining life. Now imagine the astronaut is greeted by the sight of red colored trees and grass. Such a scenario could be more reality than science fiction because of the variances in photosynthesis theorized to exist in other parts of the Milky Way Galaxy.

Photosynthesis occurs in plants when they use sunlight to create foods from carbon dioxide and water. This process of converting energy from sunlight into chemical energy produces oxygen and causes chlorophyll to form. It is chlorophyll that is responsible for imbuing plants with a healthy green color. The reason this happens is because chlorophyll absorbs more blue and red light waves and fewer green light waves from sunlight. Reflecting the green light waves is what causes the plants to appear green to the human eye.

When the Sun in our solar system radiates light, it reaches the Earth in a particular distribution of colors. As this sunlight passes through the Earth’s atmosphere, the various gases that comprise the atmosphere filter out certain colors before they strike the surface of the Earth. Much of the color that is not absorbed in the atmosphere is red, blue or green. Plants tend to absorb a greater amount of red and blue rays and reflect back green.

Some scientists think that plant life growing under the rays of an extraterrestrial sun could reflect colors other than green after the photosynthesis process is completed. The color that is most commonly visible on alien plants correlates with how colors are distributed in the light radiated by the parent star that strikes the surface of the extraterrestrial world.

The spectral type of a main sequence star can have a direct impact on the coloring of plants. For that reason, coloring can vary from star to star if the spectral type for each star also shows some variance. In a scientific paper published in the March, 2007 issue of Astrobiology magazine, a team of scientists examined how light emitted by another sun would appear from the vantage point of a planet orbiting that host star. Nancy Kiang, author of the paper, said the scientific team determined that the atmosphere of any extrasolar Earth-like planet would feature a chemical composition that is compatible with the chemical composition of its host star. How light from that star is seen on the planet’s surface would be affected by how it is filtered through the atmosphere as it reaches the surface.

Kiang, who works with the Goddard Institute for Space Sciences at NASA, and her team conducted an extensive study where they modeled how sunlight would reach the surface of Earth-sized planets that are hospitable to life from stars of varying spectral types. Kiang’s team speculated that each planet could experience different dominant colors that emerge in plants through photosynthesis based on how hot or cool the sun is that is anchoring that solar system.

Plant life existing on other worlds is not guaranteed to mimic the appearance of plants we are accustomed to seeing on Earth. Planets revolving around a blue star could feature plant life that has a dominant color of yellow or orange and this could lend to forests that boast autumn type colors throughout the growing season on those planets. If a habitable world is located in a binary star system or multi-star system, it could cause some exotic variations in how the plant life grows and appears to the human eye after going through photosynthesis. These planets could have plants that are almost black in color.

In the case of habitable planets around red dwarf stars, all plant life would likely exist underwater. The proximity of the habitable zone around the star would make it difficult for plants to fend off ultraviolet radiation because they could not generate enough energy from infrared light through photosynthesis to create sufficient oxygen to block ultraviolet radiation penetrating the atmosphere.

The idea that differences exist in the photosynthesis process from one planet to the next changes how astrobiologists search for evidence of life on other worlds outside our own solar system. It reinforces the idea that plant and animal life alike have evolved and adapted to fit the unique conditions of this Earth as well as reinforcing the notion that life on other habitable planets would evolve to survive and thrive in a similar manner on other planets. What do you think? Is photosynthesis the same on all planets or do you think it will be vastly different?

Kiang, N., Siefert, J., Govindjee, ., & Blankenship, R. (2007). Spectral Signatures of Photosynthesis. I. Review of Earth Organisms Astrobiology, 7 (1), 222-251 DOI: 10.1089/ast.2006.0105

Kiang, N., Segura, A., Tinetti, G., Govindjee, ., Blankenship, R., Cohen, M., Siefert, J., Crisp, D., & Meadows, V. (2007). Spectral Signatures of Photosynthesis. II. Coevolution with Other Stars And The Atmosphere on Extrasolar Worlds Astrobiology, 7 (1), 252-274 DOI: 10.1089/ast.2006.0108