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Black trees in a world of
two Suns?

Posted: 19 April 2011

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Scientists studying what plant life might be like on an Earth-like planet with two or three suns have found that they might appear black or grey if the system is dominated by a red dwarf.

Plants on Earth use a process called photosynthesis to convert sunlight into energy, outputting oxygen as a “waste” product, which of course is essential for animal life. If a planet existed in a system with two or more stars, there would potentially be multiple sources of energy available to drive photosynthesis.

An Earth twin with two or more suns may host plants with black rather than green foliage. Image: NASA/JPL-Caltech

“The temperature of a star determines its colour and, hence, the colour of light used for photosynthesis,” says Jack O’Malley-James from the University of St Andrews. “Depending on the colours of their starlight, plants would evolve very differently.”

Since red dwarf stars are a lot dimmer than Sun-like stars, plant-life would need to absorb a lot more sunlight in order to maintain the same level of photosynthesis that we are familiar with today. Therefore, suggests O’Malley-James, plants would evolve darker leaves to draw in more light, absorbing across the entire visible wavelength range in order to soak up as much light as possible.

O’Malley-James tells Astronomy Now that if the binary system comprises a Sun-like star and a red dwarf, then some plants would develop an affinity for the red dwarf star while others would prefer to collect energy from the Sun-like star. The result: a planet with a mixture of green and black-leaved foliage.

“A different type of star might also affect the “machinery” of photosynthesis,” says O’Malley-James. “They may also be able to use infrared or ultraviolet radiation to drive photosynthesis. For planets orbiting two stars like our own, harmful radiation from intense stellar flares could lead to plants that develop their own UV-blocking sun-screens, or photosynthesizing microorganisms that can move in response to a sudden flare.”

The study was prompted by the growing population of exoplanets orbiting different types of star, and the occurrence of stars found in multi-star systems – over 25 percent of Sun-like stars and 50 percent of red dwarfs are found in multi-star systems. Understanding the signatures of vegetation under different solar conditions will better equip astronomers with the tools necessary to seek out life on other planets.