Imagine a world where alien plants thrive under the light of a red dwarf star. Exoplanets provide the conditions for developing such plant life, even if evolution never progresses to complex organisms and animals. These extraterrestrial plants could be unlike anything we’ve ever seen, challenging our understanding of photosynthesis.
“It’s natural to ask if photosynthesis happens in a range of visible light— 400 to 700 nanometers—and you take a star that’s fainter, cooler, and redder, is there enough light to support photosynthesis?” – Thomas Haworth, Physicist at the Queen Mary University of London
Challenging Our Understanding of Photosynthesis
Many of the rocky exoplanets discovered so far orbit red dwarf stars, the most abundant type of star in our galaxy. These stars emit fainter, redder light than our sun. This raises a pertinent question: can photosynthesis occur under such conditions? Thomas Haworth, a physicist at the Queen Mary University of London, suggests that the answer might be “yes, sometimes.” This conclusion is encouraging, meaning life might adapt differently under redder suns.
The “Red Edge” Dilemma
Most terrestrial plants, including leafy vegetation, mosses, and cyanobacteria, use photosynthesis to convert sunlight and carbon dioxide into energy and oxygen. They rely on chlorophyll pigments to transform solar energy into chemical energy. The green color of plants results from chlorophyll, which absorbs sunlight from violet-blue to orange-red. However, chlorophyll doesn’t absorb many photons at longer, redder wavelengths beyond 700 nanometers – the wavelengths that small red dwarf stars emit most of their light. This presents a problem for photosynthetic species under the light of such stars.
Envisioning Alien Photosynthesis
Haworth and biologist Christopher Duffy endeavored to understand how extraterrestrial photosynthesis might function under unusual conditions. They developed a general model of photosynthesis that wasn’t tied to any particular species. The researchers concluded that organisms with extremely efficient antennae could absorb dim light redder than 700 nm, but oxygenic photosynthesis might be challenging. As a result, these organisms might have to expend a lot of energy to maintain the photosynthetic machinery, limiting their evolutionary progression.
The Diversity of Photosynthesis on Earth
While green plants dominate the Earth, there are species on our planet that follow different rules. Subterranean microbes can produce “dark oxygen” in the absence of light. Purple and green sulfur bacteria conduct photosynthesis without oxygen, relying on other pigments and gases, particularly sulfur. These organisms use infrared light for energy, which falls within the range of red dwarfs’ starlight. This diversity of photosynthesis on Earth might provide clues about what to expect in alien worlds.
Looking for Signs of Life
Detecting signs of life from a distance of 1,000 light-years requires a challenging effort. Astrobiologists look for particular chemical markers in the atmospheres of exoplanets that may suggest biological processes such as respiration or decay. However, I would like to point out that these markers can also result from non-biological sources. Therefore, the existence of these gases does not provide conclusive evidence of the presence of life.
Keeping an Open Mind
Nathalie Cabrol, an astrobiologist and director of the SETI Institute’s Carl Sagan Center, argues that scientists should not limit their perspective to life as we know it. Considering the possibility of life forms that can survive without oxygenic photosynthesis could mean expanding the habitable zone rather than narrowing it. Cabrol aptly says, “We need to keep our minds open.”