Could Photosynthesis Occur on Other Earthlike Planets?

When viewed from space, Earth appears as a breathtaking paradise with its deep ocean blues and vibrant green vegetation covering every continent, even Antarctica. However, is it possible that our perception of an Earthlike planet automatically having oxygen-producing plant life is biased?

Ariel Anbar, an astrobiologist at Arizona State University in Tempe, explains that photosynthesis, the process of converting light energy into chemical energy in the form of hydrocarbon compounds, may or may not result in the production of molecular oxygen (O2).

About 2.4 billion years ago, the emergence of oxygenic photosynthesis is believed to have played a major role in the Great Oxidation Event, leading to the greening of Earth. This event essentially paved the way for the development of complex oxygen-breathing life on our planet.

Anbar emphasizes that the evolution of oxygenic photosynthesis was crucial for the evolution of complex life on Earth. Complex life requires the energy obtained from aerobic respiration, and the level of O2 in Earth’s atmosphere could not have reached its current state without the biological production of oxygenic photosynthesis.

However, in 1996, scientists discovered that Acaryochloris marina, a marine cyanobacterium, uses chlorophyll d instead of chlorophyll a in its photosynthesis process. Chlorophyll a absorbs light in the visible spectrum, while chlorophyll d’s absorption wavelength extends farther into the red, almost into the near infrared spectrum. Nancy Kiang, a biometeorologist at the NASA Goddard Institute for Space Studies, explains that this discovery indicates a possibility for adapting photosynthesis to stars with more infrared and less visible light than our Sun.

Kiang further states that anoxygenic photosynthesis already utilizes near-infrared photons and raises the question of how far oxygenic photosynthesis can operate into the near infrared spectrum.

How can we be certain that photosynthesis will occur on other planets?

According to Kiang, the fact that photosynthesis emerged early in Earth’s history suggests that its occurrence is not too difficult. Anbar also supports this view, stating that it is logical to presume that evolution would eventually exploit the energy available from light.

Are there planets that can have high levels of oxidation without photosynthesis?

Anbar suggests that planets with a significant amount of water orbiting close to a red dwarf star could experience high levels of oxidation. The intense ultraviolet radiation emitted by red dwarfs would break up water vapor, releasing molecular oxygen. However, on such worlds, the presence of O2 would not necessarily indicate the presence of life.

Anbar explains that while oxygenic photosynthesis produces O2, reactions with volcanic gases and other materials from the Earth’s interior consume oxygen. He also suggests that the evolution of the solid Earth played a crucial role in building O2 in our atmosphere until about 2.4 billion years ago.

How can we avoid false negatives in our search for life?

Anbar suggests that we must consider the possibility that many worlds have evolved oxygenic photosynthesis, but the solid planet consumes all the produced O2, preventing its accumulation in the atmosphere. To avoid false negative results, we need to understand the evolution of the interiors of Earth-like planets and develop methods to predict which worlds are more likely to have oxygen as a biosignature.

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