Photosynthesis might occur differently on exoplanets with atmospheres unlike the Earth’s, according to William Bains, Sara Seager, and Andras Zsom from MIT. The researchers outline their ideas in a recent paper published in the journal Life, as part of a special issue on Planetary Exploration: Habitats and Terrestrial Analogs.
Scientists have hypothesized that so-called Super Earths—planets with a mass of up to roughly 10 Earth masses—are likely to retain their early hydrogen atmospheres. Due to a strong greenhouse effect on these worlds, their habitable zones might be much more extended than those of more Earth-like planets.
Bains and his co-authors worked out the energetics and some of the chemistry of a photosynthetic pathway—they call it hydrogenic photosynthesis—that might exist on such a world. The reaction would use methane, water, and light to produce organic molecules and hydrogen. The energy requirement for hydrogenic photosynthesis would be less than for oxygenic photosynthesis (the pathway that produces many green leaved-trees on Earth), meaning that life could be supported with less light than on Earth.
Since this type of photosynthetic life would also use longer-wavelength (near-infrared) light than plants on Earth do, it would not show the same spectral features—such as the so-called “red-edge” effect—that terrestrial vegetation does. We could expect planets with this type of life to exist at distances of up to 10 AU (Astronomical Unit, 1 AU = Sun-Earth distance) from their central star, on Super Earth planets with atmospheres up to 30 times denser than Earth’s. This is very exciting, because this type of life would be different from life as we know it, yet plausible. Bains and colleagues also provide us with clues as to how life could use a metabolic pathway unknown on Earth.
Would this type of photosynthesis preclude the possibility of complex life? The authors of the paper don’t think so, based on the fact that some terrestrial organisms spend large parts of their life cycle without oxygen, and are able to gain substantial amounts of energy from other energy-rich compounds.
Would we be able to detect life on a planet with hydrogenic photosynthesis? It would be difficult, as the likely waste product from the metabolic reaction would be hydrogen, which we would have a hard time distinguishing remotely from the hydrogen in the planet’s atmosphere. Other possible waste products are hydrogen sulfide and ammonia. Hydrogen sulfide is also a common product of volcanic outgassing, but ammonia is not. So if it were detected, it might be a signature of life processes on a distant planet.