To Find Evidence of Life on Exoplanets, Scientists Should Search for "Purple Earths"

"Retinal is a relatively simple chemical compared to chlorophyll. It has an isoprenoid structure and there is evidence for the presence of these compounds on the early Earth, as early as 2.5-3.7 billion years ago. Retinal's absorption occurs in the yellow-green part of the visible spectrum where a lot of the solar energy is found, and it is complementary to chlorophyll's absorption in the flanking blue and red regions of the spectrum. Retinal-based phototrophy is much simpler than chlorophyll-dependent photosynthesis, requiring only the retinal proteins, a membrane vesicle and ATP synthase to convert light energy into chemical energy (ATP). It seems reasonable that the simpler retinal-dependent photosynthesis evolved earlier than the more complex chlorophyll-dependent photosynthesis."

"Our work is concerned with the subset of exoplanets that may be habitable and whose spectral signatures could one day be analyzed for signs of life. The VRE as a biosignature is informed by just one type of organism—oxygen-producing photosynthesizers like plants and algae. This type of life is dominant on our planet today, but it was not always so and may not be the case on all exoplanets. While we do expect life elsewhere to have some universal characteristics, we maximize our chances of success in the search for life by considering the diverse characteristics organisms elsewhere may have."

"The retinal light-harvesting we discuss in our paper would produce a signature distinct from the VRE. While vegetation has a distinctive "red-edge," caused by strong absorption of red light and reflection of infrared light, the purple membrane bacteriorhodopsins absorb green light most strongly, producing a "green-edge." The characteristics of this signature would differ between organisms suspended in water or on land, just as with ordinary photosynthesizers. If retinal-based phototrophs existed at high enough abundance on an exoplanet, this signature would be embedded in that planet's reflected light spectrum and could potentially be seen by future advanced space telescopes (which would also be searching for the VRE, oxygen, methane, and other potential biosignatures, too)."