In a quest to find life outside the solar system, astrobiologists have started searching for colors associated with living things. Now, the range of what to look for has been expanded, giving us a better chance to find life that is not as we know it.
The oxygen in Earth's atmosphere is a product of photosynthesis. If all life disappeared tomorrow, oxygen would react with rocks until it disappeared. Consequently, if we detect oxygen in the atmosphere of a planet orbiting another star, it probably has, or very recently had, widespread life. Other signatures of life, such as methane, are less definitive, but would at least mark a planet as worthy of further investigation.
However, there is extensive debate about how similar, chemically speaking, life on other worlds would be to what we see around us. We can't assume that oxygen production will be part of their repertoire. Edward Schwieterman, a University of Washington doctoral student, wondered what the prospects would be for picking up signs of pigments from widespread organisms that, like the first life forms on this planet, don't photosynthesize oxygen.
Although the search for signs of life is still in its infancy, astrobiologists making plans to study life with future generations of telescopes have focused on the “red edge." This is based on the fact that leaves absorb plenty of visible light but reflect in the infrared, making them bright in wavelengths too long to see. Satellites that use infrared photography see forests as very bright, so a heavily forested planet would look suddenly brighter if we shifted from studying it in the visible light spectrum to the infrared.
The various types of chlorophyll make green the dominant color of life on our planet. However, species that use light for other purposes “will produce reflectance, or brightness, signatures different than those of land vegetation like trees,” said Schwieterman. “This could push us to broaden our conception of what surface biosignatures might look like.”
Using computer simulations, Schwieterman looked at widespread Earthly pigments, such as those used to protect living things from excess sunlight, to see whether they would create a recognizable signature on distant worlds. To subsequently find such a signature will not be simple. We don't know which pigments we should be looking for and a planet with several widespread pigments would be very hard to distinguish from a bare, rocky world.
Nevertheless, in a paper in Astrobiology, Schwieterman argues, “Given sufficient surface coverage, nonphotosynthetic pigments could significantly impact the disk-averaged spectrum of a planet.” Since the best vision we can hope for, at least for many decades, is to see distant planets as a single pixel, the average across the disk is all we will get.
While Schwieterman admits that identifying nonphotosynthetic pigments from colors alone will be hard, the university has created a database of the spectra of pigments that future astronomers should look out for, and how they might be affected when viewed through an atmosphere.