Sunlight is an abundant energy source on Earth's surface that life very quickly figured out how to use as a fuel. On another habitable world, star light is likely to be an equally rich free energy source. So, if life were to evolve on another planet, it would not be surprising if it came up with something like photosynthesis to take advantage of this free food. Photosynthesis has the power to change the composition of gases that make up an atmosphere, and vegetation itself has unique reflectance properties that can change the appearance of a planet's surface. By exploring how plant life might work on other worlds, we bring ourselves a step closer to knowing how to find some of the biggest fingerprints that life can leave behind on a planet.
Vegetation coverage on Earth is extensive enough to influence our planet's colour. Although plants appear green to our eyes, they have a much more intense colour in the infrared part of the spectrum, beyond our visual range. When looking at Earth with infrared satellites, vegetated areas are notably bright; a feature commonly referred to as the vegetation red edge. This has led to speculation that vegetation on other worlds could be detected via a similar bright infrared signal. Detecting plant reflectance on a distant planet will much more challenging than measuring Earth's vegetation reflectance from orbit, but it will be possible with powerful telescopes and a bit of patience.
Our assumptions about the strength of the vegetation red edge are largely based on the vegetation types and distributions we see on Earth today. However, by looking back through geological time we can estimate how the strength of Earth's red edge may have varied over time. Comparing the types of plants that lived 100s millions of years ago, or the lichens and microbial photosynthesisers that dominated the planet over a billion years ago, to the measured infrared brightnesses of similar species living today, we can predict how the strength of the red edge may have changed on Earth over time.
Interestingly, Earth's red edge strength has generally been increasing over geological time, but in periods when it was hotter and wetter (like the Cretaceous period) it may have been a little brighter than it is today, which suggests that hot, humid jungle planets could be particularly good candidates for searches for alien plant life.
Some exoplanets have more than one sun. How would having two or more light sources, which could shine with different strengths at different times, influence the development of photosynthesis on a habitable world?
'Life and Light: Exotic Photosynthesis in Binary and Multiple Star Systems' explores this thought experiment, using plant life on Earth as a guide to how plant might evolve to live in the very different light environments of binary star worlds.