Plants have the edge in the search for life beyond Earth

How the changing face of vegetation on Earth has altered the strength of our planet’s own ‘red edge’. Image credit: Jack O’Malley-James/Wendy Kenigsberg/Brand Communications.

The reflected light from vegetation, particularly on older, hotter planets, could give away the existence of life elsewhere in the Universe, new research from scientists at the Carl Sagan Institute at Cornell University have shown.

There are numerous different signals that could betray the presence of life on an exoplanet, one of which is a spike in infrared light visible in the planet’s spectrum and caused by light reflecting from vegetation.

Plant-life absorbs visible light, using photosynthesis to convert that light into energy. Visible light is more likely to bounce around within the leaf to give it the best chance of absorbing the useful wavelengths, whereas infrared light is more likely to be reflected and transmitted. (More green light is reflected than other visible wavelengths, but the variation is no more than a few percent.) Leaves are comprised of water-filled cells surrounded by air and the structure of the cell causes some light to be internally scattered, which either exits the leaf as transmitted light below the leaf, or reflected light above the leaf. The reflected infrared light creates a sharp peak in the spectrum known as the vegetation red edge. It is still not fully understood why plants reflect infrared light, but some studies suggest that it is to avoid damage by overheating.

Past, present, and future

It is possible that the detection of the vegetation red edge on other planets would serve as evidence of life. A paper published in Astrobiology by Jack O’Malley-James and Lisa Kaltenegger from the Carl Sagan Institute at Cornell University has revealed how the vegetation red edge might have varied throughout Earth’s lifetime.

The ‘red edge’ is a peak in the spectrum of light reflected off plants, shown here between 0.8 and 1.6 microns. Image credit: Eric Brown de Colstoun.

They looked at three different epochs to model the changing vegetation on Earth: the young vegetation Earth, present-day Earth, and future Earth. Land vegetation such as mosses first appeared on Earth between 725 and 500 million years ago. Modern flowering plants and trees only evolved around 130 million years ago, and cacti are a relatively recent development, only emerging between 90 and 30 million years ago. The different types of vegetation all reflect slightly differently in infrared, with different peaks and wavelengths. The early mosses are the weakest reflectors compared to the modern cacti, which reflect up to 80% of infrared light. Overall, this results in the vegetation red edge signal gradually increasing over time.

O’Malley-James and Kaltenegger looked at two possible future scenarios; a jungle world and a desert world. As the Sun grows older, it will increase in luminosity, causing increasing surface temperatures on Earth. How exactly this will play out is unknown, but one possibility is a humid planet similar to Earth in the Cretaceous era, which would have dense global jungles. An alternative is a desert planet with cacti as the main source of vegetation. Such a world would have a high infrared reflectivity, but the high reflectivity of the sand in visible light would exceed the vegetation signal. However, both scenarios would still have a higher vegetation red edge signal than the present-day Earth.

Detecting the signal

The vegetation red edge can be measured on Earth by looking at the Earth’s spectrum via Earthshine, which is the light reflected from the non-illuminated side of the Moon. The vegetation red edge is averaged across the whole disc of the Earth and is weak and difficult to detect as the infrared reflectivity only changes by a few percent. It will be even more challenging to detect on other planets.

It cannot be resolved with current telescopes, but it is possible that upcoming ground-based telescopes like the Extremely Large Telescope, or proposed space-based telescope concepts like LUVOIR [Large UV Optical InfraRed telescope] would be able to observe widespread surface features like the vegetation red edge,OMalley-James tells Astrobiology Magazine.

How different parts of a leaf reflect infrared light. Image credit: Jeff Carns.

Understanding how the vegetation red edge varies through time is also more complicated than modeling the increased signal from evolving vegetation. O’Malley-James and Kaltenegger also had to account for changing geology over time. The land, ocean and ice fractions have changed throughout history, which will also alter the vegetation cover. For example, ice ages result in less vegetation. However, highly vegetated periods like the Cretaceous would have had much higher infrared signals than even the present-day Earth. Cloud cover can also mask signs of vegetation and it is even possible that other photosynthetic organisms, such as algae, can exhibit a red edge.

Distinguishing between different types of red edge would be more challenging than the already challenging observations required to detect a red edge feature in the first place,says O’Malley-James. However, other information, like a strong visible reflectance feature that corresponds to pigments found only in photosynthetic bacteria, for example, could help us to tentatively distinguish photosynthesising microbes from leafy plants. This of course assumes that plants on another planet would have evolved to have the same colors and pigments as on Earth, which adds another potential source of uncertainty!”

The best targets for future telescopes will be older, hotter planets like desert worlds, or planets that have Cretaceous-style jungles. Desert planets can also exist across a wider range of distances from their star than Earth-like planets, which would make them more common and therefore prime targets in the search for extrasolar vegetation.