Earth’s surface is covered with green vegetation and cyanobacteria, which absorb a lot of red light and reflect a lot of visible and infrared light from the sun. This contrast between plants’ reflection and absorption leads to a unique feature of life on our planet called the “red edge.”
The red edge is seen in astronomical spectra, where the starlight reflected off an object (like a planet) is separated into its component colors. Plants absorb so much red light but barely any infrared light, so there’s a steep cliff in the spectra at that wavelength. Satellites observing Earth use this feature to trace vegetation growth, and astrobiologists may be able to look for this feature on planets around other stars as a sign of life.
New research published in Frontiers in Astronomy and Space Sciences uses numerical models of the chemistry and physics of photosynthesis to find the optimal wavelengths of light for a plant to absorb around different stars.
On Earth, life interacts with light because of a pigment called Chlorophyll a, a chemical compound that harvests light for photosynthesis. This pigment absorbs light for plants to use as energy for their biologic processes. Biologists think plants rely on chlorophyll a because it maximizes a plant’s energy production by maximizing the energy collected from the Sun while minimizing the energy needed to do photosynthesis. (The plant needs to do the hard work to make the pigments and find a way to store energy, all chemical reactions that require energy!)
But what if the sun spewed a different color of light? Would chlorophyll a still be the best pigment for the job? Likely not, since a plant around another star would need to be tuned to its star’s light to be as efficient as possible. This means if we look for a red edge around a different kind of star, we may not find anything — because the edge might not be red. It might be blue, a different shade of red, or not even in visible light at all.
In the new study, the team from NASA Ames, NASA Goddard, and the University of Washington considered factors like the amount of light at different wavelengths emitted by the star, the effect of an Earth-like atmosphere, and the energy cost for cells to do photosynthesis. Their goal was to figure out where future telescopes should look for the "red" edge, using it as a sign of extraterrestrial biology and life.
Using chemistry and physics equations, they created a numerical model to determine the ideal wavelength for plants to do photosynthesis around different kinds of stars. They compared their model’s results to Earth vegetation and they were able to reproduce the absorption spectra of plants like spinach. By using their model on a known plant like spinach, they could check their calculations were working correctly. Brighter, hotter stars than our Sun, like F type stars, which burn ~50% hotter than the sun, would have plants absorbing at higher energy wavelengths, creating a blue edge. Cooler stars, like K or M stars, would absorb mainly at shorter, lower energy wavelengths, creating a far-red or even near-infrared edge.
Interestingly, these results show that the edge happens in visible light for all but the very coldest stars, which burn at only half the temperature of the Sun or less. With a whole spectrum of light to choose from, plants still find optimal energy production in the small range of light visible to humans. In their models, the researchers also found that plants around any star aren’t limited by the amount of energy available — instead, constraints on their growth likely come from other factors like suitable terrain and nutrients.
These models improve on past studies, which assumed light from a star could be modeled as a simple curve, by using detailed spectra of different star types. They also assumed an Earth-like atmosphere, which may actually pose a problem for other exoplanets that could have very different compositions. The atmosphere also influences what kind of light a plant at the surface has to work with, since it absorbs some of the light from a star.
Although there are more complexities that could be added to the models — such as different atmospheres and leaf shapes —this research is already a great start for a search for alien plant life. This information could even help scientists find vegetation on other planets in the next few decades with upcoming space telescopes like HabEx and LUVOIR. These two space telescopes should be able to provide the first spectra of atmospheres of planets like our modern Earth, maybe even finding an exoplanet's red — or blue — edge.