Researchers at the Astrobiology Center of National Institutes of Natural Science in Japan have been looking into what alien plants might look like. Their conclusion: They are not going to be much different from the earthly equivalent – at least in infrared light.

Plants are capable of performing photosynthesis thanks to a substance known as chlorophyll, which is capable of absorbing visible light. Chlorophyll is transparent in infrared, but the cellular structure of plants is not, with each cell acting as a corner reflector when observed in infrared.

This capability gives rise to the “red edge”, a region of the electromagnetic spectrum where plants suddenly become very reflective. The red edge has been suggested as a way to spot plants on exoplanets, but some have said that the correct infrared region would depend on the star. The Japanese researchers, however, think the picture is a bit more complicated.  

As they discuss in Scientific Reports, the red edge on Earth and on exoplanets around red dwarfs (which emit a lot of red and infrared light) could be very similar, at least for some time. The team claim that if plant life evolved in water on those exoplanets like it did on Earth, then it will probably look similar. Only the blue-green wavelength can penetrate through meters of water to reach the primordial “exoplants”.

It is possible for plants to then have infrared photosynthesis, but only once they have established themselves on land. They are likely to have all evolved from something similar to Earth algae. "It is too risky to utilize IR-radiation during water-to-land evolution," lead author Kenji Takizawa said in a statement.

The team focused on red dwarfs because they are some of the most common stars in the universe and are exciting targets for study. TRAPPIST-1 and Proxima b are both red dwarfs. They could be our best bet at finding life beyond the Solar System. 

Future surveys of exoplanets might need to take this into account. Observations focused on the red edge will need to look at many different wavelengths to potentially catch a glimpse of alien plants. Our tech is not there quite yet, but it’s good to know that researchers are keeping this in mind.