Is Red the Signature of Life-Bearing Planets?

A new Open Access paper in Scientific Reports suggests that we may soon to able to look for a signature in the spectrum of reflected light. The marker they discuss is something seen on Earth, the ‘red edge’. It’s something a few astronomers have talked about, but one argument has been how red the red edge is. Under the light of different stars, wouldn’t it make sense to use different parts of the electromagnetic spectrum?

What is a Red Edge?

Earth’s surface does not uniformly reflect light. It absorbs more light at certain wavelengths than others. Where there’s a lot of plant coverage, there’s a lot of blue and red absorbed by chlorophyll for photosynthesis, and the unused portion of the visible light spectrum, green, is reflected back.
But it’s not just green that gets reflected. While plants use red, they don’t use the near infra-red, so they’re a lot more reflective in that part of the spectrum too. This change is something you can see for yourself if you fit an R720 filter to your camera. The abrupt difference in reflection between red and near infrared is called the red edge.

People have used this red edge as a measure of plant chlorophyll content in surveys. Because this is all visual data, it’s the sort of thing satellites can look for, allowing them to search for wildfire damage or biomass in hard to reach areas. For similar reasons, astronomers can use it when surveying light reflected from exoplanets, to see if there is a red edge in it. If there is, then it will be interesting, given what we know about its origins on Earth.

Why does the Red Edge exist?

The Red Edge is an example of evolution not always finding the best solution because it can’t plan for the future. Instead, organisms can only work with what conditions they have now. For the earliest photosynthetic organisms that evolved in Earth’s oceans, that means no infrared light. This is because infrared radiation is blocked by more than a metre of water. Unless life could find a very stable and shallow body of water then infrared would not be an option.

This is important because, while infrared is useful once you’re on land, first you have to get there. Takizawa and his team argue that using infrared during the transition phase is risky. There are big rewards, but even a small increase in water cover would starve an organism of light. Organisms using just our range of visible light might be less efficient on land, but are more robust in water and so much more likely to survive.

This means that the plants that we see today still carry the baggage of their pioneering ancestors. This explains why Earth’s vegetation still produces a distinctive red edge. They can’t erase their evolutionary history.

Could the light of other stars make a difference?

That explains the colour of vegetation on Earth, but does similar life have to obey the same rules under other stars? Takizawa’s team are interested in M-class stars, stars at the red end of the spectrum. Cooler and redder, their peak radiation is much more to the infrared end of the electromagnetic spectrum. Could alternative forms of photosynthesis make use of the much more plentiful infrared light under alien skies?
Plants use light by trapping it in small packets called photons. The power in photons is related to their wavelength. Blue photons have a short wavelength so have more energy. Ultraviolet is shorter and so has even more energy, but is so powerful it can break carbon-carbon bonds. This is lethal to carbon-based life so it can’t really be used much. This puts a cap on photosynthetic systems at the short wavelength end of the spectrum.

Red photons have a much longer wavelength and so have less energy, but a lot more of them make their way down to the surface. Chlorophyll uses two photons to generate the energy to photosynthesise. Its biochemistry means the green photons cannot be used.

Takizawa and his team looked to see if shifting the distributions of photons towards the redder end of the spectrum would make a difference to two-photon photosynthesis. They found that, theoretically, more use of the infrared was possible, but energy required from two photons meant there wasn’t much more of the spectrum to use. That means if there is life on other planets using two-photon photosynthesis, then you’d expect the red edge to be roughly where you find it on Earth.

But, they also modelled three- and four-photon photosynthesis. Using more photons means that the photons themselves can be of lower energy, meaning more infrared. If this method of photosynthesis can evolve then the red edge could be shifted quite a bit into the infrared. But this does rely on organisms evolving entirely new pigments for photosynthesis.

Is there time for infrared plants to evolve under an M-class star?

While a four-photon photosynthesis system might be possible in theory, it takes time to evolve. We don’t know how much time, but we can say it hasn’t evolved in 500 million years on Earth. However, we’re dealing with a sample size of one, so maybe other planets might catch a lucky break. On the other hand, it might take a long time if ever.

While this sounds pessimistic, astronomical timescales usually dwarf evolutionary timescales. Earth has been around for four and a half billion years. M-class stars burn cooler and longer than our Sun. One estimate is that some exoplanets might be in a star’s habitable zone for over fifty billion years, compared to Earth which might have six to eight billion habitable years. That doesn’t mean that other planets have had fifty billion years to evolve lifeforms, the universe isn’t fourteen billion years old, but it does mean that other life on other planets might have had much more time to come up with solutions.

The opposite side is that it also has longer to run into problems. A commentator on the Centauri Dreams blog notes that Earth has only been ‘earth-like’, as we’d call it, for a relatively short time. It’s possible in the longer term that Earth is going to change again, in which case ‘earth-like’ might be just a phase some planets go through. So while a red edge might mean there’s life, it might not be life as we know it.

Does a red edge have to be vegetation?

Another discussion of the red edge and life recently came up in the Monthly Notices of the Royal Astronomical Society. It’s an interesting paper because it proposes that if there is intelligent life in the universe we might well see a sharp red edge on some exoplanets.

The reason is power. Specifically solar power. Lingam and Loeb say that if an advanced civilisation exists then tidally locked planets, planets where only one side ever faces the star, are an opportunity. In this situation, it makes sense to cover the sunny side of the planet with solar panelling and then store energy on the cool side. This would produce an artificial red edge, at the limits of the wavelengths used to generate power.

Should a red edge be found in the light spectrum of another star, the location of the edge might be the clue that not only have we found somewhere where life was, but it might be intelligent life.
Extrasolar botany might well seem fanciful, but exoplanets have only been observed for thirty years. In the next thirty botanical research into photosynthetic mechanisms could prove highly influential when exobiologists finally have their first data.