Pedagoguery

One of the current goals of many scientists is to identify life on other planets. Given that the number of exoplanets is currently in the hundreds, and the fact that powerful new telescopes dedicated to this task are being designed and funded, it seems like a goal that can be achieved in our lifetimes. Certain biomarkers can be easily determined from spectroscopy: the presence of oxygen with water vapor, ozone, and oxygen with methane are examples. However the color of plants on the surface of a planet would also be a major biomarker. What color would plants on other planets be? The answer, of course, is, it depends.

One of the major determining factors in the color of a photosynthetic organism is the spectrum of the light that it receives. The peak wavelength of sunlight at the top of the Earth’s atmosphere is 583 nanometers (nm), which lies in the yellow-green part of the spectrum. However, this is not the light that plants on the Earth’s surface see. The atmosphere of our planet is not transparent at all wavelengths. Oxygen, water vapor, carbon dioxide, and ozone all combine to alter the spectrum at the surface. For instance, ozone filters out much of the ultraviolet, while also broadly but weakly absorbing across the entire visual spectrum. Oxygen has two strong absorption lines at 687 and 761 nm, which combined with a water absorption line at 700 nm sharply defines the lower limit of the visual range. As a result, the peak wavelength of the surface is shifted to 685 nm, which is in the red part of the spectrum.

On Earth, the dominant forms of photosynthetic organisms primarily use three different pigments to accomplish this task. Two of them are closely related: Chlorophyll a and chlorophyll b. The third is a family of pigments called carotenoids. Both chlorophylls have a dual peaked absorption spectrum, with a strong peak in the blue, and a weaker peak in the red. Most of the yellow and green is reflected, hence the green color of the plants that we see. Carotenoids are the pigments you see in the autumn on the leaves deciduous trees as the chlorophyll is degraded. Because they absorb mostly in the blue and green parts of the spectrum, they look red, orange, and yellow.

How plants look to us now, however, is not how they have always looked. The first photosynthetic organism to evolve on Earth were bacteria around 3.4 billion years ago. They utilized mostly red and infrared light, and thus probably looked purple in color. At this time, Earth’s atmosphere was very different than it is today, and the bacteria were aquatic, meaning that the light they received was much different than current plants receive at the surface. In addition, they did not produce oxygen. Instead, they probably produced sulfur or sulfate compounds.

Around 2.7 billion years ago, the first photosynthetic organisms that produced oxygen evolved. These were the cyanobacteria. They used a combination of pigments including carotenoids and phycobillins. Pycobillins absorb primarily in the yellow and longer wavelengths, thus resulting in the blue color of these bacteria.

These examples mean that we have to take into account the star, the atmosphere, and the evolutionary age of the planet to be able to guess the color of the plant life. To do this, I will take four examples. The first is a mature M class star, or a red dwarf. The planet would probably lie a mere 0.07 astronomical units (AU) away from the star, or 7% of the radius of Earth’s orbit around the sun. The second example is a young M class star, with a planet at 0.16 AU. The third example would be an F class star with a planet at 1.69 AU.

For the mature M class star, assuming an atmospheric composition similar to that of Earth, the peak of light at the surface would be 1044 nm – well into the infrared. Very little available energy would be found in the visible part of the spectrum, but photons below a certain threshold of energy would not be useful for photosynthetic reactions because they would not be energetic enough. As a result, the plants would probably need to soak up as much of the available higher energy photons as they could, resulting in pigments that to our eyes would look black.

Young M class stars tend to emit violent ultraviolet flares, potentially frying any life on a nearby planet not shielded. The most likely shielding would be water, so we would expect photosynthetic life to be aquatic. At a depth of about 9 meters, there would be sufficient protection from UV, while still providing enough light to allow photosynthesis. The peak of radiation at the surface (not counting flares) would be 1045nm, once again in the near-infrared. However, water tends to absorb longer wavelengths preferentially, so the peak at nine meters would be much shorter. The plants would still need to absorb most of the available energy, so they might still be black, but they might end up being purple like early photosynthetic bacteria on Earth.

For the F class star, the situation is more straightforward. The peak of light on the surface is at 451 nm, in the blue part of the spectrum. These highly energetic photons are quite useful for photosynthetic organisms, but there are so many of them that it might be damaging to the plant. A couple of possibilities suggest themselves. If the plant can utilize the blue photons without damage, it would not need longer wavelength photons, and would probably look yellow to us. If it needed protection, it would develop a pigment that reflected much of the blue light, and would thus have a bluish tint.

In the final analysis, much of this is informed speculation, and nature has many surprises in store for us. However, being open to the possibilities allows us to recognize ambiguous signs when we encounter them.

Next time, the chaotic birth of planetary systems.