What do plants look like on other exoplanets?

2024 Author: Seth Attwood | [email protected]. Last modified: 2023-12-16 15:55

The search for extraterrestrial life is no longer the domain of science fiction or UFO hunters. Perhaps modern technologies have not yet reached the required level, but with their help we are already able to detect the physical and chemical manifestations of the fundamental processes underlying living things.

Astronomers have discovered more than 200 planets orbiting stars outside the solar system. So far we cannot give an unambiguous answer about the likelihood of the existence of life on them, but this is only a matter of time. In July 2007, after analyzing the starlight that passed through the exoplanet's atmosphere, astronomers confirmed the presence of water on it. Telescopes are now being developed that will make it possible to search for traces of life on planets such as the Earth by their spectra.

One of the important factors affecting the spectrum of light reflected by a planet may be the process of photosynthesis. But is this possible in other worlds? Quite! On Earth, photosynthesis is the basis for almost all living things. Despite the fact that some organisms have learned to live at elevated temperatures in methane and in ocean hydrothermal vents, we owe the richness of ecosystems on the surface of our planet to sunlight.

On the one hand, in the process of photosynthesis, oxygen is produced, which, together with the ozone formed from it, can be found in the atmosphere of the planet. On the other hand, the color of a planet may indicate the presence of special pigments, such as chlorophyll, on its surface. Almost a century ago, having noticed the seasonal darkening of the surface of Mars, astronomers suspected the presence of plants on it. Attempts have been made to detect signs of green plants in the spectrum of light reflected from the planet's surface. But the doubtfulness of this approach was seen even by the writer Herbert Wells, who in his "War of the Worlds" remarked: "Obviously, the vegetable kingdom of Mars, in contrast to the earthly one, where green predominates, has a blood-red color." We now know that there are no plants on Mars, and the appearance of darker areas on the surface is associated with dust storms. Wells himself was convinced that the color of Mars is not least determined by the plants that cover its surface.

Even on Earth, photosynthetic organisms are not limited to green: some plants have red leaves, and various algae and photosynthetic bacteria shimmer with all the colors of the rainbow. And purple bacteria use infrared radiation from the Sun in addition to visible light. So what will prevail on other planets? And how can we see this? The answer depends on the mechanisms by which the alien photosynthesis assimilates the light of its star, which differs in the nature of the radiation from the Sun. In addition, a different composition of the atmosphere also affects the spectral composition of the radiation incident on the planet's surface.

Stars of spectral class M (red dwarfs) shine faintly, so plants on Earth-like planets near them must be black in order to absorb as much light as possible. Young M stars scorch the surface of planets with ultraviolet flares, so organisms there must be aquatic. Our Sun is class G. And near F-class stars, plants receive too much light and must reflect a significant part of it.

To imagine what photosynthesis will be like in other worlds, you first need to understand how plants carry it out on Earth. The energy spectrum of sunlight has a peak in the blue-green region, which made scientists wonder for a long time why plants do not absorb the most available green light, but, on the contrary, reflect it? It turned out that the process of photosynthesis depends not so much on the total amount of solar energy, but on the energy of individual photons and the number of photons that make up light.

Each blue photon carries more energy than a red one, but the sun predominantly emits red ones. Plants use blue photons because of their quality, and red ones because of their quantity. The wavelength of green light lies exactly between red and blue, but green photons do not differ in availability or energy, so plants do not use them.

During photosynthesis to fix one carbon atom (derived from carbon dioxide, CO₂) in a sugar molecule, at least eight photons are required, and for the cleavage of a hydrogen-oxygen bond in a water molecule (H₂O) - just one. In this case, a free electron appears, which is necessary for further reaction. In total, for the formation of one oxygen molecule (O₂) four such bonds need to be broken. For the second reaction to form a sugar molecule, at least four more photons are required. It should be noted that a photon must have some minimum energy in order to take part in photosynthesis.

The way in which plants absorb sunlight is truly one of the wonders of nature. Photosynthetic pigments do not occur as individual molecules. They form clusters consisting, as it were, of many antennas, each of which is tuned to perceive photons of a certain wavelength. Chlorophyll primarily absorbs red and blue light, while the carotenoid pigments that give fall foliage red and yellow perceive a different shade of blue. All the energy collected by these pigments is delivered to the chlorophyll molecule located in the reaction center, where water splits to form oxygen.

A complex of molecules in a reaction center can carry out chemical reactions only if it receives red photons or an equivalent amount of energy in some other form. To use the blue photons, antenna pigments convert their high energy into lower energy, just as a series of step-down transformers reduce 100,000 volts of a power line to a 220 volt wall outlet. The process begins when a blue photon strikes a pigment that absorbs blue light and transfers energy to one of the electrons in its molecule. When an electron returns to its original state, it emits this energy, but due to heat and vibrational losses, less than it absorbed.

However, the pigment molecule gives up the received energy not in the form of a photon, but in the form of an electrical interaction with another pigment molecule, which is able to absorb the energy of a lower level. In turn, the second pigment releases even less energy, and this process continues until the energy of the original blue photon drops to the level of red.

The reaction center, as the receiving end of the cascade, is adapted to absorb available photons with minimal energy. On the surface of our planet, red photons are the most numerous and at the same time have the lowest energy among photons in the visible spectrum.

But for underwater photosynthesizers, red photons don't have to be the most abundant. The area of light used for photosynthesis changes with depth as water, dissolved substances in it, and organisms in the upper layers filter the light. The result is a clear stratification of living forms in accordance with their set of pigments. Organisms from deeper layers of water have pigments that are tuned to the light of those colors that were not absorbed by the layers above. For example, algae and cyanea have the pigments phycocyanin and phycoerythrin, which absorb green and yellow photons. In anoxygenic (i.e.non-oxygen-producing) bacteria are bacteriochlorophyll, which absorbs light from the far red and near infrared (IR) regions, which is only able to penetrate the gloomy depths of water.

Organisms that have adapted to low light tend to grow more slowly because they have to work harder to absorb all the light available to them. On the planet's surface, where light is abundant, it would be disadvantageous for plants to produce excess pigments, so they selectively use colors. The same evolutionary principles should work in other planetary systems as well.

Just as aquatic creatures have adapted to light filtered by water, land dwellers have adapted to light filtered by atmospheric gases. In the upper part of the earth's atmosphere, the most abundant photons are yellow, with a wavelength of 560-590 nm. The number of photons gradually decreases towards long waves and abruptly breaks off towards short ones. As sunlight passes through the upper atmosphere, water vapor absorbs IR in several bands longer than 700 nm. Oxygen produces a narrow range of absorption lines near 687 and 761 nm. Everyone knows that ozone (Oh₃) in the stratosphere actively absorbs ultraviolet (UV) light, but it also absorbs slightly in the visible region of the spectrum.

So, our atmosphere leaves windows through which radiation can reach the planet's surface. The range of visible radiation is limited on the blue side by a sharp cutoff of the solar spectrum in the short wavelength region and UV absorption by ozone. The red border is defined by oxygen absorption lines. The peak of the number of photons is shifted from yellow to red (about 685 nm) due to the extensive absorption of ozone in the visible region.

Plants are adapted to this spectrum, which is mainly determined by oxygen. But it must be remembered that the plants themselves supply oxygen to the atmosphere. When the first photosynthetic organisms appeared on Earth, there was little oxygen in the atmosphere, so plants had to use pigments other than chlorophyll. Only after a lapse of time, when photosynthesis changed the composition of the atmosphere, chlorophyll became the optimal pigment.

Reliable fossil evidence of photosynthesis is about 3.4 billion years old, but earlier fossil remains show signs of this process. The first photosynthetic organisms had to be underwater, in part because water is a good solvent for biochemical reactions, and also because it provides protection from solar UV radiation, which was important in the absence of an atmospheric ozone layer. Such organisms were underwater bacteria that absorbed infrared photons. Their chemical reactions included hydrogen, hydrogen sulfide, iron, but not water; therefore, they did not emit oxygen. And only 2, 7 billion years ago, cyanobacteria in the oceans began oxygenic photosynthesis with the release of oxygen. The amount of oxygen and the ozone layer gradually increased, allowing red and brown algae to rise to the surface. And when the water level in shallow waters was sufficient to protect against UV, green algae appeared. They had few phycobiliproteins and were better adapted to bright light near the water surface. 2 billion years after oxygen began to accumulate in the atmosphere, the descendants of green algae - plants - appeared on land.

The flora has undergone significant changes - the variety of forms has rapidly increased: from mosses and liverworts to vascular plants with high crowns, which absorb more light and are adapted to different climatic zones. The conical crowns of coniferous trees effectively absorb light in high latitudes, where the sun hardly rises above the horizon. Shade-loving plants produce anthocyanin to protect against bright light. Green chlorophyll is not only well adapted to the modern composition of the atmosphere, but also helps to maintain it, keeping our planet green. It is possible that the next step in evolution will give an advantage to an organism that lives in the shade under the crowns of trees and uses phycobilins to absorb green and yellow light. But the inhabitants of the upper tier, apparently, will remain green.

Painting the world red

While searching for photosynthetic pigments on planets in other stellar systems, astronomers should remember that these objects are at different stages of evolution. For example, they may encounter a planet similar to Earth, say, 2 billion years ago. It should also be borne in mind that alien photosynthetic organisms may have properties that are not characteristic of their terrestrial "relatives". For example, they are able to split water molecules using longer wavelength photons.

The longest wavelength organism on Earth is the purple anoxygenic bacterium, which uses infrared radiation with a wavelength of about 1015 nm. The record holders among oxygenic organisms are marine cyanobacteria, which absorb at 720 nm. There is no upper limit to the wavelength that is determined by the laws of physics. It's just that the photosynthesizing system has to use a larger number of long-wavelength photons compared to short-wavelength ones.

The limiting factor is not the variety of pigments, but the spectrum of light reaching the planet's surface, which in turn depends on the type of star. Astronomers classify stars based on their color, depending on their temperature, size, and age. Not all stars exist long enough for life to arise and develop on neighboring planets. The stars are long-lived (in order of decreasing temperature) of spectral classes F, G, K, and M. The sun belongs to class G. F-class stars are larger and brighter than the Sun, they burn, emitting a brighter blue light and burn out in about 2 billion years. Class K and M stars are smaller in diameter, fainter, redder and classified as long-lived.

Around each star there is a so-called "life zone" - a range of orbits, being on which the planets have the temperature necessary for the existence of liquid water. In the solar system, such a zone is a ring bounded by the orbits of Mars and Earth. Hot F stars have a life zone farther from the star, while cooler K and M stars have it closer. Planets in the life zone of F-, G- and K-stars receive about the same amount of visible light as the Earth receives from the Sun. It is likely that life could arise on them based on the same oxygenic photosynthesis as on Earth, although the color of the pigments may be shifted within the visible range.

M-type stars, the so-called red dwarfs, are of particular interest to scientists as they are the most common type of stars in our Galaxy. They emit noticeably less visible light than the Sun: the intensity peak in their spectrum occurs in the near-IR. John Raven, a biologist at the University of Dundee in Scotland, and Ray Wolstencroft, an astronomer at the Royal Observatory in Edinburgh, have suggested that oxygenic photosynthesis is theoretically possible using near-infrared photons. In this case, organisms will have to use three or even four IR photons to break a water molecule, while terrestrial plants use only two photons, which can be likened to the steps of a rocket that impart energy to an electron to carry out a chemical reaction.

Young M stars exhibit powerful UV flares that can only be avoided underwater. But the water column also absorbs other parts of the spectrum, so the organisms located at depth will be sorely lacking in light. If so, then photosynthesis on these planets may not develop. As the M-star ages, the amount of emitted ultraviolet radiation decreases, at the later stages of evolution it becomes less than our Sun emits. During this period, there is no need for a protective ozone layer, and life on the surface of planets can flourish even if it does not produce oxygen.

Thus, astronomers should consider four possible scenarios depending on the type and age of the star.

Anaerobic Ocean Life. A star in the planetary system is young, of any type. Organisms may not produce oxygen. The atmosphere can be composed of other gases such as methane.

Aerobic Ocean Life. The star is no longer young, of any type. Enough time has passed since the onset of oxygenic photosynthesis for the accumulation of oxygen in the atmosphere.

Aerobic land life. The star is mature, of any type. The land is covered with plants. Life on Earth is just at this stage.

Anaerobic land life. A faint M star with weak UV radiation. Plants cover the land but may not produce oxygen.

Naturally, the manifestations of photosynthetic organisms in each of these cases will be different. The experience of shooting our planet from satellites suggests that it is impossible to detect life in the depths of the ocean using a telescope: the first two scenarios do not promise us color signs of life. The only chance to find it is to search for atmospheric gases of organic origin. Therefore, researchers using color methods to search for alien life will have to focus on studying land plants with oxygenic photosynthesis on planets near F-, G- and K-stars, or on planets of M-stars, but with any type of photosynthesis.

Signs of life

Substances that, in addition to the color of plants, can be a sign of the presence of life

Oxygen (O₂) and water (H₂O) … Even on a lifeless planet, the light from the parent star destroys water vapor molecules and produces a small amount of oxygen in the atmosphere. But this gas quickly dissolves in water and also oxidizes rocks and volcanic gases. Therefore, if a lot of oxygen is seen on a planet with liquid water, it means that additional sources produce it, most likely photosynthesis.

Ozone (O₃) … In the stratosphere of the Earth, ultraviolet light destroys oxygen molecules, which, when combined, form ozone. Together with liquid water, ozone is an important indicator of life. While oxygen is visible in the visible spectrum, ozone is visible in infrared, which is easier to detect with some telescopes.

Methane (CH₄) plus oxygen, or seasonal cycles … The combination of oxygen and methane is difficult to obtain without photosynthesis. Seasonal fluctuations in methane concentration are also a sure sign of life. And on a dead planet, the concentration of methane is almost constant: it only decreases slowly as sunlight breaks down molecules

Chloromethane (CH₃Cl) … On Earth, this gas is formed by burning plants (mainly in forest fires) and by exposure to sunlight on plankton and chlorine in seawater. Oxidation destroys it. But the relatively weak emission of M-stars can allow this gas to accumulate in an amount available for registration.

Nitrous oxide (N₂O) … When organisms decay, nitrogen is released in the form of an oxide. Non-biological sources of this gas are negligible.

Black is the new green

Regardless of the characteristics of the planet, photosynthetic pigments must satisfy the same requirements as on Earth: absorb photons with the shortest wavelength (high-energy), with the longest wavelength (which the reaction center uses), or the most available. To understand how the type of star determines the color of plants, it was necessary to combine the efforts of researchers from different specialties.

Starlight passing

The color of plants depends on the spectrum of starlight, which astronomers can easily observe, and the absorption of light by air and water, which the author and her colleagues have modeled based on the likely composition of the atmosphere and the properties of life. Image "In the world of science"

Martin Cohen, an astronomer at the University of California, Berkeley, collected data on an F-star (Bootes sigma), a K-star (epsilon Eridani), an actively flaring M-star (AD Leo), and a hypothetical calm M-star with temperature 3100 ° C. Astronomer Antigona Segura of the National Autonomous University in Mexico City has conducted computer simulations of the behavior of Earth-like planets in the life zone around these stars. Using models by Alexander Pavlov of the University of Arizona and James Kasting of the University of Pennsylvania, Segura studied the interaction of radiation from stars with the likely components of planetary atmospheres (assuming that volcanoes emit the same gases on them as on Earth), trying to figure out the chemical composition atmospheres both lacking oxygen and with its content close to that of the earth.

Using Segura's results, University College London physicist Giovanna Tinetti calculated the absorption of radiation in planetary atmospheres using David Crisp's model at the Jet Propulsion Laboratory in Pasadena, California, which was used to estimate the illumination of solar panels on Mars rovers. Interpreting these calculations required the combined efforts of five experts: microbiologist Janet Siefert at Rice University, biochemists Robert Blankenship at Washington University in St. Louis and Govindjee at University of Illinois at Urbana, planetologist and Champaigne. (Victoria Meadows) from Washington State University and me, a biometeorologist from NASA's Goddard Space Research Institute.

We concluded that blue rays with a peak at 451 nm mostly reach the surfaces of planets near F-class stars. Near K-stars, the peak is located at 667 nm, this is the red region of the spectrum, which resembles the situation on Earth. In this case, ozone plays an important role, making the light of F-stars bluer, and the light of K-stars redder than it actually is. It turns out that radiation suitable for photosynthesis in this case lies in the visible region of the spectrum, as on Earth.

Thus, plants on planets near F and K stars can have almost the same color as those on Earth. But in F stars, the flux of energy-rich blue photons is too intense, so plants must at least partially reflect them using shielding pigments like anthocyanin, which will give the plants a bluish coloration. However, they can only use blue photons for photosynthesis. In this case, all light in the range from green to red should be reflected. This will result in a distinctive blue cutoff in the reflected light spectrum that can be easily spotted with a telescope.

The wide temperature range for M stars suggests a variety of colors for their planets. Orbiting a calm M-star, the planet receives half the energy that the Earth does from the Sun. And although this, in principle, is enough for life - this is 60 times more than is required for shade-loving plants on Earth - most of the photons coming from these stars belong to the near-IR region of the spectrum. But evolution should lead to the emergence of a variety of pigments that can perceive the entire spectrum of visible and infrared light. Plants that absorb almost all radiation may even appear black.

Small purple dot

The history of life on Earth shows that early marine photosynthetic organisms on planets near class F, G, and K stars could live in a primary oxygen-free atmosphere and develop a system of oxygenic photosynthesis, which would later lead to the appearance of terrestrial plants. The situation with M-class stars is more complicated. The results of our calculations indicate that the optimal place for photosynthesizers is 9 m under water: a layer of such depth traps destructive ultraviolet light, but allows enough visible light to pass through. Of course, we will not notice these organisms in our telescopes, but they could become the basis of land life. In principle, on planets near M stars, plant life, using various pigments, can be almost as diverse as on Earth.

But will future space telescopes allow us to see traces of life on these planets? The answer depends on what will be the ratio of water surface to land on the planet. In telescopes of the first generation, the planets will look like points, and a detailed study of their surface is out of the question. All that scientists will get is the total spectrum of reflected light. Based on his calculations, Tinetti argues that in order to identify plants on this spectrum, at least 20% of the planet's surface must be land covered with plants and not covered by clouds. On the other hand, the larger the sea area, the more oxygen the marine photosynthesizers release into the atmosphere. Therefore, the more pronounced the pigment bioindicators, the more difficult it is to notice oxygen bioindicators, and vice versa. Astronomers will be able to detect either one or the other, but not both.

Planet seekers

The European Space Agency (ESA) plans to launch the Darwin spacecraft in the next 10 years to study the spectra of terrestrial exoplanets. NASA's Earth-Like Planet Seeker will do the same if the agency gets funding. The COROT spacecraft, launched by ESA in December 2006, and the Kepler spacecraft, scheduled by NASA for launch in 2009, are designed to search for faint decreases in the brightness of stars as Earth-like planets pass in front of them. NASA's SIM spacecraft will look for faint vibrations of stars under the influence of planets.

The presence of life on other planets - real life, not just fossils or microbes that barely survive in extreme conditions - may be discovered in the very near future. But which stars should we study first? Will we be able to register the spectra of planets located close to stars, which is especially important in the case of M stars? In what ranges and with what resolution should our telescopes observe? Understanding the basics of photosynthesis will help us create new instruments and interpret the data we receive. Problems of such complexity can be solved only at the intersection of various sciences. So far we are only at the beginning of the path. The very possibility of searching for extraterrestrial life depends on how deeply we understand the basics of life here on Earth.