We can observe the existence of red edges in the spectrum of the earth’s celestial bodies. Since plants absorb a lot of red light, but rarely absorb infrared light, the spectral curve of this band will show a “steep slope.” Satellites above the earth will use this feature to track the growth of vegetation, and astrobiologists may also be able to look for this feature on other planets as one of the signs of life.
In a new study recently published in “Frontiers in Astronomy and Space Science”, scientists used a variety of chemical and physical models of photosynthesis to find the best wavelengths that are most suitable for plants to absorb around different stars.
Life on earth interacts with sunlight through a chemical substance called “chlorophyll a”. The substance can capture light and be used for photosynthesis. The light they absorb can be used by plants as energy for biological processes. Biologists believe that the reason why plants rely on chlorophyll a is because it maximizes the energy absorbed from sunlight and minimizes the energy required for photosynthesis, so that the plant’s energy output rate reaches the highest level.
But what if the color of the light from the sun is different? Is chlorophyll a still the best chemical for this task? Probably not, because plants that rely on other stars for growth also need to be adjusted according to the corresponding light to maximize energy efficiency. This means that if we are to look for the red edge effect on planets around other stars, we will probably get nothing, because these planets are not necessarily “red” edges, they may be blue edges, or red in another hue. The edge may not even be in the visible light range.
In the figure, we can see which kind of light is absorbed the most by plants around different stars. Among them, F-type stars are the brightest and M-type stars are the dimmer.
The researchers for this new study are made up of scientists from NASA Ames Research Center, NASA Goddard Space Flight Center, and the University of Washington. They considered a variety of factors, such as the amount of light at each wavelength in the light of a star, the impact of an atmosphere similar to the Earth, and the energy consumption of cells for photosynthesis. Their goal is to find out whether future telescopes should look for the “red” edge as a sign of life outside the system and search for it.
Using a series of chemical and physical equations, they built multiple models to determine the best wavelength for photosynthesis of plants around different types of stars. Then the results of these models were compared with the vegetation of the earth, and the absorption spectra of plants such as spinach were reconstructed. By applying these models to well-known plants such as spinach, they can check whether their calculations are correct. It was found that around stars that are brighter and hotter than the sun (such as F-type stars whose temperature is half higher than that of the sun, plants tend to absorb light with higher energy, resulting in a “blue edge”; Around stars (such as K-type and M-type stars), plants mainly absorb light with lower energy, and the edges produced are redder, even close to infrared light.
Interestingly, with the exception of the lowest temperature class of stars (the temperature is only half of the sun, or even lower), the edges generated by these models are all in the visible light range. Although the spectral range is large, the light that is most suitable for plants to produce energy is still concentrated in the small visible light band. The researchers also found in the model that no matter what kind of star is around, the growth of plants will not be limited by the amount of energy. On the contrary, the influence caused by factors such as land and nutrients is greater.
These models have been improved on the basis of previous research. Scientists previously believed that using the detailed spectra of different types of stars, the light emitted by stars can be built as a simple curve model. In addition, they have been making inferences based on the Earth’s atmosphere before, but the atmospheric composition of exoplanets may be completely different from that of the Earth. Since the atmosphere absorbs part of the light emitted by the stars, the atmosphere also affects the light absorbed by the plants on the planet’s surface.
Although there are more and more complex factors that can be added to these models, such as different atmospheric composition, different leaf shapes, etc., this research has laid a good foundation for searching for alien plants. In the next few decades, using next-generation space telescopes such as HabEx and LUVOIR, this information may really help scientists find alien vegetation. These two space telescopes should be able to provide us with information on the atmospheric spectra of terrestrial planets, and may even find red edge (or blue edge) effects on exoplanets. (leaf)