What color is the living planet?

  In 1968, when the Apollo 8 spacecraft orbited the moon, the astronauts photographed the first color photograph of the Earth rising from the moon’s horizon. On the gray lunar surface, a blue-and-white hemisphere emerges in the night sky, which is the sight of the earth seen from the moon. Four years later, the Apollo 17 astronauts first captured a panoramic picture of the Earth. In space, the earth resembles a “blue marble”. Since then, people have used “blue marble” to refer to high-resolution Earth photographs taken by satellites.
  These pictures of the Earth taken from space let us know that the earth is a blue planet. According to the survey, people have a more rational love for blue. For example, a survey of British people shows that blue is the first place with a 33% vote, which is the favorite color, and the second one is red. Only 15% of the votes, is this the effect of the Earth as a blue planet?
  In major space exploration organizations, such as NASA, the European Space Agency, and the search for extraterrestrial civilization programs, etc., they also use blue as an important color when designing their own flag.
  In contrast to the yellow Venus, the red Mars, the grayish Jupiter and Saturn, in many people’s minds, the blue has become a symbol of a living planet, the blue planet is the cradle of life and the oasis of the universe. So, will the planet of life be blue?
  Does the blue planet herald life?
  Many people know that the earth is blue because 71% of the earth’s surface is covered with water, which scatters and reflects blue light. The presence of liquid water seems to imply that there will be life. But this conclusion is not entirely correct. On some other planets, blue is the forbidden zone of life.
  About 63 light years from Earth, there is an exoplanet called HD 189733b, which is a huge blue gaseous planet, 13% larger than Jupiter. Its atmosphere also contains a lot of water vapor, similar to the sky in the summer. Dark blue, it looks very quiet, but if you regard this planet as a friendly livable planet, it is a big mistake.
  Because HD 189733b is very close to the star, the orbital period only needs 2.2 Earth days, resulting in its atmospheric temperature exceeding 700 °C. It is blowing a strong hurricane every day. The wind speed is as high as 2 kilometers per second, which is about 7 times of the speed of sound. Space travellers are instantly caught in the vortex around the planet. Raining on this planet is also fatal, and it will make your body riddled with holes. Because the planet is under the “glass rain”, the glassy silicate particles are at a speed of 11,000 kilometers per hour with the hurricane. The planet travels through the atmosphere. In short, this is a really crazy planet.
  So why is it a blue planet? The current guess is that HD 189733b has a cloud of hot minerals with a very pure atmosphere on the clouds. The clouds reflect most of the light, but the molecular hydrogen in the upper atmosphere preferentially reflects blue light, which ultimately leads to a very beautiful blue appearance on the planet. Of course, the blue of HD 189733b is very different from the blue of the earth. The blue of the earth is light blue, while the HD 189733b is dark blue.
  We don’t even need to go outside the solar system to find blue planets. In fact, the Earth is not the only blue planet in the solar system. Uranus and Neptune are also blue planets, but their blue color is neither derived from the reflection of blue light by the ocean nor from the reflection of blue light from the atmosphere. The blue color of these ice giants is mainly due to the small amount of methane in the upper atmosphere. Methane molecules absorb red and infrared spectra of different wavelength bands and reflect blue light.
  Therefore, if a planet is blue, it does not necessarily mean that it is suitable for living.
  What color is extraterrestrial life?
  If you don’t start with the color of the planet, but explore the color of life itself, what color will the life of the alien planet be?
  On Earth, the main cause of photosynthesis in plants is chlorophyll. The light absorbed by chlorophyll is mainly blue-violet and red light, and reflects green light. Therefore, on the surface of the Earth’s ocean, a large number of micro-tourism creatures will produce a huge green light. On land, plants can also produce a lot of green.
  If we can find green imprints in the Earth’s reflections and then look for the same signs from the distant planetary spectrum, we can consider whether these planets may exist.
  Some scientists have tried to capture the Earth’s reflections reflected back from the moon to see how much of these green tones are in the spectrum. This is a very complicated study. They must consider which part of the earth’s light is shining on the moon, and also consider how much earth light the moon will reflect back. When these Earth’s reflections return to Earth again, the photons will be absorbed and reflected again by the Earth’s atmosphere, which will also affect the experimental data of the scientists.
  Even with the difficulties, some scientists have successfully captured the Earth’s reflections. But to their disappointment, the possibility of finding green light reflected by photosynthetic organisms from the Earth’s reflections is very small, because this green light seems to account for only a small fraction of the total spectrum.
  In addition, another fatal problem is that the color of alien life may not be green. Now even on Earth, there are a variety of pigments involved in photosynthesis that absorb light from different spectra. Carotenoids give plants a red, orange or yellow glow. Some microorganisms that use carotenoids and chlorophyll at the same time can even adjust the proportion of the two according to the environment, and then adjust their own color. For example, in cold environments, they turn brown and green, and when the weather is hot, they turn yellow and orange. Another class, called phycobilin, also allows plants to appear blue or red.
  In different geological periods, life on Earth will also have different colors. Some scientists believe that in prehistoric times, the earth may be purple. Because the microbes that ruled the earth at the time used a photoreceptor called retinal. In terms of molecular structure, retinal is simpler than chlorophyll and absorbs green light. This visible light contains most of the energy in sunlight, making it more adaptable to the Earth’s early high-radiation environment. Today, some halophilic archaea are still using photosynthesis pigments for photosynthesis, and these species are also resistant to intense ultraviolet radiation. Since retinal absorbs green light, it reflects red light and blue light, and the two mix and produce purple. Therefore, 3 billion years ago, the earliest biological hotspots on the earth were dyed with a unique purple color. Thus, if the outer planet is in the early stages of evolution, we might have to use purple as the basic color of life, looking for the purple wavelength.
  Or the color of alien life should be black. Today, in the Milky Way, most stars have less mass, temperature, and brightness than the Sun. In these star systems, if there are plants, they get very little light, and they tend to absorb all the light in the spectrum for photosynthesis. So these plants are likely to be black.
  The truth is that trying to judge whether a planet has life through color is a very narrow idea. Moreover, scientists now have no sensitive instruments that can detect the color of reflections on exoplanets.
  Looking for color, not entirely without effect
  , of course, this does not mean in terms of the search for extraterrestrial life, color is useless. Because the biosphere changes the color structure of the planet and changes the spectrum of the planet in various ways.
  A very famous example is the “red vegetation” effect, which means that in a certain band of the spectrum, the reflectivity of the earth’s vegetation rises sharply. For example, for infrared light with a wavelength greater than 0.7 micron, the reflectivity of the plant can reach about 50%, and the intensity is more than 10 times that of red visible light.
  Although it is useful for humans to study the red edge effect of plants, remote sensing satellites can distinguish the earth’s jungles, grasslands and crops through the red edge effect. However, 70% of the Earth’s surface is covered by the ocean, and even on land, about 70% of it is covered by clouds. Therefore, if you look at the Earth from an alien planet, you may only see less than 10% of the plant’s red edge effect. However, if the alien planet is not a landform like the Earth, such as a larger land area, the red edge effect of the plant may be more pronounced.
  The biosphere also causes seasonal changes in the color of the planet’s surface. As the seasons change, the creature grows or dies, causing a change in the color of the creature. For example, in 2008, the Casatoki volcanic eruption in the Northeast Pacific Ocean, the volcanic ash rich in iron, caused the explosive growth of plankton in just a few days, resulting in a large area of ​​the earth covered by green, from space. It can also see more than 2 million square kilometers of Pacific Green waters. Therefore, we can also find the signal from life from the sudden changes in the color of the exoplanet.
  In short, exploring the color of the outer planet and finding the alien life will still be one of the means used by astronomers, but it will not be as simple as we think, so just look for a blue planet. Or just finding a green spectrum from the reflected light of the planet to find extraterrestrial life is not enough. The color of a living world is far more complicated than we think.