The concept of a plurality of inhabited worlds originated in antiquity and revived in modern times on a natural science basis. By the end of the 19th century, few educated people doubted that neighboring planets were inhabited by other forms of life. And the science of aliens arose by itself - xenology.
MARSIAN OPTIONS
The "father" of xenology, probably, should be recognized by the English science fiction writer Herbert Wells. Of course, before him, various scientists, writers and even clergymen tried to imagine what the hypothetical inhabitants of the Moon, Venus and Mars look like, but their considerations were based only on imagination, often constrained by religious dogmas. Wells proceeded from the information that science gave him. The result was the article "Creatures that Live on Mars" (1907). Scientist knew that the force of gravity on Mars is less than on Earth, the air there is more rarefied, and the climate is colder and drier. In such conditions, the Martians must be tall, huge chests and large heads; their bodies are covered with feathers or fur. They probably have hands, but this is not necessary: tentacles or a trunk can serve as a grasping organ.
Wells' reconstruction made an impression: even the leader of the world proletariat, Vladimir Lenin, once spoke about it, who, however, believed that aliens must be humanoids.
Later, scientists found that conditions on Mars are much more severe, which does not contribute to the development of highly organized life. But maybe there is vegetation there? One of those who believed in the Martian flora was the Soviet astronomer Gavriil Tikhov, who founded astrobotany. He studied terrestrial plants, common in the Far North, and took the spectra of the light they reflected. Comparing then the result obtained with the spectra of the neighboring planet, he "proved" that analogs of our alpine conifers and junipers prevail there.
MULTIPLE WORLDS
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Unfortunately, the xenological reconstructions of Wells and Tikhov remained in history as scientific curiosities, since the interplanetary vehicles that studied the planets of the solar system have established that there is no alien life in the immediate space environment. There remains a timid hope to find someday microbes in the underground rivers of Mars or in the subglacial ocean of Europa, the moon of Jupiter, but such a discovery is unlikely to satisfy those who dream of contact with "brothers in mind."
As for exoplanets (planets near other stars), their existence was considered unproven, therefore, there was not enough material to build xenological hypotheses. The first exoplanet was discovered in 1995, and it was "hot Jupiter" near the star 51st Pegasus, recently officially named Dimidium. To date, 3635 planets have been discovered in 2726 systems.
Of course, scientists pay the most attention to the planets located in the "habitable zone", that is, at such a distance from the star, at which the heat received is sufficient for the existence of water in a liquid state. Why is it important? Because we know only one form of life - earthly, and it could not have arisen without water, which serves as a universal solvent. Accordingly, scientists believe that the likelihood of the appearance of a biosphere on a planet with bodies of water is much higher than anywhere else. Today, astronomers know 44 terrestrial exoplanets and 1514 gas giants that are in the "habitable zone" of their stars.
THE SCIENCE OF ALIEN
In May 2005, National Geographic and Channel 4 International TV channels released the popular science film Alien Worlds. It features two large-scale xenological reconstructions prepared by a team of scientists, including such celebrities as evolutionist Simon Morris, planetary scientist Christopher McKay, astronomer Seth Shostak. In October of the same year, the materials used to create the reconstructions were exhibited at the Science of Aliens exhibition in London.
Scientists have chosen two models as a basis for theoretical research.
The first model is the exoplanet Aurelia, whose characteristics are comparable to those on Earth, but which revolves around a red "dwarf". This type of star is very common in the Galaxy; they are colder than the sun and burn more slowly (it is believed that the lifespan of some of them may reach 10 trillion years!). It is clear that the "habitable zone" of red "dwarfs" is narrow and is closer to the star than in our solar system. However, such proximity leads to the fact that, due to the tidal influence, the rotation of the planet around the axis will be synchronized with its rotation around the star - that is, the planet will always be turned to its star on one side, like the Moon to the Earth. As a result, the illuminated hemisphere will always be hot, water will boil out there, and the shaded one will always be cold, there will be ice and temperatures close to absolute zero. Because of this, the strongest winds will always blow between the hemispheres. Previously, it was believed that in such difficult conditions the emergence of life is fundamentally impossible, but on the model of Aurelia, scientists were able to prove that this is not so.
The second model is the exoplanet Blue Moon, orbiting a gas giant the size of Jupiter. Scientists have suggested that such a world could be almost completely covered with water and have an atmosphere whose surface pressure is three times higher than that of the earth. Climatic fluctuations there are minimal, but for local life forms there is an opportunity to actively master the air: for example, “celestial whales” can live on the Blue Moon.
NEAREST NEIGHBORS
In 2013, astronomer Mikko Tuomi noticed a recurring anomaly in long-term observations of the nearest star, Proxima Centauri, and suggested that this indicates the presence of an exoplanet. For verification, the specialists of the European Southern Observatory, located in Chile, launched the Red Dot project in January 2016, and on August 24, the discovery of the world, which has so far been codenamed Proxima b, was officially announced. The exoplanet turned out to be relatively small: its mass is estimated at 1.27 Earth masses. It rotates so close to its star that the year on it is 11 Earth days, but due to the low luminosity of Proxima, the conditions there correspond to the model of Aurelia.
Immediately there were many publications dedicated to the possible options for life on Proxima b. The main problem is the radiation of Proxima, because an exoplanet, even in a "quiet" time, receives from it 30 times more ultraviolet radiation than the Earth from the Sun, and X-rays - 250 times more. Nevertheless, scientists believe that the biosphere can adapt to such harsh conditions: from the deadly rays, local creatures can hide in caves or under water. In addition, there are life forms on Earth (for example, coral polyps) that have learned to re-emit the energy of the Sun through biofluorescence. If the inhabitants of the exoplanet have also mastered this technique, their presence can be detected by radiation at certain wavelengths, which scientists are going to do in the near future.
Although it seems that xenology cannot be classified as a real science, because it operates only with imaginary models, its purpose is to formulate criteria by which it will be possible to distinguish between inhabited and dead worlds. And then the question of the prevalence of alien life will be resolved by itself.
Anton Pervushin