Life in the universe is a natural phenomenon. It may occur anywhere, where there are favorable conditions for its genesis and development. Although there is no proof of the existence of life on other planets so far, scientists constantly try to define a possible number of the manned worlds in the galaxy as well as in the observed universe by using various scholarly methods. These days, modern science throws all its effort into the searches for extraterrestrial civilizations, which involve scientists of different fields such as astrophysics, radiophysics, biology, cybernetics, linguistics, archeology etc. Some scholars, however, assert that the existence of life is impossible in our galaxy, not to speak of the existence of reasonable life. The aim of this paper is to study different views regarding the existence of extraterrestrial life.
The number of the manned worlds depends on a combination of astronomical, biological, climatic and other factors that are necessary for life existence. Only with a favorable combination of these factors, there is an opportunity for an advanced civilization to develop on a planet. First of all, for the emergence and further activity of proteinaceous organisms on the planet, suitable temperature conditions are required. For this purpose, the planet has to approximate a dwelling zone. The Earth is a bright example of a successful location of an orbit in relation to the central star. Its neighbors in space, Venus and Mars, are already out of this zone, with Venus being too close to the Sun and Mars being too far from the admissible limit. As Halley states in his book How Likely is Extraterrestrial Life?, “Venus has a crushing pressure, a very high temperature, and a sulfuric acid atmosphere” . Besides, a planet suitable for life needs to have an orbit that would be close to circular. In this case, there are no huge temperature swings on its surface. Star radiation has to remain sufficiently constant throughout hundreds of millions or even billions of years. Therefore, advanced living beings can be met only on the planets that are moving round old stars similar to the Sun of the late spectral classes – F, G, and K.
Another important condition for maintaining a highly developed life on the planet is the celestial body mass. If the mass of the planet is small, it is not able to hold an atmosphere. For example, Mercury, the mass of which is 18 times smaller than that of the Earth, is almost devoid of the gas shell. On the other hand, a massive planet like Jupiter, the mass of which is almost 318 times greater than that the mass of the Earth, will hold an extended and dense atmosphere that is extremely rich in hydrogen. Such an atmosphere is unsuitable for protein life. Therefore, for life on a planet to exist, its mass may be a few times less than the mass of the Earth, and it should not exceed the mass of the Earth by more than 10 times.
However, favorable astronomical conditions are not the only prerequisite for successful maintenance of life on a planet. A number of other factors are necessary as well. First of all, every planet suitable for life should have a powerful liquid cover – the hydrosphere. On the Earth, this is the ocean. The existence of a habitable zone depends on the premise that the planet should not be too close to its sun, but not too far, so that the liquid remains liquid, and not boiled or frozen.
In the galaxy, there are about 200 billion stars, including about 2 billion similar to our sun. Of these, at least every tenth star transforms into a planet. The total number of planets located in the habitable zone is about 200 million. The number of planets with the conditions favorable for life can be estimated at about 2 million. However, on most of the planets, life came to a standstill for some reasons, and only on 20 thousand planets, it received further development. Nevertheless, it is not necessary that life has become reasonable on a planet once evolved. The probability of such an outcome is less than one-thousandth. Under this condition, there can be only 20 planets with intelligent inhabitants in the galaxy.
Thus, on about 20 planets in the galaxy, intelligent beings might appear one day. Nevertheless, only on 10 of them, civilization could survive. For modern astronomical instruments, a part of the universe with a radius of about 15 billion light-years is available. In this part, there are about 100 billion star “islands” – galaxies. It is not necessary that each of them has alien civilizations. Even if out of each ten galaxies, only one has a habitable planet with intelligent beings, the total number of civilizations in the observable universe will be 10 billion.
It is extremely difficult to estimate the number of habitable planets in the galaxy. The discovery of giant clouds in the galaxy with organic molecules indicates that in the cosmos, there are natural reservoirs for the construction of life substance – nucleic and amino acids – in case suitable conditions appear on certain planets. Recent studies of meteorites from Mars allow drawing a conclusion about the existence of some form of life on Mars. This conclusion is based on the known fact of the existence of a dry river bed on Mars. In addition, recent studies revealed frozen water beneath the surface of Mars. Organic molecules were found in the substance of comets, which can also be regarded as primary concentrators of organic and synthesis reactions of organic molecules, particularly when approaching the sun and other similar stars. Researches over the past 20 years have shown that certain living microorganisms are extremely resistant to adverse conditions and are able to survive in the vacuum of space. The combination of all these facts leads to the conclusion that life was not necessarily born on the Earth or even on Mars, but rather populated the planet as soon as suitable conditions occurred after the Earth cooled down. Thus, the terrestrial biosphere is not unique and the only one in the galaxy or universe.
According to Stuart Kallen, for the creation of intelligent life, several factors are necessary. These factors include single-celled bacteria, cells, special cells that allow life to form, and intelligent life with a special language. Thus, the existence of the intelligent life on planets other than Earth is unlikely, because a combination of several factors is needed for its development. Scientists believe that a key factor for the emergence of life in the universe is liquid water. Astronomers assert that it is necessary to look for water to find evidence of extraterrestrial life. Liquid water, unlike other liquids, is a universal solvent. It is capable of dissolving a striking number of various chemicals. It is also a perfect environment for the evolvement of more complex molecules. Besides, the water molecule is extremely simple. It can be found everywhere in the universe, while others solvents are rare.
Carbon is the second likely component of intelligent life. The carbon atom is tetravalent. This means that it can bind with four other atoms, creating thereby the molecules of incredible complexity. In particular, it easily forms a long carbon chain – the main element of hydrocarbon compounds and all organic chemistry. The third pillar of intelligent life is the DNA molecule that is able to replicate. In chemistry, self-copying molecules are extremely rare. It took hundreds of millions of years for the first DNA molecules to evolve (most likely in the depths of the ocean). The most likely areas where the first DNA molecules on the planet could evolve accidentally at the beginning of the Earth’s history are the volcanic outlets at the bottom of the ocean. The activity of these hot springs could serve as a source of energy for the first molecules of DNA and the first cells long before photosynthesis and first plants appeared. There are no other known carbon-containing molecules except DNA that are capable of self-replication. However, most likely, the other self-copying molecules in the universe will look like DNA molecules. In this case, for the emergence of intelligent life, three most important components are required - liquid water, hydrocarbon compounds, and self-replicating molecules like DNA.
Detecting distant civilizations and, furthermore, establishing contact with them is practically impossible. The repetition of a long chain of random events and circumstances that led to the appearance of a man on the Earth is unlikely. Thus, life on other planets is unlikely to develop in a way similar to the Earth scenario. Therefore, the results of scientific researches lead scientists to an extremely important idea - the planet Earth is practically a unique carrier of reason in the galaxy.
Scientists believe that there are probably very few extraterrestrial civilizations in the galaxy. If they are really only ten, it can be assumed that three civilizations are ahead of our development, three civilizations reached the Earth’s technical level, and three civilizations lag behind in the development. Those civilizations that have risen to the highest level of scientific and technological progress should represent the special interest. Scientists believe that the most appropriate way of establishing the contact between civilizations separated by an unimaginable space abyss is the reception and transmission of radio-frequency signals. This communication has a great advantage. The signal propagates with the maximum possible speed equal to the speed of light.
According to the theory of the Great Filter, alien life really exists. However, intelligent life is unable to move forward technologically far enough to invent a long-range cosmic connection or space travel. Though modern space ships, satellites, and radios state that people are close to the point of no return, a barrier might occur in the form of a disaster that either destroys the Earth or leads to the degradation of technology. Catastrophic natural disasters periodically occur on the Earth. Therefore, it is possible that similar disasters are destroying worlds everywhere, directing intelligent life back into the Stone Age before the technology is able to adequately develop. There is also a bright side to this theory. Some scientists believe that the Earth will be the first to pass through the filter. Thus, it is likely that people will become a dominant race in outer space.
Some scientists believe that the existence of extraterrestrial life is impossible. The search for traces of highly developed civilizations with the help of the infrared space telescope WISE has not led to success. After having studied nearly one hundred thousand galaxies with the telescope, researchers from the University of Pennsylvania concluded that none of them shows an abnormally high intensity of radiation in the middle infrared range, which should accompany the activities of civilizations of type III according to Kardashev scale. Out of 100 million objects in the WISE database, investigators chose about 100 thousand most promising galaxies. Although about 50 galaxies have uncharacteristically high levels of radiation, the authors acknowledged that they could not find the type III civilizations.
High technical development of civilizations is associated with the release of a large amount of energy into the cosmic space. For more than 80 years, powerful radio stations, and later television stations have continuously sent their signals in the air. Thus, the general level of radio emission from Earth has increased significantly. In terms of power and the level of radio emission, the Earth became strikingly different from other planets in the solar system. Similarly, people might find a technically advanced civilization based on their power activity and emission level. However, scientists state that the absence of radio emission is the evidence of the fact that extraterrestrial life does not exist.
Nevertheless, the opposite might have been the case. Advanced civilizations might exist, and they might send some special signals to the vastness of the Universe. However, scientists still do not have the equipment to receive these signals. People need to develop systematic sending of signals to the stars of the galaxy. Today, scientists have created powerful generators of light beams – lasers. Soon, laser technologies will surpass the radio as a way of transmitting information over long distances. Thus, mankind will use lasers for interstellar communication. However, according to Stuart Kallen, if to stick to the hypothesis of the existence of Earth-like civilizations that can send reasonable signals, the signals from civilizations that are within the galaxy would have been detected. As they are not observed, it must be acknowledged that technologically advanced civilizations do not exist. Thus, the Earth can be stated as the only civilization in the galaxy.
In recent years, a clear pattern can be traced – the farther humanity penetrates into space, the more people change their minds about the next existence of extraterrestrial civilizations. It is most likely that the next planetary systems, which are home for thinking beings, are located at the distance of many thousands of light years. To date, the determination of the number of habitable planets is a difficult task. Some scientists state that according to the calculations, there might be about ten planets in the galaxy with the existence of extraterrestrial life. However, a civilization that would be technologically similar to the Earth is unlikely. It might be located at a distance of several thousand light years from the Earth. There is no civilization using radio communication in our galaxy. Many scientists assert that if people cannot trace radiation in space, the alien life does not exist. This is confirmed by many years of unsuccessful attempts to produce radio signals of an artificial origin. Considering that humanity owns radio for about 100 years, it can be argued that in a radius of 100-150 light years, there are no similar civilizations. In such a way, there are still no signs of extraterrestrial life in the galaxy except for the Earth. Nevertheless, most of the scientists do not lose hope to discover alien life.