The possibility of life. The only planet in the Solar System where life is known is Earth. There is a large number of factors combined, which make life possible in our system and on our planet. First, the Sun meets all the requirements for a star system suitable for life. The spectral class of the star should be in a range from mid-K to late F in Harvard classification, which corresponds to temperatures from 4,000 K to 7,000 K. Temperature of the Sun is about 5,777 K, thus it is a G2 star (Phillips, 1995). The Sun has a stable habitable zone where planets can maintain liquid water on their surface. Venus is located on the inner edge of the habitable zone, and Jupiter is located on the outer edge. Thereby, Earth has the most suitable location to retain liquid water. The Sun also has a low stellar variation, which is important for stable luminosity and it has a high metallicity. The amount of metals in a star correlates with the number of metals in a protoplanetary disk. The higher concentration of metals, the higher possibility of planet formation. Stars with such conditions could maintain planets, which also have some specific requirements to be suitable for life. Suitable for life planets should have middle or high mass. Higher mass helps to hold the atmosphere and they are likely to have an iron core, which is important for the magnetic field formation. The planet has to rotate quickly enough to have a stable day-night cycle and to produce a magnetic field. The orbit should be spherical. Living organisms require some environmental factors, such as liquid water, nutrients, energy for metabolism, and conducive physical conditions. Today there is no known planet except the Earth that meets all those requirements. Nonetheless, to this day there is a great number of Earth-like exoplanets found in habitable zones of various stars. According to a report based on Kepler space mission data, there are about 40 billion Earth-like planets in the Milky Way (Petigura, 2013). Kepler is one of the most famous and active working space telescopes in low orbit, which is commonly used for search for extrasolar planets.
What Can Be Found in Space?
Various organic substances were found in outer space (sugars, alcohol, amino acid glycine, formaldehyde, isopropyl cyanide, etc). At the beginning of this century, there have been over 100 organic molecules identified in space. In 1970 amino acid glycine was found in outer space. Glycine is one of the most important molecules for living organisms, a component of most proteins. In 2002 the presence of glycine in space was confirmed. All this leads to the idea that life can exist not only on Earth.
The unique Earth. All confrontations on the issue of the possibility of extraterrestrial life existence are based on the question Does Earth occupy a unique position in the Universe? On the one hand, some famous scientists such as Carl Sagan and Steven Hawking believe that Earth is not unique and life is widely spread in the Universe. Hawking in his lecture (1996) said that the fact that life happened to occur on Earth, is not however surprising or unlikely, it is just an application of the Weak Anthropic Principle: if life had appeared instead on another planet, we would be asking why it had occurred there.
On the other hand, there is a large number of scientists who believe that process of life formation is too complicated to occur simultaneously in different parts of the universe. Astronomer Donald Brownlee and paleontologist Peter Ward (2000) suggested a theory of the Rare Earth. It is also often called the Unique Earth. They used the Drake equation (more details will be discussed below) to prove that the existence of an Earth-like planet in the Universe would be an extremely rare phenomenon. All the factors, which make life possible in our system and on our planet, were analyzed. They concluded that stars like our Sun (spectral class G) are rare in the Milky Way; they make up only 5% of all-stars. These stars should locate both far from the middle of the galaxy (stars with too high metallicity) and far from the outer edge (stars with low metallicity). The most important part of the Unique Earth hypothesis is interaction with other celestial bodies. Collision with the body the size of Mars in the early stages of planets formation is important for the formation of tectonic plates. The availability of satellites with large size increases the chances of highly organized organisms to survive. Satellite plays a function of an asteroid shield.
Thus, it seems clear that correct values of hundreds of planets and star systems parameters are needed to make the existence of highly organized life possible. It remains a chance that somewhere in the universe there is an Earth-like planet with a highly organized life. Nevertheless, the possibility that such a planet exists close enough to the sun and that we can ever reach it or contact inhabitants is practically zero.
Universe Development Through Years
Astronomer Erik Zackrisson with colleagues created a complicated and powerful computer model, which shows the process of Universe development through 13.8 billion years. They concluded that Earth might be the only planet in the universe suitable for life (Hall, 2016).
Besides the direct confrontation over the Earth's uniqueness, some scientists criticize the assumption that life can exist only on Earth-like planets. The phenomenon called Carbon chauvinism will be discussed below.
Methods for extraterrestrial life search. The scientific search for extraterrestrial life is being carried both in the Solar system and in Extrasolar systems. Extrasolar search is based on the search of exoplanets within the habitable zone of their star, which may be suitable for life's existence. According to "Interactive Extra-solar Planets Catalog (Schneider, 2011) there are 3487 planets in 2611 planetary systems, including 590 multiple-planet systems as of 3 August 2016. Most planets have been discovered by the Kepler space telescope using the transit method. Besides the transit method, there are some other methods widely used by scientists: direct imaging, Doppler method, gravitational microlensing, pulsar timing, and some others. Gravitational microlensing is considered a perspective method today. The effect of microlensing occurs when the gravitational field of a star acts like a lens and magnifies the light of another star, the distant one.
Search in the Solar system carries out both directly and indirectly. Direct search is a search for any biosignatures by studying surfaces of planets, natural satellites, and meteorites. There were several controversial reports about finding microbial life on Mars (Crenson, 2006; Webster, 2014), but none was confirmed. One of the tasks of the Curiosity rover filled with a variety of scientific instruments was to find any biosignatures on Mars. The rover landed on Mars in August 2012 and since then, he transferred a large amount of scientific data, but no evidence of the existence of life on Mars.
The chemical composition of the planet's atmosphere and even dust nebulae in outer space is easy to determine by spectroscopy. Cosmic dust contains a high amount of complex organic matter: sugars, alcohol, and even amino acids (Kwok, 2011). For example, in 2012 astronomers detected sugar molecule glycolaldehyde in a distant star system about 400 light-years from Earth (Than, 2012).
However, not always scientists directly search for biosignatures, sometimes they send into outer space some tips on how to find Earth. This is the indirect search. SETI (search for extraterrestrial intelligence) is the most famous project for search for intelligent extraterrestrial life.
Another interesting indirect method of search for extraterrestrial life is laboratory simulation. Learning how terrestrial extremophiles behave in conditions close to the conditions on the surface of other planets, can help to manage where and how to search them in space.
What we are looking for. Most living organisms on Earth require soft climate and temperature conditions, liquid water, and oxygen. Earth is the only planet in the Solar system, which meets the requirements above. Despite the fact that Mars and Venus are in the habitable zone of the Sun, Mars is too cold and Venus is too hot for Earth life forms. However, there are some organisms called extremophiles, which can live in much more extreme conditions. Many microorganisms can live in an environment with very high or low temperatures (extremophiles), without light, oxygen, and water (Pikuta, 2007). It explains why microorganisms may be resident in rock samples, such as meteorites. Today one of the areas of the search for extraterrestrial life is connected with the study of meteorites that fall to Earth, as well as the study of soil samples on other planets and moons.
On the one hand, there are many arguments that the last common ancestor to life on Earth was thermophilic (Stetter, 1996). On the other hand, it is also argued that life was originated in cold environments (Levy et al., 2000). Nevertheless, it is clear that the first living organisms were extremophiles and our search in outer space should be aimed at finding organisms with similar characteristics.
Mars always was considered as the most probable place in the Solar system to find extraterrestrial life because of the presence of water (ice caps at the poles). The planet is cold, and life forms living in or near the surface would have to be cold-adapted. Today the most perspective is the search of extraterrestrial life forms under the ice core of the Europa, a moon of Jupiter. Despite the fact, there is no light, the subsurface ocean is able to support lithoautotrophic processes in living organisms. Unique ecosystems with numerous lakes with a wide range of temperature, salinity, and oxygen level exist on Earth in Antarctica. Moreover, a variety of microorganisms including Archaea live in such conditions (Cavicchioli, 2002). The Antarctic subsurface lake system is highly similar to those on Europa and they can be used as model environments.
While it is clear that most extremophiles are microorganisms, there are some multicellular organisms that can live in such extreme conditions. Tardigrades, water micro-animals, can survive in such extreme conditions which would be fatal for almost all known living organisms. They are tolerant to both extremely high and low temperature, pressure, and ionizing radiations at doses hundreds of times higher than lethal doses for humans. They can exist without food and water for about 30 years in a state of suspended animation and then they come back to life. They are even able to survive in a vacuum in outer space (Jonsson et al, 2008). It seems that some important things about extraterrestrial life we can find studying extent life on Earth.
The differences between life and intelligent life. Talking about extraterrestrial life, we should understand that extraterrestrial life does not mean extraterrestrial intelligence. While life can exist in various forms from the smallest microorganisms to intelligent hominids, extraterrestrial intelligence is a narrower concept. It is unknown whether the appearance of intelligent species is mandatory in the course of the evolution of life.
Astronomer Frank Drake created an equation to estimate the number of advanced and intelligent civilizations in the Milky Way.
R average rate of star formation
fs fraction of good stars that have planetary systems
ne number of planets around these stars with the habitable zone
fl fraction of those planets where life develops
fi fraction of living species that develop intelligence
fc fraction of intelligent species with communications technology
L lifetime of the communicative phase
Drake estimated that a number of intelligent technological civilizations in our galaxy is between 2 and 280 million (Burchell, 2006). In accorda...
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