Extraterrestrial life is life that does not originate from the planet Earth. It is also called alien life. These still hypothetical forms of life can range from simple organisms like bacteria to beings much more complex than humans. It was also proposed the possibility that viruses may exist in extraterrestrial environments
The development and research of hypotheses about extraterrestrial life is known as "exobiology" or "astrobiology", although astrobiology also considers life based on Earth in its astronomical context. Many scientists consider that extraterrestrial life is plausible, but there is as yet no direct evidence of its existence.
Since the mid-twentieth century, there has been a continuous search for signs of extraterrestrial life, from radio telescopes used to detect possible signs of extraterrestrial civilizations, to telescopes used to search for potentially habitable extra-solar planets. The theme also played an important role in science fiction.
The hypothesis of alien life forms, such as bacteria, was raised to exist in the Solar System and throughout the universe. This hypothesis is based on the vast dimension and consistent physical laws of the observable universe. According to this argument, made by scientists like Carl Sagan and Stephen Hawking, it would be unlikely that life did not exist somewhere off planet Earth.
This argument is embodied in the principle of Copernicus, which states that the earth does not occupy a unique position in the universe, and in the principle of mediocrity, which suggests that there is nothing special about life on earth. Life may have arisen independently in many places throughout the Universe. Alternatively life may also develop less frequently, but spread among habitable planets through panspermia or exogenesis.
In any case, the complex organic molecules necessary for the formation of life may have formed in the protoplanetary disk of dust grains around the Sun before the formation of the Earth based on studies of computational models. According to these studies, this same process may also occur around other stars that maintain a planetary system. Among the suggested locations in which life may have developed in the past are the planets Venus and Mars in Europe, one of the moons of Jupiter, and in Titan and Enceladus , two of Saturn's moons. In May 2011, NASA scientists reported that Enceladus "is emerging as the most habitable place beyond Earth in the Solar System for life as we know it."
Since the 1950s, scientists have been promoting the idea that "habitable zones" are the most likely places for life to be found. A study published in 2016 suggests that old stellar clouds may be the best places for an advanced civilization to survive in a galaxy. Long life stars in these clusters and the relative ease of "jumping" from one stellar system to the next could provide a safe space for any technologically savvy species that can leave their home and establish outposts around other stars.
Several discoveries in this type of zone since 2007 have stimulated estimates of frequencies of Earth-like habitats, with numbers reaching many billions. By 2013, however, only a small number of planets were discovered in these areas. Nevertheless, on November 4, 2013, astronomers reported, based on data from the Kepler space mission, that there could be about 40 billion Earth-sized planets orbiting in habitable zones of Sun-like stars and red dwarfs only in the Milky Way, and 11 billion of them may be orbiting stars similar to the Sun. The planet of this type may be 12 light-years away, according to scientists. Astrobiologists have also considered "following the energy" of "potential habitats".
Biochemical and morphological bases
All life on Earth is based on 26 chemical elements. However, about 95% of this life is built on just six of these elements: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur, abbreviated as CHONPS. These six elements form the basic "building blocks" of virtually all life on Earth, while most of the remaining elements are found only in trace amounts.
Life on Earth requires water as the solvent in which biochemical reactions occur. Enough amounts of carbon and other elements, along with water, can allow the formation of organisms on other planets with a chemical composition and a temperature range similar to that of life on Earth. Rocky planets, such as Earth, are formed in a process that allows them to have similar compositions to Earth.
Because of their relative abundance and usefulness in maintaining life, many have the hypothesis that life forms elsewhere in the universe would use these same basic materials to form. However, other elements and solvents can provide a foundation for life. Life forms based on ammonia (instead of water) have been suggested, although this solution seems worse than water.
From the chemical point of view, life is fundamentally a self-replicating reaction, but it could arise under a large number of conditions and with several possible ingredients, although carbon-oxygen within the temperature range with liquid water seems a more propitious environment. Suggestions were made even though self-replication reactions of some sort could occur within the plasma of a star, though this would be very unconventional. Life on the surface of a neutron star, based on nuclear reactions, has also been suggested. However, communication with such creatures would be difficult because the time scales involved are much faster.
Several preconceived ideas about the characteristics of life outside Earth have been questioned. For example, a NASA scientist suggests that the color of photosynthetic pigments of hypothetical life on extrasolar planets may not be green.
The attempt to define limited characteristics challenges certain notions about morphological needs. Skeletons, which are essential for large terrestrial organisms according to experts in the field of gravitational biology, are almost certain to replicate elsewhere in the universe, one way or another. The assumption of radical diversity among putative extraterrestrials is not yet settled. While many exobiologists claim that the extremely heterogeneous nature of life on Earth heralds an even greater variety in outer space, others point out that convergent evolution can dictate substantial similarities between life on Earth and the extraterrestrial life. These two schools of thought are called "divergent" and "convergent", respectively.
Direct Search
NASA's Kepler spacecraft aims to find exoplanets.
Scientists are searching directly for bioassinatures within the Solar System, conducting studies on the surface of Mars and meteors that have fallen on Earth. At present, there is no concrete plan for exploring Europe in search of life. In 2008, a joint mission from NASA and the European Space Agency was announced that it would have studies that included Europe. However, in 2011 NASA was forced to deprioralize the mission due to a lack of funding and it is possible that ESA will take over the mission alone.
There is some limited evidence that microbial life might possibly exist (or have existed) on Mars. An informal survey conducted at the conference in which the European Space Agency presented its findings on methane in the atmosphere of Mars and indicated that 75% of those present agreed that the bacteria already lived on Mars. In November 2011, NASA launched the Mars Science Laboratory (MSL), which is designed to look for past or present evidence of habitability on Mars using a variety of scientific instruments. MSL landed on Mars at Gale Crater in August 2012.
In August 2011, NASA's findings, based on studies of meteorites found on Earth, suggest components of DNA and RNA (adenine, guanine, and related organic molecules), the building blocks for life as we know it. be formed in extraterrestrial environments in outer space.
In October 2011, scientists reported that cosmic dust contains complex organic matter ("amorphous organic solids with a mixed aromatic-aliphatic structure"), which can be created naturally and rapidly by stars. One of the scientists suggest that these compounds may have been related to the development of life on Earth said, "if this is the case, life on Earth may have started easier, as these organic products can serve as basic ingredients for the life."
In August 2012, astronomers at the University of Copenhagen reported the detection of a specific sugar molecule, glycolaldehyde, on a planet located in a system of distant stars. The molecule was found around the binary protostares IRAS 16293-2422, which are located 400 light years from Earth. Glycolaldehyde is needed to form ribonucleic acid, or RNA, which has a DNA-like function. This finding suggests that complex organic molecules can form in stellar systems before the formation of planets and eventually enter the young planets at the beginning of their formation.
In September 2012, NASA scientists reported that polycyclic aromatic hydrocarbons, subject to the conditions of the interstellar medium, are transformed, through hydrogenation, oxygenation and hydroxyalkylation, into more complex organic products - "a step on the way to amino acids and nucleotides, protein and DNA raw materials, respectively. " Moreover, as a result of these transformations, PAHs lose their spectroscopic signature, which may be one of the reasons for the "lack of detection of PAH in the ice of cosmic dust, especially cold outer regions with dense clouds or superior molecular layers of protoplanetary discs. "
On February 21, 2014, NASA announced a very up-to-date database to track the polycyclic aromatic hydrocarbons in the universe. According to scientists, more than 20% of the carbon in the universe can be associated with PAHs, possible starting materials for life formation. The PAHs appear to have been formed soon after the Big Bang, are scattered throughout the universe and are associated with new stars and extrasolar planets.
