Life in the universe: different theories and hypotheses compared
Since the dawn of civilization, man has always been driven by curiosity about whether there is life on other planets in the Universe and especially whether there are intelligent creatures. Throughout the centuries, numerous philosophers, theologians and scientists have addressed the issue bringing arguments for and against the existence of extraterrestrial life.
The first proponent of the plurality of inhabited worlds was the Greek philosopher Anaximander of Miletus (who lived in the 6th century B.C.). According to him, the source of all known things was an Undetermined Element, and therefore it followed that there could exist (simultaneously) several worlds, different from one another.
Epicurus (341-270 B.C.E.) also believed that there were infinite inhabited worlds since everything is made up of an infinite variety of atoms that break away from the Infinite to momentarily assume a precarious order. Such atoms could then also pass from one world to another, generating very different living things from time to time. Epicurus' theses were supported in the Roman world by Lucretius (1st century B.C.).
Generally, however, Western philosophical thought rejected the thesis of the plurality of worlds and the question of extraterrestrial life. Few thinkers argued in favor of the existence of alien life. The dominant rationalism sanctioned the impossibility of speculating on matters on which it is not possible to rely on tangible experience and evidence. Moreover, the pluralist conception was considered heretical because it implicitly opposed the dogmas of the Christian religion, which sanctions a single act of creation on a single world.
The first scientific basis for the idea of the possible existence of alien life was laid in the 17th century as a result of the observations and work of Nicholas Copernicus (1473 - 1543) and Galileo (1564 - 1642).
Mercury, Jupiter, Venus, Mars and Saturn thus appeared clearly as planets, and mountains and craters began to be sighted on the Moon. The planet Mars in particular immediately drew much attention to itself as sea-like and continent-like expanses, clouds and sandstorms could be observed on its surface, and above all two "polar caps" that seemed to melt in "summer" and then reconstitute themselves in "winter." Astronomer Schiapparelli (director of the Milan Observatory) in the last century identified an intricate network of lines connecting the "seas"; these lines, therefore called "channels," also would split during the course of some Martian seasons. From these observations therefore arose the idea that the planet might be inhabited by intelligent creatures, moreover skilled in hydraulic engineering. In reality, subsequent observations made with more powerful and precise telescopes made it clear that the "channels" are natural and highly irregular structures, and that the "seas" do not contain water but are empty craters.
In contemporary times the observation of the planets of the solar system has become more refined and precise through the use of increasingly powerful telescopes and especially through the use of radio telescopes and probes sent into space.
The Viking 1 and 2 probes (during missions that lasted from 1976 to 1982) took soil samples from Mars to see if there were any traces of biological life on the surface of this planet; experiments prepared for this purpose, however, proved negative. Similarly, the Voyager 1 and 2 probes launched in 1977 to fly over Jupiter, Saturn, Uranus and Neptune did not discover any traces of biological life. In the course of those missions, however, the presence of complex organic molecules (methane, ethane, acetylene, hydrogen cyanide, ammonia) was nonetheless discovered both in the atmosphere of Titan - Saturn's satellite - and in the wake and core of comets. Such molecules are originated by the chemical transformation of simple molecules (water, ammonia ...) present on stellar dust grains operated by the bombardment of ultraviolet cosmic rays; under favorable chemical conditions they can form even more complex organic compounds (proteins, DNA) necessary for the development of biological life.
In this regard, some scholars believe that comets would play an important role as space transporters of molecules essential for the development of life and hypothesize that repeated comet "bombardment" of planets favorable in characteristics for the development of life (adequate surface temperature, continuous presence of liquid water for billions of years, the right amount of carbon atoms for the development of complex organic molecules) may have an inseminating effect of simple organic molecules tending to favor the development of simple life forms.
Since research to date leads us to conclude that no simple or intelligent life forms exist in the solar system, the field of investigation has shifted to stars. Since the star outside the solar system closest to us is a good 40 light-years away, searches are carried out exclusively by means of radio telescopes. Such instruments analyze electromagnetic waves from space and interpret them according to wavelength.
The exploration of such a vast space is just in its infancy, and so, while waiting for some significant results, the theoretical debate about the possible existence of extraterrestrial life forms somewhere in the Universe is still very lively.
The debate is also complicated by the fact that within the general issue there are others at various levels that need to be clearly distinguished.
First, surprising as it may be, a satisfactory definition of the term "life" has not yet been coined. For physicists, life can be defined as a sufficiently organized collection of matter and energy. Since this definition does not necessarily include the presence of blood and flesh, hypothetically one could conceive of alien life in the form of interstellar clouds of high density or as a set of intersecting magnetic fields. While such hypotheses have the merit of stimulating the mind to go beyond the usual boundaries, they imply a very important negative consequence: even if we encounter alien life of such an unusual nature we might not recognize it as such. More practically then, extraterrestrial life is researched according to a basic biological model similar to that of the Earth: a physical structure organized according to a precise pattern, whose existence is regulated by the expression of information contained in DNA or RNA molecules, capable of replication and evolution.
In general, most scholars believe it is possible that a simple form of life, similar to Earth's microorganisms, could have developed somewhere in the Universe. Opinions diverge more regarding the possible existence of intelligent alien life ...
