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We have always been curious about the odds of existence of life elsewhere in the universe. We are curious to know if we really are special? And is the occurrence of life a rare happening?
We can classify the likelihood of existence of life elsewhere in two separate areas of possibilities: First the odds of intelligent or complex life to exist, and second the odds of unintelligent or less complex life to exist.
Possibility of unintelligent life to exist
The existence of less complex life is very probable. As the micro organisms and less-developed life forms can develop easily, as they do not need complex chemical combinations. Microorganisms can exist in the extreme conditions like boiling hot acidic springs. This greatly increases the odds their existence elsewhere, even within our solar system.
The surface of the mars clearly suggests that there was water on its surface in the past. As the water dried up on the surface, it is still supposed to exist below the surface of the planet. Viking probe sent to mars has confirmed it, this greatly increases the odds of existence of microorganisms beneath the surface of mars.
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An even surface of sulphuric acid cloud exists above the Venus's surface, and it hypothesized that microorganisms might exist in this cloud. Though this is not proved yet in any way.
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Europa is one of the moons of the Jupiter. It is externally covered with ice, and there is a strong possibility of a liquid water Ocean beneath this surface of the ice. Smooth surface of this planet supports this hypothesis, as a hard base would result in a surface with many curves, as there are mountains on the earth. There is a strong possibility of various varieties of microorganisms in the water ocean beneath the ice layer.
Micro orgasmic life is also presumed to exist on two moons of Saturn named Enceladus and Titan, but it is not proved yet.
Possibility intelligent life to exist elsewhere
We can put forward two cases, one for the extraterrestrial intelligent life, and other against the existence of intelligent life beyond the earth.
Case for intelligent life to exist elsewhere
The possibility of intelligent life to exist elsewhere is supported by:
The vastness of the universe
Universe is incredibly vast. There are about 200 to 400 billion stars in our galaxy milky way, and there are about 100 to 200 billion galaxies in the observable universe. These 100 to 200 galaxies in observable must consist of millions or billions of stars depending upon their type, and most of them will have planets orbiting around them. This greatly increases the chances of existence of intelligent life in the universe other than the earth, as every star means the possibility of life to exist.
In 1961, Dr Frank Drake formulated an equation which estimates the odds of existence of life in the observable universe. Its results show that only in our galaxy, there might be 10,000 planets with intelligent life on them. This equation is purely based on estimation and not on evidence or proof for which it is often criticized.
Constant physical and chemical laws
Physical and chemical laws responsible for origination of life, or life-supporting conditions on the earth are valid through-out the observable universe. Which suggests that formation of the life, and life supporting conditions is not a unique property residing only to the earth.
Life on the earth is mostly composed of carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. It is generally hypothesized that life forms existing elsewhere in the universe might be composed of these essential elements, but solvent (as water on earth) and other elements might vary. It is also probable that the essential elements responsible for life on the earth may also vary, and could be substituted by elements with similar chemical composition depending on environment of planet, but the chemical laws remain the same. It is suggested that complex organic materials needed for life might have been formed in dense gas disc surrounding the sun before the formation of earth. This is a process which can occur in most of the stars, thus creation of a life-supporting planet like the earth cannot be an isolated incident.
Discovery of earth-like planets beyond our solar system
Planets like Gliese 581 c, g, d and Kepler-22b are discovered. They are located in livable zones just like the earth, which contain water and probably have temperatures well within living range.
Above are the fictional images of Gliese 581 c and Kepler-22b.
Case against intelligent life to exist elsewhere
Physicist Enrico Fermi suggested this paradox in 1950, the main arguments presented were:
♦ Intelligent civilization, as on earth would have developed a way of communication. If their existence is obvious in such a great amount, then why haven’t they contacted us yet?
♦ Ideal placement and all other things leading earth to inhabit life were unique, and could not happen elsewhere.
The drake equation suggests that large number of civilized extraterrestrial life forms can exist just in our galaxy, but still there is no evidence for it and there are no contacts made from within the galaxy. As among civilized life forms many would be much more civilized than others, and will have all the necessary ways to make contact. So why isn’t there any contact yet?
We can answer this paradox with many possible answers, but we cannot be certain of them, due to our less knowledge of the universe.
The possible answers include:
♦ Number of civilized nations might be much fewer. Therefore, no contact is yet made.
♦ Intelligent life forms might destroy themselves periodically with atomic wars, etc.,
♦ Nature might keep destroying intelligent civilizations by asteroids, comets etc, as dinosaurs were once destroyed in this way.
♦ There might exist intelligent life but not developed enough to contact, or they don’t want to contact, or might consider it dangerous to contact.
♦ They might be using different communication equipment or methods than ones used on the earth.
Peter ward and Donald Brownlee
Two professors at the university of Washington Peter ward and Donald Brownlee have also suggested that life on earth is a unique event, and is incredibly unlikely to occur throughout the universe. They proposed few observations to explain the uniqueness of the earth, like Jupiter’s orbit is oriented in that it protects us from comets, etc., and the moon is placed in such a way that it even out the axis of earth. Many other such observations were proposed.
Conclusively, at present we don't know for certain whether extraterrestrial life exists or not, perhaps time would answer this question. As we get enough technological advancement, we would be able to answer this question certainly. Alternatively, we can get an answer before that, in case we are contacted by extraterrestrial life.