The latest hypothesis is that there are no signs of any aliens out there because they are extinct.
Even if there were life on other planets, it would probably die out due to certain environmental conditions. So goes the latest point of view about why no alien life could be found throughout the universe.
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The extreme heat and cold would kill off any life that happened to exist on another planet. The universe has many planets in it that may be habitable. From all appearances they ought to be filled with life forms that interact with each other as well as their surroundings.
The most primitive forms of life are rather sensitive and insecure in their biological makeup. Thus they tend not to survive the onslaughts of the environment.
The instability of early habitats makes the viability of life on these planets a highly unlikely probability. In order to create an atmosphere like earth, the life forms will have to naturally recycle the carbon dioxide and water as well as other greenhouse gases and thereby generate a climate fit for respiration.
"The universe is probably filled with habitable planets, so many scientists think it should be teeming with aliens," said Dr Aditya Chopra from the ANU Research School of Earth Sciences and lead author on the paper, which is published in the journal Astrobiology. It can be downloaded here (PDF).
"Early life is fragile, so we believe it rarely evolves quickly enough to survive. Most early planetary environments are unstable. To produce a habitable planet, life forms need to regulate greenhouse gases such as water and carbon dioxide to keep surface temperatures stable."
There is a possibility that about four billion years ago, Earth, Mars and Venus were all capable of supporting life.
Nevertheless, about a billion years into their history, Mars became a frozen ice world while Venus became a seething arena of hot gases. Only the planet Earth managed to retain its ability to house the delicate and fragile thing called life. And there too it was a miracle of sorts.
Due to the highly unstable environment, life forms just couldn’t survive on Mars and Venus. The life forms that were present on earth most likely made the environment one that was worthy of nurturing the biosphere.
Thus the reason behind why alien life forms haven’t been found yet isn’t any lack of primitive and primordial self-replicating agents. Life may sprout anywhere where the initial conditions are right.
But for life to take root and grow, an atmosphere is needed. That means the self-regulation of the climate of the planet via the input and output of the life forms.
The physicist Enrico Fermi reminded scientists that no alien life had been found yet. Fermi’s paradox may be resolved by stating the fact that extinction of all other life forms took place early on in the universe.
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The fossils of microbes may still exist on far off planets. But such complex species as dinosaurs and human beings require a long time to evolve.