Many years ago, Carl Sagan predicted there could be as many as 10,000 advanced extraterrestrial civilizations in our galaxy, but astrobiologists have figured out why we haven’t heard from Aliens. The Reason: Aliens are silent because they are all dead.
After nearly 60 years of searching without success, more astrobiologists are thinking that life on earth was a lucky series of evolutionary accidents and that we were lucky to come together at the right time and will never be repeated. What are the odds it will be repeated? Highly unlikely.
Earth is a typical rocky planet, in an average solar system, nestled in the spiral arm of an ordinary galaxy and there galaxies with hundreds of billions of stars, the law of averages dictates that intelligent life must exist somewhere. So, why haven’t we found it yet?
The real reason we haven’t found life on other planets or been visited by aliens is that the existence of life would have been likely to be brief and become extinct very quickly, say astrobiologists from The Australian National University (ANU). “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 Astrobiology. “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.”
They speculate that about four billion years ago Earth, Venus and Mars were probably suitable for life. However, a billion years or so after formation, Venus turned into a hothouse and Mars froze into an icebox. Early microbial life on Venus and Mars, if there was any, failed to stabilize the rapidly changing environment, said co-author Associate Professor Charley Lineweaver from the ANU Planetary Science Institute.
Dr Chopra said their theory solved a puzzle.”The mystery of why we haven’t yet found signs of aliens may have less to do with the likelihood of the origin of life or intelligence and have more to do with the rarity of the rapid emergence of biological regulation of feedback cycles on planetary surfaces,” he said. Wet, rocky planets, with the ingredients and energy sources required for life seem to be ubiquitous, however, as physicist Enrico Fermi pointed out in 1950, no signs of surviving extra-terrestrial life have been found.
The Fermi paradox or Fermi’s paradox, named after Enrico Fermi, is the apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilizations, such as in the Drake equation, and the lack of evidence for such civilizations. The basic points of the argument, made by physicists Enrico Fermi (1901–1954) and Michael H. Hart (born 1932), are:
- The Sun is a typical star, and there are billions of stars in the galaxy that are billions of years older.
- With high probability, some of these stars will have Earth-like planets, and if the Earth is typical, some might develop intelligent life.
- Some of these civilizations might develop interstellar travel, a step the Earth is investigating now.
- Even at the slow pace of currently envisioned interstellar travel, the Milky Way galaxy could be completely traversed in about a million years.
Here’s in what Aditya Chopra and Charles H. Lineweaver wrote in their paper entitled, “The Case for a Gaian Bottleneck: The Biology of Habitability”
The prerequisites and ingredients for life seem to be abundantly available in the Universe. However, the Universe does not seem to be teeming with life. The most common explanation for this is a low probability for the emergence of life (an emergence bottleneck), notionally due to the intricacies of the molecular recipe. Here, we present an alternative Gaian bottleneck explanation: If life emerges on a planet, it only rarely evolves quickly enough to regulate greenhouse gases and albedo, thereby maintaining surface temperatures compatible with liquid water and habitability. Such a Gaian bottleneck suggests that (i) extinction is the cosmic default for most life that has ever emerged on the surfaces of wet rocky planets in the Universe and (ii) rocky planets need to be inhabited to remain habitable. In the Gaian bottleneck model, the maintenance of planetary habitability is a property more associated with an unusually rapid evolution of biological regulation of surface volatiles than with the luminosity and distance to the host star.