In 1950, physicist Enrico Fermi raised a very important question about the Universe and the existence of extra-terrestrial life.Given the size and age of the Universe, he stated, and the statistical probability of life emerging in other solar systems, why is it that humanity has not seen any indications of intelligent life in the cosmos?
This query, known as the Fermi Paradox, continues to haunt us to this day.
If, indeed, there are billions of star systems in our galaxy, and the conditions needed for life are not so rare, then where are all the aliens?
According to a recent paper by researchers at Australian National University's Research School of Earth Sciences, the answer may be simple: They're all dead. In what the research teams calls the "Gaian Bottleneck," the solution to this paradox may be that life is so fragile that most of it simply doesn't make it.
To put this in perspective, let's first consider some of the numbers. As of the penning of this article, scientists have discovered a total of 2,049 planets in 1,297 planetary systems, including 507 multiple planetary systems. In addition, a report issued in 2013 by the Proceedings of the National Academy of Sciences of the USA indicated that, based on Kepler mission data, there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs within the Milky Way, and that 11 billion of these may be orbiting Sun-like stars.
So really, there should be no shortage of alien civilizations out there. And given that some scientists estimate that our galaxy is over 13 billion years old, there's been no shortage of time for some of that life to evolve and crate all the necessary technology to reach out and find us. But according to Dr. Aditya Chopra, the lead author on the ANU paper, one needs take into account that the evolutionary process is filled with its share of hurdles.
"Early life is fragile, so we believe it rarely evolves quickly enough to survive," he says. "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."
Consider our Solar System. We all know that planet Earth has all the right elements to give rise to life as we know it. It sits within the Sun's so-called Goldilocks Zone (aka habitable zone), it has liquid water on its surface, an atmosphere, and a magnetosphere to protect this atmosphere and ensure that life on the surface isn't exposed to too much radiation. As such, Earth is the only place in our Solar System where life is known to thrive.
But what about Venus and Mars? Both of these planets sit within the Sun's Goldilocks Zone and might have had microbial life on them at one time. But roughly 3 billion years ago, when life on Earth was beginning to convert the Earth's primordial atmosphere by producing oxygen, Venus and Mars both underwent cataclysmic change.
Whereas Venus experienced a runaway Greenhouse Effect and became the hot, hostile world it is today, Mars lost its atmosphere and surface water and became the cold, desiccated place it is today. So whereas Earth's microbial life played a key role in stabilizing our environment, any lifeforms on Venus and Mars would have been wiped out by the sudden temperature extremes.
