Given the size and age of the universe, he said, 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 there are indeed 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 team 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 writing of this article, scientists have discovered 2,049 planets in 1,297 planetary systems, including 507 with multiple planets. 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-size 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 has been no shortage of time for some of that life to evolve and create 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 Earth has 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 each 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 while 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.
NASA Goddard Spaceflight CenterVenus transiting the sun as seen from Earth.In other words, when considering the likelihood of life in the cosmos, we need to look beyond the mere statistics and consider whether it may come down to an "emergence bottleneck." Essentially, those planets where lifeforms fail to emerge quickly enough, thus stabilizing the planet and paving the way for more life, will be doomed to remain uninhabited.
In their report, "The Case for a Gaian Bottleneck: The Biology of Habitability" — which appears in the first issue of Astrobiology for 2016 — Chopra and his associates summarize their argument as follows:
While potentially depressing, this theory does offer a resolution to the Fermi Paradox. Given the sheer number of warm, wet terrestrial planets in the Milky Way Galaxy, there ought to be at least a few thousand civilizations kicking around. And of those, surely there are a few that have climbed their way up the Kardashev Scale and built something like a Dyson Sphere, or at least some flying saucers!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.
And yet, not only have we not detected any signs of life in other solar systems, but the Search for Extra Terrestrial Intelligence hasn't detecting any radio waves from other star systems since its inception. The only explanations for this are that either life is far more rare than we think or we aren't looking in the right places. If life is indeed so rare, an emergence bottleneck may be why life has been so hard to find.
But if we simply aren't looking in the right places, it means our methodology needs to change. So far, all of our searches have been for the "low-hanging fruit" of alien life — looking for signs of it on warm, watery planets like our own. Perhaps life does exist out there, but in more complex and exotic forms that we have yet to consider. Or, as is often suggested, it is possible that extraterrestrial life is taking great pains to avoid us.
Regardless, Fermi's Paradox has endured for over 50 years, and it will continue to endure until such time that we make contact with an extraterrestrial civilization. In the meantime, all we can do is speculate. To quote Arthur C. Clarke: "Two possibilities exist: either we are alone in the Universe or we are not. Both are equally terrifying." LINK
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