Scientific Method / Science & Exploration

Habitable exoplanets are bad news for humanity

Intelligent life hasn't contacted us yet, possibly because it self-destructs.

Let’s hope it’s barren.
NASA Ames/SETI Institute/JPL-CalTech

Last week, scientists announced the discovery of Kepler-186f, a planet 492 light years away in the Cygnus constellation. Kepler-186f is special because it marks the first planet almost exactly the same size as Earth orbiting in the “habitable zone,” the distance from a star in which we might expect liquid water—and perhaps life.

What did not make the news, however, is that this discovery also slightly increases how much credence we give to the possibility of our own near-term extinction. This is because of a concept known as the Great Filter.

The Great Filter is an argument that attempts to resolve the Fermi Paradox: why have we not found aliens (or why have they not found us), despite the existence of hundreds of billions of exosolar systems in our galactic neighborhood in which life might evolve? As the namesake physicist Enrico Fermi noted, it seems rather extraordinary that not a single extraterrestrial signal or engineering project has been detected (UFO conspiracy theorists notwithstanding).

This apparent absence of thriving extraterrestrial civilizations suggests that at least one of the steps from humble planet to interstellar civilization is exceedingly unlikely. The absence could be because intelligent life is extremely rare, or because intelligent life has a tendency to go extinct. This bottleneck for the emergence of alien civilizations from any one of those billions of planets is referred to as the Great Filter.

Are we alone?

What exactly is causing this bottleneck has been the subject of debate for more than 50 years. Explanations could include a paucity of Earth-like planets or self-replicating molecules. Other possibilities could be an improbable jump from simple prokaryotic life (cells without specialized parts) to more complex eukaryotic life—after all, this transition took well over a billion years on Earth.

Proponents of this “Rare Earth” hypothesis also argue that the evolution of complex life requires an exceedingly large number of perfect conditions. In addition to Earth being in the habitable zone of the Sun, our star must be far enough away from the galactic center to avoid destructive radiation, our gas giants must be massive enough to sweep asteroids from Earth’s trajectory, and our unusually large Moon stabilizes the axial tilt that gives us different seasons.


Or is the filter ahead of us?

While the emergence of intelligent life could be rare, the silence could also be the result of intelligent life emerging frequently but subsequently failing to survive for long. Might every sufficiently advanced civilization stumble across a suicidal technology or settle into an unsustainable trajectory? We know that a Great Filter prevents the emergence of prosperous interstellar civilizations, but we don’t know whether or not we already cleared it in humanity’s past or if it awaits us in the future.


Esteemed scientists such as Astronomer Royal Martin Rees at the Cambridge Centre for the Study of Existential Risk point to advances in biotechnology as being potentially catastrophic. Others, such as Stephen Hawking, Max Tegmark, and Stuart Russell, also with the Cambridge Centre, have expressed serious concern about the exotic but understudied possibility of machine superintelligence.

Let’s hope Kepler-186f is barren

When the Fermi Paradox was initially proposed, it was thought that planets themselves were rare. Since then, however, the tools of astronomy have revealed the existence of hundreds of exoplanets. That seems to be just the tip of the iceberg.



In the case of Kepler-186f, we still have many reasons to think intelligent life might not emerge. The atmosphere might be too thin to prevent freezing or the planet might be tidally locked, causing a relatively static environment. Discovery of these hostile conditions should be cause for celebration. As philosopher Nick Bostrom once said:


Andrew Snyder-Beattie works at the University of Oxford. This article was originally published on The Conversation.

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