Last week, we reported on the astounding confirmation that all solar systems in the Galaxy probably have planets, and that Earthlike planets are more common than previously thought. While this seems like good news for SETI-enthusiasts, the revelation is actually quite disturbing.

Given that we have yet to meet any extraterrestrials, the finding could mean that basic life may be very common — but that it gets snuffed out before having a chance to leave the cradle. That could be very bad news for humans.



To find out more about more about this grim possibility, we talked to two experts on the subject: economist and futurist Robin Hanson from George Mason University, and philosopher Nick Bostrom from Oxford University.

Most io9 readers are familiar with the Fermi Paradox: the observation that our Galaxy is so old that it could have been colonized many times over by now by an advanced civilization. But because we find ourselves in a Galaxy that appears completely unperturbed by intelligent life, we're forced to come up with explanations as to why.

There are nearly as many theories for the so-called Great Silence as there are people who think about it. Some believe that it's because most advanced civilizations couldn't be bothered to make the effort, given the immense timescales, distances, and costs involved. Or that the conditions to spark and support life in the Universe are extremely rare. Others say there's a kind of Star Trekkian "prime directive" in effect, a galactic convention that precludes advanced life from interfering with other civilizations.


And still others suggest that life somehow gets snuffed out along the way, by a kind of cosmological filter that prevents it from advancing beyond a critical stage.

The Great Filter


This theory, called the Great Filter, was put forth by GMU's Robin Hanson. His idea offers a rather elegant, if not disturbing, solution to the Fermi Paradox. Hanson suggests that there's some kind of evolutionary hurdle that's preventing life from advancing to the stage where it can go interstellar. There's something out there, says Hanson, that's preventing "dead matter" from giving rise, in time, to "expanding lasting life."

Given the news from NASA's Kepler team last week, we spoke to Robin Hanson about their discovery and how it relates to his hypothesis – and what it might mean for human civilization moving forward.


"If our descendants survive for a million years, there's a good chance they'll change our galaxy in quite visible ways," Hanson told io9. At the same time, however, he reminds us that even though there are a vast number of planets out there where alien life might possibly have arisen, alien life has not visibly changed our galaxy. "Thus the chance that any one planet gives rise to a galaxy-changing civilization must be very low," Hanson says. "So a great filter must lie between dead planets and galaxy-changing civilizations — either many unlikely steps are required, or there are a few very difficult steps."

And this is the crux of the problem: We don't know what it is or where in the evolution of life the Great Filter resides. It could be behind us, which would mean that we are in fact cosmological freaks, and that the future is completely open to us — including the possibility that we might become an interstellar species. Or, it might be ahead of us, and we may well be doomed. The biggest nightmare is the idea that all sufficiently advanced civilizations destroy themselves before reaching the stage where they can start traveling through the Galaxy.


Hanson wonders how far human civilization has made it through the filter — that is, what fraction of inhabited planets make it to our level of development. "The more of the great filter that lies ahead, instead of in our past, the bigger the chance that we'll destroy ourselves," he says. "Yes, there are ways to fail the filter without dying — maybe we'll just become incapable of leaving our solar system." But since the possibility of killing ourselves must be a part of our future filter, warns Hanson, "the bigger our future filter is, the bigger that chance [becomes]."

It's for this reason that Hanson believes it is bad news to discover that habitable planets are more common than we thought. "This means that it was easier to get to where we are than we thought, which suggests that we are more likely to kill ourselves in the future." Subsequently, he suggests that we should be even more careful to watch for and avoid such disaster scenarios.

How hard is evolution?

Indeed, the idea that the Milky Way is teeming with rocky, habitable planets blows the Rare Earth Hypothesis right out of the water. Our planet is clearly not unique or special, in the cosmological scale of things. Consequently, given that there are trillions of stars in the Galaxy, and that intelligent life could have emerged as long as 5 billion years ago, we are suddenly confronted with the realization that the Great Filter likely awaits us in the future. It just seems highly improbable that it could be otherwise.

Hanson is not the only person who shares this concern. Oxford philosopher Nick Bostrom is likewise worried – but he's not convinced that the filter awaits us in the future. "This is only slightly bad news for us," he told io9.



Bostrom contends that the main uncertainty is not in the number of habitable planets, but rather in the likelihood of intelligent life arising on any suitable Earth-like planet. For all we know, Bostrom argues, the chance of intelligence evolving could be extremely small, which would explain why we haven't yet spotted any extraterrestrial civilizations.

If we actually discover evidence that complex life has evolved independently on another planet, that would be much worse news, Bostrom tells io9. That discovery would mean that it's easier to develop life on an Earthlike planet, which would mean the limiting factor is at a later stage. And that, in turn, would make it more likely that we'll face a major pitfall between our current situation and the technologically advanced stage where we can engage in large-scale space colonization.

And the more likely it is that the difficult part comes later, the likelier it is that we'll "succumb to come existential catastrophe – perhaps related to some dangerous technological advance that almost all sufficiently advanced civilizations [fall prey to]."


Indeed, this is this concern that has led Bostrom to think and write extensively on the subject of existential risks. The suggestion that human extinction awaits us in the future is obviously quite discouraging –- but the good news is, we don't know enough to say that for certain.

It's not outrageous to suggest that complex life is indeed a very rare fixture of the Universe. As Bostrom noted, there are some stages in the evolution of life that could serve as powerful filters, including the emergence of reproductive molecules (RNA), simple single-cell life (prokaryotes), or complex single-cell life (eukaryotes).

Life, but not as we know it



It's also conceivable that our conceptions of futuristic star-hopping civilizations are completely wrong. If anything, we may be guilty of anthropomorphizing aliens, and projecting our own goals onto them. Perhaps advanced life, for whatever reason, has no interest in colonizing the Galaxy or making its presence known to us.

The trouble with this thinking, though, is that this has to apply to every single advanced civilization that has ever emerged in the Galaxy to date; all it would take is just one civilization to start a colonization wave to upset this notion.

All in all, we'll have mixed feelings if and when we discover life on another planet. On the one hand, it'll be nice to know we're not alone in the cosmos. But on the other hand, life on other planets would be evidence that the biggest barrier to interplanetary colonization is still ahead of us — and that we're more likely to wipe ourselves out before we get that far.


Top image via NASA. Inset images via Cosmos Magazine and Nature.