Earlier this week we reported on the astounding revelation that 22% of sunlike stars in the Milky Way are orbited by potentially habitable, Earth-sized worlds. Given that there may be billions upon billions of life-friendly planets out there, it's time to revise the numbers in the Drake Equation and estimate how many communicable alien civilizations may be out there.

Convergent Evolution

For example, Fermilab physicist Don Lincoln, author of the recently released Alien Universe: Extraterrestrial Life in Our Minds and in the Cosmos, argues that intelligent life is likely to arise from similar environmental, chemical, and evolutionary processes as humanity. 

Lincoln points to carbon-based life on Earth — which he says is not an accident or some kind of intergalactic anomaly. Carbon atoms can handle four chemical bonds, unlike single-bonding hydrogen atoms. But it’s relatively easy to swap those bonds around. 

It’s also very unlikely, says Lincoln, that technically advanced civilizations like ours could have developed on a planet without land masses, like a so-called water world. He believes it's unlikely that intelligent dolphins will ever develop the technology for spaceflight. "There could be alien cavemen underwater," he says. "But truly, you can't smelt metal." 

Indeed, to get to our level of development, a species would probably have to be terrestrial. And if it’s terrestrial, it would likely have to face the same sort of evolutionary pressures that our ancestors did. That doesn’t mean, of course, that all intelligent civs are descended from primates. But they may all take similar paths on their evolutionary journey, a well-documented phenomenon evolutionary biologists refer to as convergent evolution — those cases in which organisms not closely related independently acquire some characteristic or characteristics in common; mutation in evolution may be random, but selection is not.

Examples include physical traits that have evolved independently (e.g. the eye), ecological niches (e.g. pack predators), and even scientific and technological innovations (e.g. language, writing, mathematics, the domestication of plants and animals, and basic tools and weapons). Looking off-world, it’s not unreasonable to think about similar examples of convergent evolution; there may be certain ecological and sociological niches that are not Earth-specific or human-specific and are archetypal throughout the universe. 

Sure, this smacks of determinism — but hey, physics is the ultimate arbiters of all things.

Simon Conway Morris, in his book, Inevitable Humans in a Lonely Universe, describes life's "eerie" ability to repeatedly navigate to a single solution. "Eyes, brains, tools, even culture: all are very much on the cards," he writes. "So if these are all evolutionary inevitabilities, where are our counterparts across the galaxy? The tape of life can only run on a suitable planet, and it seems that such Earth-like planets may be much rarer than hoped. Inevitable humans, yes, but in a lonely Universe." 

Morris clearly subscribes to the Rare Earth Hypothesis, the suggestion that the conditions for life on this planet are excruciatingly precise and difficult to replicate elsewhere. Personally, I don’t necessarily disagree that life on Earth arose from a perfect storm of conditions, but I do take exception to the idea that life is still rare across the cosmos. Even if we’re one in a million — or hell, one in a billion — there should still be countless technological civilizations throughout the universe. And the ones that do make it may look eerily familiar to us.

So What Do We Know About ET?

So if we take all these assumptions and methodologies into account, what do we really know about alien civilizations? Based on our own experience we can start to make some really, really broad brush strokes.
First, we can assume that a certain subset of technological civilizations go through similar developmental states, including stone age and agrarian culture, industrialization, globalization (cultural, economic, and political — and in that order), and an information age.

Unfortunately, we can’t extrapolate beyond that because we ourselves have not progressed into the next phase, whatever that may be. And in fact, owing to the threat of existential risks like artificial superintelligence and molecular assembling nanotechnology (just to name a few), there may not even be a next phase. Sadly, we can’t even make the assumption that advanced ETIs engage in space travel and interstellar colonization; we have yet to see any evidence of this, so we can’t extrapolate that far — a repugnant conclusion derived from the infamous Fermi Paradox.

On a similar note, we can theorize about the presence of developmental mechanisms that constrain and give directionality to the evolution of organisms and society itself. This idea, that of the “megatrajectory,” was proposed by A. H. Knoll and R. K. Bambach in their 2000 collaboration, "Directionality in the History of Life.” They argued on behalf of a middle road that encompasses both contingent and convergent features of biological evolution — one that may be attainable through the idea of the megatrajectory:
We believe that six broad megatrajectories capture the essence of vectoral change in the history of life. The megatrajectories for a logical sequence dictated by the necessity for complexity level N to exist before N+1 can evolve...In the view offered here, each megatrajectory adds new and qualitatively distinct dimensions to the way life utilizes ecospace.
According to Knoll and Bambach, the six megatrajectories outlined by biological evolution thus far are:
1. The origin of life to the "Last Common Ancestor"
2. Prokaryote diversification
3. Unicellular eukaryote diversification
4. Multicellular organisms
5. Land organisms
6. Appearance of intelligence and technology
Interestingly, cosmologists Milan Ćirković and Robert J. Bradbury took the megatrajectory idea one step further by speculating about a seventh megatrajectory: postbiological evolution triggered by the emergence of artificial intelligence and the invention of several key technologies like molecular nanoassembling and stellar uplifting.
Similarly, historian of science Steven J. Dick, in his 2003 paper "Cultural Evolution, the Postbiological Universe and SETI," posited a central concept of cultural evolution he called the Intelligence Principle:
The maintenance, improvement and perpetuation of knowledge and intelligence is the central driving force of cultural evolution, and that to the extent intelligence can be improved, it will be improved.
Dick made the case that broad speculations about the developmental tendencies of advanced civilizations could be made through the application of this principle.

Alien Tech and Exopolitics

Which leads nicely to the next point: We can also make some guesses about alien technological innovations. There may in fact be some universal technological archetypes and scientific breakthroughs that are common to alien civs, including the rough chronological order in which these advancements are developed.
For example, modern cosmology cannot arise before the advent of telescopes. Similarly, microbiology can’t progress without the development of microscopes. Scientific advancements also piggyback off each other. For example, Newtonian dynamics had to precede Einsteinian Relativity, and neo-Darwinianism was fused from traditional Darwinism and Mendelian genetics. 

It may even be possible for us to speculate the existence of common cultural and meta-ethical characteristics of advanced societies, namely the notion that technological societies independently reach the same conclusions about ethics, morality, and social imperatives. 

This last point is perhaps the trickiest of all seeing as we’re far from consensus here on Earth, not to mention the problem of social constructivism (the idea that groups construct knowledge for one another, and that the natural world has a small or non-existent role in the construction of knowledge, including science). 

The jury is still out on which political system is the “best” or most effective — or if any single political system will ever “win out” in the end. If there even is such a thing. Here on Earth there’s a tension between democracy, authoritarianism, capitalism, and collectivism. Perhaps alien civilizations experiment with these concepts, too. Or more conceptually, it’s conceivable that the smooth governance of intelligent and (mostly) autonomous agents pass certain thresholds (both in terms of size and sophistication) that necessitate political paradigm shifts. These shifts may be common to all (or most) civilizations across the cosmos. As an example, our civilization shifted from monarchism/authoritarianism to democracies just prior to (and during) to the onset of the industrial revolution. 

As an aside, ants also undergo these sorts of paradigmatic changes — and biologists have compared the scaling of these networks to human institutions.

Caveats Galore

Again, this is highly speculative, theoretical stuff. Aside from our little old selves, we have no empirical evidence to back any of these claims. It’s possible, for example, that we are in fact the only ones around, and that we somehow pierced through the Great Filter. It’s also possible that many of the assumptions I presented here are false. Maybe we’re the anomaly when it comes to technological civilizations (which would violate the self-sampling assumption), or that there’s a plurality of alien-types (both in terms of biological/morphological makeup and civilizational types) that far exceeds our imagination — and the limits of our current science. 

And as noted, we cannot extrapolate beyond our current state of technological and social development when making inferences about ETIs. But with each advancing step we take, we should assume that extraterrestrials, both past and present, have gone through similar stages.


The Drake Equation

 The Drake Equation goes like this: R * fp * ne * fl * fi * fc * L = N where:
  • R = the average rate of star formation in our galaxy
  • fp = the fraction of those stars that have planets
  • ne = the average number of planets that can potentially support life per star that has planets
  • fl = the fraction of planets that could support life that actually develop life at some point
  • fi = the fraction of planets with life that actually go on to develop intelligent life (civilizations)
  • fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
  • L = the length of time for which such civilizations release detectable signals into space
We don't know the precise numbers to fill in these variables, but we're getting a better idea. Let's go with the following assumptions:

Average rate of star formation in our galaxy: Drake originally went with the number 1 — a very conservative estimate — but it's probably closer to 7, so we'll go with that (R=7).

Fraction of those stars that have planets: Drake thought that only 50% of stars host planets, but we now know that 100% of stars have planets (fp=1).

Average number of planets that can potentially support life per star that has planets: Here's where we get to plug in the new data. According to the new study, 1-in-5 sunlike stars host an Earth-sized planet in the habitable zone. Depending on the total number of stars in the Milky Way, that could be as high as 11 billion planets, or a figure of roughly 20%. But there's also red dwarfs to consider. Recent surveys have shown that upwards of 40% of these dim stars host Earth-like planets in their habitable zones. That means there's as many as 40-60 billion habitable planets orbiting red dwarfs. That said, we do not know if red dwarfs can harbor life. These systems have poor magnetic fields, are tidally locked, and experience poor levels of "good" radiation. So let's prepare the equation to account for both possibilities. So our figures will be 34% (rl=0.34) in the optimistic case and 4% (fl=0.04) in the pessimistic case. Drake himself gave values between 1-5.

Fraction of planets that could support life that actually develop life at some point: This is a tough one, and we haven't got a clue. Drake thought it was 100%, but that can't possibly be right. Still, life on Earth started almost immediately once the proper conditions were established, so it's probably not a figure that's close to zero. Let's go with a number established by Charles Lineweaver who estimated that 13% (fl=.13) of planets have sprouted life.

Fraction of planets with life that actually go on to develop intelligent life (civilizations): Another tough one. There have been billions of species on Earth, yet only one has developed the capacity for radio communication. It's not immediately obvious that evolution favors human-like intelligence, preferring instead other sorts of adaptations. What's more, it not obvious that all human-like intelligences go on to form technological civilizations. Most estimates place this figure somewhere between 50 and 100% — but that seems absurdly high. Let's go with something more reasonable, like 1-in-10 (fi=0.1).

Fraction of civilizations that develop a technology that releases detectable signs of their existence into space: This one's probably quite high. Even pre-atomic civilizations are capable of this. Let's go with a figure of 80% (fc = 0.8). Drake himself said this figure should be between 10-20%.

Length of time for which such civilizations release detectable signals into space: This variable is challenging to assess because we ourselves are about to go radio silent. But let's assume that civilizations will engage in METI efforts and deliberately send signals into spaceregardless of the potential risks. In fact, we're already doing this. But there's also the possibility that technological civs are short-lived owing to existential risks, including nuclear war, artificial superintelligence, and molecular assembling nanotechnology. Humanity could conceivably wipe itself off the intergalactic map at some point in the coming decades. But should we survive our technologies and the accompanying technological Singularity, we could be around for a tremendously long period of time — something on the order of millions of years. But because that's highly speculative, and because the Fermi Paradox and the Great Filter hypothesis tells us otherwise, let's go with 200 years (L=200). Drake gave values between 1,000 and 100,000 years.

Okay, with that out of the way, let's do some math:
7 x 1 x 0.34 x 0.13 x 0.1 x 0.8 x 200 = 5.

So, if both sunlike stars and red dwarfs host habitable planets, we get a figure of five communicable civilizations in our galaxy (including ourselves). This implies that, probabilistically speaking, our nearest radio-capable neighbor is 22,000 lightyears away. That's highly discouraging, to say the least.

But it gets considerably worse if we exclude red dwarfs. In that case, our equation reads:
7 x 1 x 0.04 x 0.13 x 0.1 x 0.8 x 200 = 0.58.

Which is a tough pill to swallow considering that we're here! What this might imply, however — aside from the fact that our variables may be wrong — is that we may in fact be the only ones in the Milky Way right now who are spewing radio signals out into the cosmos.

As for Frank Drake, he came up with values for N ranging between 1,000 and 100,000 civilizations in the Milky Way.
Top photo: Milky Way Galaxy by Jacob Marchio.