“Two possibilities exist: either we are alone in the Universe or we are not. Both are equally terrifying.” — Arthur C. Clarke
The universe is… vast, to put it lightly. How small and inconsequential are we in comparison? We inhabit but one of the eight planets orbiting a single star. The Milky Way galaxy contains 100 to 400 billion stars, and there are likely over 170 billion galaxies in the observable universe. To believe that we alone constitute the only life forms in this fathomless emptiness smacks of a narcissism that heralds back to when we thought the Sun — and the entire cosmos — revolved around the Earth.
Does that mean life must exist somewhere in the universe? No, but with trillions of planets out there, the odds that at least one other world is inhabited by a civilization don’t seem all that unreasonable. But just what are those odds, exactly?
First, we’ll dial back our scope to the odds of detecting intelligent life in our galaxy. Why? Our closest neighbor galaxy, Andromeda, is 2.5 million light-years away — that means that if we were able to travel at the speed of light, it would take 2.5 million years to get there. We expect to detect intelligent life based on signals they would send out — radio waves, the likes of which Earth has been broadcasting into space for the past century. Our signals have only extended about 100 light years away from the Earth, and any alien entity just receiving word from us now would be hearing a message that is already 100 years old. Between the signal degradation with increasing distance and the immense time scale, other galaxies are believed to simply be too far away for us to detect any signs of life from them.
The Drake Equation
So what, then, are the odds of us finding intelligent life in our galaxy? To answer this question, we turn to the Drake Equation. In 1961, astronomer and SETI founder Frank Drake came up with a formula that could estimate the number of intelligent civilizations that may exist in the Milky Way. That equation is:
N = the number of civilizations in our galaxy with which radio-communication might be possible
R* = the average rate of star formation in our galaxy
fp = the fraction of the stars in our galaxy that are orbited by planets
ne = for every star that has planets, the average number of planets it has that can potentially support life
fl = the fraction of planets that could support life that actually go on to develop life
fi = the fraction of planets with life that develop intelligent life (i.e. civilizations)
fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space (i.e. radio waves)
L = how long such a civilization releases detectable signals into space
Let’s plug in some numbers and find out how many civilizations may — or may not — be out there, waiting to hear from us.
R* = According to NASA and the European Space Agency, the current rate of star formation in our galaxy is about 7 per year.
fp = While Drake initially estimated that only one fifth to one half of all stars formed will have planets, current estimates suggest that every star in the milky way is orbited by at least one planet. That means this value is equal to 1.
ne = We’ll be basing this figure on the number of planets that orbit their parent star in the “Goldilocks zone” — a region far enough from the star that water won’t boil away, but not so far that it remains frozen. Not too hot, not too cold; just right. We’ll also only consider stars that are similar to our sun and red dwarf stars, since various theories suggest that stars of other types may lack habitable zones, and planets that are roughly the size of the Earth, which would rule out gas giants like Jupiter that lack a solid surface.
We’re making an educated guess here, but based on data from the Kepler space observatory from November 2013, there could be as many as 40 billion planets that meet our criteria. Assuming the galaxy has, say, 200 billion stars, then 40 billion out of 200 billion gives us a ratio of 0.2.
fl = Now, this is pure speculation. How can we possibly know how many worlds that can support life actually go on to develop life? We only have one point of reference — Earth — which isn’t enough to come anywhere close to drawing conclusive evidence. Drake initially argued that this value should be 1 — that life will eventually develop at some point. Or, as Jeff Goldblum might say, “Life, uh… finds a way.”
We’ll use a more conservative figure drawn on a statistical argument made by astrobiologists Charles H. Lineweaver and Tamara M. Davis in 2002. Based on how long it took for life to evolve on Earth, they estimated that at least 13% of suitable planets that have existed for over one billion years will develop life. Thus, we’ll use the minimum of 0.13 as our value.
fi = Now we dive even deeper into speculation. How can we possibly know whether life will evolve into intelligent life? Most of the life we are looking for in our solar system is expected to be bacteria — hardly what I’d call intelligent. Again, Drake’s original estimates suggested that all life will eventually evolve into intelligent life, and many scientists still hold to this idea. They point to the generally increasing complexity of life over evolutionary time as an argument that intelligence is an almost guaranteed evolutionary trait. However, others point to the billions of species that have existed on Earth and highlight the fact that only one has become intelligent so far. We’ll side with Drake on this one and use a value of 1, but we’ll come back to this point later.
fc = If intelligent life originates, will it develop the technology to send out radio waves into space? Again, pure speculation, and Drake estimated that 10% to 20% would. We’ll use the midpoint of 0.15 as our value.
L = Civilizations have risen and fallen on Earth in the past, but since their technologies have mostly been carried over, we wouldn’t count the Roman Empire as a different civilization than our modern one, for instance. We didn’t have to reinvent the bound book after the fall of the Roman Empire. To date, our civilization has only been releasing radio signals into space for a century, but we’ll continue to do so until some cataclysmic event either destroys humanity entirely or sends us back to the Stone Age. A sufficiently advanced civilization may have the technology to overcome any threat to its survival and exist for billions of years. Alternatively, an arms race could lead an advanced civilization to complete and total self-destruction in a few hundred years — we presently have enough nuclear warheads on Earth to wipe ourselves out entirely. We’ll then use a middle-ground value of 10,000,000 years.
Running the Numbers
With our values established, let’s run the math:
The answer? 273,000 intelligent civilizations may be out there, broadcasting radio waves into space, just waiting to be discovered by us.
Now, as you may have noticed, the further down the equation we go, the more our values become based on guesses than facts. If we alter any of those values by a significant degree — such as the odds of life developing into intelligent life — we get a drastically different answer. If life isn’t guaranteed to evolve intelligence, and the odds are instead one in a billion as some would suggest, then there exist 0.0003 communicative civilizations in the Milky Way — meaning we are alone. Upper estimates that use more generous values than the ones we’ve plugged in here result in as many as 36.4 million civilizations.
Drake’s purpose in developing his equation was never to arrive at a firm answer. It was to stir scientific inquiry. Every variable in his equation is a separate discussion point, and as new discoveries continue to be made, these blurry estimates have been coming into greater focus. While the wide range of answers to the equation have caused some to criticize it as useless, the optimistic results — that there are possibly hundreds of thousands, even millions of civilizations out there — have motivated research and funding into the search for extraterrestrial intelligence.
But here lies the problem. If there do exist an abundance of intelligent civilizations out there in our galaxy… where are they all? Why haven’t we heard from them yet? Why haven’t any more advanced civilizations revealed themselves to us? Well, these questions lead to the Fermi paradox, and that’s a topic for another week.