The Drake Equation Revisited: Part I
The Drake equation was developed as a means of predicting the likelihood of detecting other intelligent civilizations in our galaxy. At the NASA forum, Frank Drake, who formulated the equation 42 years ago, moderated a debate between paleontologist Peter Ward, co-author of the book Rare Earth, and astronomer David Grinspoon, author of the forthcoming book Lonely Planets: The Natural Philosophy of Alien Life.
In this installment of the series, Dr. Drake explains the history and the content of his famous equation. Dr. Drake is the director for the Center for the Study of Life in the Universe at the SETI Institute in Mountain View, CA. He is also chairman emeritus of the SETI Institute board of trustees and professor emeritus of astronomy and astrophysics at the University of California at Santa Cruz.
Frank Drake: It’s a pleasure and honor to be with all of you exobiologists tonight. When I started in this game there were no exobiologists. So just seeing you all out there is a lot of progress.
I want to start out by giving you a little bit of the history and a brief description of the equation. This all began shortly after I conducted the first search for extraterrestrial intelligent radio signals at the National Radio Astronomy Observatory in Green Bank. That was in 1960. At that very same time, a very seminal paper was published by Philip Morrison and Giuseppe Cocconi, pointing out what I had already realized, and that was that we had the ability to detect reasonable signs of intelligent technology across the distances which separated the stars.
This in one way opened the door to detecting life, in this case specifically intelligent life, beyond Earth. A great new window of possibilities was opened, which was slowly recognized at the time and, of course, is widely recognized today. It’s expressed by the great growth in the field of astrobiology.
Shortly after this, the National Academy of Sciences wanted to convene a small meeting to examine this whole question and propose where we should go from here. They asked me to do this and, indeed, just about 42 years ago at this time I sponsored the first such meeting at Green Bank. I was the entire scientific and local organizing committee, but it wasn’t a hard task because I invited every person in the world we knew of who was interested in working in this subject – all twelve of them. And all twelve of them showed up.
As I planned the meeting, I realized a few day ahead of time we needed an agenda. And so I wrote down all the things you needed to know to predict how hard it’s going to be to detect extraterrestrial life. And looking at them it became pretty evident that if you multiplied all these together, you got a number, N, which is the number of detectable civilizations in our galaxy. This, of course, was aimed at the radio search, and not to search for primordial or primitive life forms.
Well, what is the equation? It encapsulates our understanding of the evolution of our galaxy and of our solar system. We know our galaxy is about 14 billion years old and that stars have been created at almost a constant rate. And since very early on those newly formed stars have been accompanied by planetary system – at least in some cases.
So the whole equation is based on a continuous production of new planetary systems, and then presumably life, intelligent life and technology-using life. And that tells us, of course, that the number of detectable civilizations is going to be proportional to the rate of star formation, which we write as R*, because the more stars you make, the more civilizations there will be, eventually. That’s an easy one.
We’ve known for a long time that about 20 new stars are produced per year in our galaxy, and that has been the case for many billions of years now. But over time, we’ve become a little bit more sophisticated in defining what this factor means. Twenty stars per year are produced, but we realize not all of them could produce an intelligent species. Some burn out their hydrogen cores extremely fast, in literally millions of years – no time to evolve intelligent species.
If we take all of those away, the fast-burning stars, we’re left with 19 stars per year. Of those, about four are like the sun. So is the right number for R* four per year? This is still one of the big questions in astrobiology, and one of the challenging ones.
What are the other 15? They’re all very small red dwarf stars, stars known as M dwarfs to astronomers. For a long time we believed that they could not be abodes of intelligent life because, indeed, they could have planetary systems (although none has ever been detected yet), but even if there were planets there, they would be so close to the star that, just as our moon keeps one face to the Earth, they would keep one face to their star. And we thought this would create a situation where, on the dark side of these planets, it would be so cold that the atmosphere would freeze out. There would be no atmosphere and therefore no possibility of life arising.
Well, now that belief has been challenged and it has been shown that with a properly massive atmosphere this freezing out of the atmosphere will not occur. So perhaps the M stars and their planets, after all, are abodes of life.
So what is R*? Well maybe it’s 4, and maybe it’s 19 stars per year.
|A red dwarf star.|
If we multiply by the fraction of those stars which have planets, fp, we get the rate of production of new planetary systems per year. So what is that? Well, for a long time we have had nothing but theories to go on. We thought perhaps 50 percent of the stars had planets. That was based on the fact that half the stars are binary systems, and the other half must have something else, something small, a planet.
Of course, one of the great discoveries of the last century, which only ended about three years ago, was the detection of other planetary systems. This is one of the greatest discoveries in the history of science. We now know of over 100 such systems. Most all of them have what you might call “giant Jupiters” in them, not planets suitable for life on Earth. But we know this is the tip of the iceberg, because this is the only kind of planet we can detect. About 5 percent of the stars have such planets. What do the other 95 percent have? Perhaps Earth-like planets, or planets suitable for life. We also ask whether these giant planets have habitable satellites.
If we multiply that by the next factor, which is written ne, the number of planets in the ecosphere, a term we don’t use any more – nowadays we call it the continuously habitable zone – we get the rate of production of possible life-bearing planets. This is a complex subject, very much more complex than was first imagined. Early on it was thought that the planet had to be at such a distance from the star that liquid water could exist. Not too close, not too far. You had to be Goldilocks, to give rise to life.
Now we realize that the nature of the planet can greatly affect the distance at which it can be from a star and still be habitable. A prime example is Europa, far out where it’s very, very cold on its surface, and yet there is a potential biosphere on that object. A deep atmosphere, through the greenhouse effect, can also make a planet far out from its star nevertheless habitable. So, again, this is a factor we don’t know very well.
The next factor, fl, is the fraction of potentially habitable planets that actually give rise to life. That one we seem to know something about, because the chemists have found a multitude of chemical pathways to the origins of life. Life seems inevitable on any planet with suitable characteristics. And what are those? They seem to be very simple: liquid water, organic molecules and a source of energy.
|Although its surface area is very cold, there is still a potential biosphere on Europa.|
The real question is not whether life arises, but how it really happens. The present consensus is that life does arise in a body of water, perhaps in Darwin‘s “warm little pond,” or the deep-sea vents, the froth of ocean waves – these have all been suggested – or on the molecular templates of clay minerals. We think that faction is close to one.
Our next fraction, fi, is the one which describes what fraction of systems of living things give rise to an intelligent species. This fraction is trying to give the answer to the question: Does evolution converge or diverge? There is much evidence for convergence in intelligence, including the growth in brain size seen in the fossil record, but is an intelligent brain really contingent on things we’re not quite sure about? For instance, does it require the evolution of a means of sophisticated communication, one of the possible contingent situations which may limit the frequency with which intelligence arises? That one is a big unknown.
The next, fc, is the fraction of intelligent civilizations which give rise to a technology which we might detect, or which might communicate – that’s what the “c” means. It seems that fc should be close to one. Once you have enough intelligence in a creature whose anatomy allows the use of tools, you should get technology. Technology has, in fact, developed in many places on Earth in humans independently.
The prime drivers are pretty obvious. The drivers were to provide food, leading to the development of agriculture and the tools of agriculture; to provide the ability to live in otherwise uninhabitable regions, such as artic regions, polar regions; and, of course, for the development of weapons.
At this point, you multiply this all together and you get the rate of production of detectable civilizations in the galaxy. Now, we don’t believe, being conservative, that they remain detectable forever. Perhaps they destroy themselves through nuclear folly, or through destruction of their environment. Perhaps they suffer a cosmic catastrophe, like the K/T event [the asteroid impact that caused dinosaurs to go extinct].
|An artist’s concept of an asteroid’s crash with Earth.|
More likely, at least to the optimists such as myself, they come upon the scene, they are detectable, and then they disappear, because they become more sophisticated technologically. They’ve stopped releasing energy into space. At the present time we are very detectable, primarily through our television broadcasts. But we see television going to cable, and especially to the direct-to-home transmission of television from satellites.
This is terrifying to people like me. The ordinary over-the-air television transmitter transmits a million watts. It makes a very detectable signal. The transmitters which transmit television to those little dishes on people’s homes only transmit 10 watts, far less than a million, and make a signal which is totally undetectable at interstellar distances.
So, we have to worry: Civilizations may be thriving, with a tremendous quality of life, and yet be very hard to detect. And we must account for that by saying, okay, they exist but they only last some limited amount of time, which we will call L, the longevity.
L is dominated by those civilizations with very large Ls, because L is the average lifetime of a civilization. Just as a numerical example, given 100 civilizations, if 99 are detectable for only 100 years, and 1 is detectable for a billion years, then L turns out to be 10 million years. And so L may be larger than what our intuitive thoughts might be.
So that’s the equation. But before we move on, I’ll offer a few comments that are evident but somehow not really seen. One is that every factor in the equation appears to the first power. There are no exponentials, no powers, no power laws, no logarithms. Every one is equally important. And, in the same vein, the overall error in the result is controlled by the biggest uncertainties, which are probably fi and L. Thirdly, the uncertainties grow as we go from the left to the right of the equation, from the astronomical and chemical factors to the social ones.
And, finally, we ask whether we need some other factors. I get letters every week suggesting such. Particularly that we need a factor for the ignorance of politicians. However, all the other ones so far proposed are subsumed within the traditional ones. But the future could well reveal the need for an enlarged equation.
This story has been translated into Portuguese.