In Part I of this essay, we looked at how common life might be outside of the Earth. The only type of life considered was microbial life. Most of us, however, are really interested in more advanced life forms – the type of critters we could sit down with, have a cup of coffee and discuss the meaning of life with. Unfortunately, there appears to be a vast chasm between microbe and ET. Single celled creatures appeared very early in the Earth’s history, more than 3.8 billion years ago – not long after the Earth had cooled enough to allow for liquid water. It appears that, given a well-suited environment, life can get started fairly quickly. Those early microbes thrived. After that, however, it took another 2 1/2 billion years before the first multi-cellular life appeared. It was that innovation that really seemed to set us on the evolutionary path toward ever more complex and advanced life forms. That long 2.5 billion year gap between single- and multi-celled life seems to indicates that the jump from simple to more complex life is much more difficult and unlikely than the start of life itself! Even if life is exceedingly common among the stars, complex life might still be a precious rarity.

Even when advanced life does eventually develop, how often does evolution lead to intelligence? While advanced forms like worms and bamboo are very cool, I want more. I want something I can talk to, ask questions of and maybe even learn from. There are innumerable species of plants and animals growing on or roaming this planet but only a handful are thought of as possessing intelligence. Each species develops its own strategy for survival. Some are fast, some big, some stay well hidden and some are just plain mean. Only a handful of animal species appear to have experimented with intelligence. We know that feeding a big hungry brain takes a lot of resources. If it isn’t really advantageous to have one, you’re not going to evolve one. Other survival strategies than intelligence have proven to work very well and don’t require all the resources consumed by that hunk of meat you carry around between your ears. Look at ants, cockroaches or crabgrass; all very successful but far from what we think of as smart. Arguments like these lead me to believe that intelligence is quite uncommon even among complex life forms.

How many intelligent species are then likely to inhabit our galaxy? To try and get a handle on our level of knowledge (or ignorance) concerning this question astronomer Frank Drake (currently with the SETI Institute) developed a simple equation way back in 1961 that details the factors that contribute to the current total number of intelligent civilizations. The equation has come to be famously known as The Drake Equation:
N = R* x f(p) x n(e) x f(l) x f(i) x f(c) x L
The variables in the equation are defined as:

R* – the average rate of star formation in our galaxy (stars per year).

f(p) – the fraction of stars that have planetary systems

n(e) – the average number of planets per planetary system capable of supporting life.

f(l) – the fraction of planets that can support life where life actually begins.

f(i) – the fraction of planets with life were intelligence evolves.

f(c) – the fraction of planets with intelligence that develop long distance communications (such as radio).

L – The average number of years that civilizations continue to communicate (remain radio bright).

Find the value of each variable, multiply then all together and you end up with the number of intelligent civilizations that are currently capable of communicating with us. As simple as that! As you can see, Drake wasn’t as interested in simply the number of intelligent species; he wanted to know how many we could actually contact or at least listen in on. There could be many intelligent species out there that never develop technology for communications or decide for whatever reason that they don’t want to advertise their presence. If we can’t detect them, we can’t chat with them. Here, we will use the working definition of intelligence as a civilization that has the technology capable of interstellar communications.

We currently only know even rough values for the first two variables, R* and f(p).
R* is about 10 stars/year
f(p) is somewhere around 0.3 to 0.6
After these first two variables, anyone’s guess as about as good as any other. Just for fun, let’s have a go at it and see what we come up with:
R* = 10
f(p) = 0.5 (between the current estimates)
n(e) = 1 (Not all stars are likely to have planets that are favorable for life but some could have several. In our own solar system, there are several possible candidates. So, this number is likely fairly large. Let’s just call it one.)
f(l) = 1 (My guess is that given enough time and given a proper environment, life is likely to spring into existence. Again for simplicity, call it one.)
f(i) = 0.01 (Hmmm. I’m not so sure about this one. Just because you have life, doesn’t necessarily mean you get intelligence. Let’s go with a WAG [Wild-Ass Guess] of one in a hundred?)
f(c) = 0.01 (This is another factor that I’m really unsure of. I can imagine many reasons why a civilization comprised of intelligent creatures might never develop the technologies that we have or might make a conscious decision not to let their presence be known. Again, maybe one in a hundred??)

This leaves us with the variable L, the lifetime of a communicating civilization. This is the factor that truly matters. If we take humanity here on Earth to be an average example, we’ve had radio for roughly the last century. In that time, we’ve come perilously close to annihilating ourselves on several fronts; nuclear, environmental, wars and epidemics to name just a few. It seems that a technology that is at a level capable of broadcasting over interstellar distances is also capable of destroying itself! This argues that the lifetime of such a technology is often fleetingly short. On the other hand, perhaps some civilizations are wiser than us and are able to manage the dangers inherent in their technologies. One might imagine that such a civilization could have a vastly long lifetime. So, what’s the average life? I truly wish I knew.

In our example solution to the Drake Equation, so far we have:

N = 10 x 1 x 1 x 0.5 x 0.001 x 0.001 x L

N = 0.0005 x L

To get N, the number of communicating civilizations in our galaxy, up to just one, the average lifetime of such civilizations needs to be at least 2000 years! If it’s less than that, there aren’t likely to be many, if anybody, out there to talk to. Keep in mind that we’ve been at it for only about 100 years. On the other hand, if some civilizations can find ways to survive long-term, say millions of years, there could be hundreds to thousands of civilizations out there right now. So, what is the answer? We simply don’t know. Only through doing the searches to fill in the variables in Drake’s seminal equation can we hope to get to the answer.

Claude Plymate
Engineering Physicist
National Solar Observatory


2 thoughts on “Life in the Universe~Part II: The Drake Equation

  1. Hurray for Claude! His articles are seriously fun. Not that the soprano isn’t. O love the insight and thought you put into your blogs. Im a big fan or yours, too.

  2. It’s fun for me to provide a place for Claude’s writing. Per my stat app, these blogs get a lot of traffic. Places like Arizona, Hawaii..Chili! What does that tell you?

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