The Drake Equation

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In 1961, at the Green Bank observatory in West Virginia, a small conference was held for astrophysicists. The meeting was organized by Cornell University professor and astronomer Frank Drake.

The subject of the conference was the search for extraterrestrial life. 

In preparation for the conference, he jotted down his thoughts in the form of an equation. An equation that has changed how we think about life on other worlds. 

Learn more about the Drake Equation and the variables that make it up on this episode of Everything Everywhere Daily.


In the late 1950s and early 1960s, the science of astronomy was making huge strides. In particular, the field of radio astronomy had been taking off and many new discoveries were being made. 

With these new discoveries and new observation tools, the questions of extraterrestrial intelligence began to come to the forefront and were taken more seriously by scientists. 

It was noted that the radio telescopes of the late 50s were sensitive enough to pick up radio waves that were broadcast from other civilizations. 

It was in this environment that Frank Drake created the Drake Equation. 

The Drake Equation is nothing more than an attempt to try to identify the factors which determine the number of civilizations we could potentially communicate within our galaxy. 

The Drake Equation is really nothing more than an educated guess, and there has been criticism of the equation and suggestions that it needs to be updated in light of discoveries in the last 60 years. 

With that, let’s get into the equation itself. 

The entire equation when written out would read:

N = R* x fp x ne x fl x fi x fc x L

If that sounds complicated, it really isn’t, and each of the variables can be explained in a very easy-to-understand way.

Let’s start with N, which is the entire point of the equation. N is just the number of civilizations in our galaxy we could communicate with. That is what we are trying to figure out. 

The other seven variables are all things that would determine the number of civilizations. Many of the early attempts to assign values to these variables were really nothing more than educated guesses. Since 1961 we have gotten a far better idea as to the numbers for a few of the values. 

R* is the first variable. It represents the rate of star formation in the Milky Way, or how many new stars are created every year. 

Drake’s initial guess in 1961, and it was just a guess, was there were was about 1 new star that formed every year in our galaxy. He assumed that this was a conservative estimate, and he was right.

The Milky Way has somewhere between 100 to 400 billion stars, and the current estimate is that approximately three solar masses of stars are created each year. That could be three stars like the sun, or 1 big star three times the size of the sun, or 5 stars smaller than the sun.

Here I should note that the Drake Equation is only designed for our galaxy. If you wanted to calculate this for the entire universe, you’d have to multiply everything by the estimated number of galaxies in the universe which is somewhere between 200 billion and 2 trillion. 

The second variable is fp which represents the fraction of stars that have planets. 

Of all the variables in the equation, this is the one where the most progress has been made in the last 60 years.  In 1961, no one even knew if other stars had planets, or if they did how common of an occurrence it was. 

Since then we have discovered thousands of planets around other stars. In fact, we’ve found so many that it is assumed now that pretty much all stars have planets and that it is a natural part of the formation of stars. 

That would make the value of this variable 1 or very close to 1, and really renders it irreverent. In many of the updated versions of the equation, this variable is now eliminated entirely.

In Drakes’s original 1961 estimate, he put this value at .2 to .5, which as it turns out was a very conservative estimate. 

The third variable is ne which represents the number of planets per solar system that can support life. 

Here we have made very little progress because the tools and techniques we have to detect planets can only detect very large planets which have a measurable effect on the star which it orbits. 

Our estimates of this number should improve over the next several decades as new telescopes will be built, and new techniques developed, which will allow us to find smaller planets. 

The key thing which astronomers will be looking for are planets inside the habitable zone of a star. Also known as the Goldilocks zone, it is the zone that isn’t too close to the star, like Venus, nor too far away, like Mars. 

The 2013 Kepler mission concluded that there could be 40 billion Earth-sized planets within the habitable zone of stars, which would give this a value of 0.4, assuming there are 100 billion stars. 

Drake initially thought this value would be between 1 and 5.

The fourth variable is fl which represents the fraction of habitable planets that actually develop life.

Here too, we have no real clue what this number might be because we have yet to actually find life anywhere other than Earth.

However, there is a lot of activity on this front. Researchers on the origins of life on Earth have concluded that basic single-cell life on Earth appeared almost as soon as the planet was formed and cooled. 

Evidence from Mars has shown more water and other factors necessary for life than we originally thought.

Future missions to Mars, as well as some of the moons of Jupiter, might determine if life, or the building blocks of life, were able to form outside of the Earth. 

Drake initially thought that all planets in the habitable zone would develop life, but as of right now we have no clue if that is true or not. 

It probably isn’t 100%, but it also probably isn’t zero. 

The fifth variable is fi which is the fraction of planets with life that develop intelligent life, and here we really don’t have a clue. If we haven’t even found single-cell life outside of Earth, we can’t really even make a good estimate on how likely intelligence is to arise. 

There are a host of other problems with this. What is intelligence? Would dolphins on other planets be considered intelligent? How about a dog? 

The estimates for this variable vary widely. Some people think this might be close to 1, but others think it might be very close to zero.

One hypothesis is called the Rare Earth Hypothesis. This contends that simple cellular life might in fact be very common throughout the universe, but intelligent life like humans might be extremely rare.

Life doesn’t inexorably evolve to create intelligent life. The odds of an intelligent species evolving could be billions to one. 

Drake assumed this was near 100%.

The sixth variable is fc which is the percentage of intelligent civilizations which develop the ability to communicate through space. Basically, have they developed radio. 

Again, we have no clue what this might be. Could an intelligent species just plateau at a stone-age level of technology? Or maybe even a level like ancient Rome? 

Or maybe the level of technology required is something that we haven’t figured out yet and it isn’t electromagnetic radiation. It could be that we are the ones in the equivalent of the stone age by galactic standards. 

The final variable is L, which is the lifetime of the civilization. Even if a species were to evolve to become intelligent, and if they were to develop sufficient technology, there might only be a finite period of time where they are able to broadcast. 

They could be destroyed by war, environmental collapse, get hit by a meteor, or maybe their star explodes. 

Again, this is a huge unknown and estimates are all over the place. Estimates range from a few hundred years to infinite. 

So, if you take all seven variables, and make some best-guess estimates for each, what do you get?

Well, you get pretty much anything you want. Estimates have ranged from Earth being the only intelligent, technical civilization, to there being millions of technical species. 

You might be thinking that this is all pretty useless if we can’t even guesstimate an answer. However, it was really designed to be a way of thinking about the problem and starting a discussion, not necessarily an attempt to find a solid answer. 

One of the major criticism of those who use the equation to argue that there are many advanced civilizations, is “where is everyone?” 

This is known as the Fermi Paradox, and it will be the subject of a future episode. 

The Drake Equation was the first step towards trying to get beyond science fiction and really trying to understand if we are in fact alone in our galaxy. Over time, as we learn more, we’ll have a better idea of the values of the variables in the equation, and how likely it is that there are other intelligent civilizations. 


The associate producer of Everything Everywhere Daily is Thor Thomsen.

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You can also click on the link in the show notes.