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The Drake Equation Explained: The Search for Intelligent Alien Life

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With interests in science and nature, the author explores topics from a unique and sometimes controversial perspective.

The Drake Equation gives us a way to estimate the number of intelligent alien civilizations in our galaxy.

The Drake Equation gives us a way to estimate the number of intelligent alien civilizations in our galaxy.

What Is the Drake Equation?

The Drake Equation is a mathematical calculation that attempts to predict the number of intelligent, detectable alien civilizations in our Milky Way Galaxy. Developed back in 1961 by astrophysicist Dr. Frank Drake, it provides some interesting food for thought when looking up at the stars and pondering the question: Are we alone?

In the search for intelligent alien life, the Drake Equation doesn’t give us any solid answers. The solution varies widely depending on the numbers you plug in, some of which are unknowable. But it certainly has helped to pinpoint the questions we should be asking.

It also proves that the discussion of extraterrestrial intelligence doesn’t have to be focused on the whimsical and the supernatural. This has nothing to do with extraterrestrial visitation or the UFO phenomenon. This is simply an equation that attempts to predict the chances of advanced alien life existing, somewhere out there.

This article looks at the variables in the Drake Equation and some of the potential values we could assign to them. In some cases, modern-day astronomers can make some pretty accurate guesses. In other cases, your guess is as good as anyone else’s.

The Drake Equation is written as:

N = R* • fp • ne • fl • fi • fc • L

R* = The rate of star formation in the Milky Way Galaxy.

fp = The fraction of those stars with planetary systems.

ne = The fraction of those planets capable of sustaining life.

fl= The fraction of those planets on which life actually appears.

fi= The fraction of that life which evolves to be intelligent.

fc = The fraction of the those intelligent civilizations that develop technology we can detect on Earth.

L = The length of time such civilizations survive and release emissions into space.

R* = The Formation of Stars

How many stars are out there in the Milky Way Galaxy, and how often are new stars created? Current estimates put that number between 100 and 400 billion.1

That’s a lot of stars, and obviously, stars are very important in this equation because they are required to form solar systems. Our own solar system, and our own life-giving planet, are possible because of our star, the sun. Could there be other similar systems out there?

The potential for over 100 billion stars in our galaxy is good news for anyone hoping to find proof of intelligent life, but this is only the base on which the rest of the equation rests.

fp = Number of Stars with Planets

The variable fp represents the number of stars with planets orbiting them. Not all stars have planets around them, but researchers now believe most do.2 They have found evidence of several hundred solar systems, but that is likely only the tip of the iceberg. By some estimates, there could be as many as 160 billion planets in the Milky Way.3

But, as we know, all planets are not created equal. Worlds similar to Jupiter or Saturn may not help us much. What we really need to know is the number of planets where creatures like us could live.

ne = Habitable Planets

This is represented by the ne variable of the Drake Equation. Earth-like planets are said to exist in the Goldilocks Zone4, meaning they must orbit at the appropriate distance from their star. Any closer and the planet will be too hot like our own Venus, and any further away the planet will be too cold like our Mars. Here on Earth, it’s just right.

Estimates of how many habitable worlds may exist in the Goldilocks Zone of solar systems around the Milky Way again reach into the billions. But the potential places we may find life could be even wider than that.

For example, we now know that life can thrive in the depths of our ocean where we once thought it impossible, congregated around hydrothermal vents in the Earth’s crust. This gives hope that perhaps life may exist on some worlds in our own solar system, such as beneath the frozen surface of oceans on Jupiter’s icy moon Europa.

fl = The Development of Life

Just because life can develop doesn’t mean it will. Looking up at the vastness of space on a starry night, it is tough not to wonder how it could be possible we seem to be all alone. Statistically, there must be other life in the galaxy besides us. So, where is everybody? Indeed, this is the very question put forth by Enrico Fermi back in 1950, now known as the Fermi Paradox.

So, on how many planets in the Milky Way Galaxy has life arisen? So far the only known answer is one. Researchers have long hoped to find proof of life elsewhere in our own solar system, even on a microbial level. There is some compelling evidence that it may exist, or may have in the past. But, officially, thus far it’s just us.

Intelligent life is rare on Earth. Could it be common elsewhere?

Intelligent life is rare on Earth. Could it be common elsewhere?

fi = Intelligent Life

But simply discovering other life forms somewhere out there in the galaxy isn’t really what the Drake Equation is all about, though that would certainly be pretty cool. We’re looking for intelligent life, and this is what the fi variable describes.

Even if life is abundant on other planets, only a fraction of it will advance to develop intelligence anything similar to our own. We can see this on our own Earth. Obviously, we Homo sapiens are the best nature has done, so far anyway, but there have been other human species on this planet we can call intelligent. Still, given the number of creatures that have come and gone over our Earth’s history, the likelihood of intelligent life developing appears very low.

There have been various attempts to predict the probability of intelligent life around the galaxy but they all have one flaw. That is, any theories and assumptions are based on just one example: Earth. Earth-type planets where intelligent life actually develops may be common in the Milky Way, or Earth may be unique. As of now, we really have no way of knowing.

fc = Technological Development

The next variable (fc) represents the fraction of intelligent civilizations that will develop technology that we may detect. This may mean we pick up electromagnetic radiation or other sources of communication intended for their own consumption. It may mean we pick up a signal intended for our ears. It may mean we see signs of a cataclysmic war or accident that impacted their solar system.

This variable is perhaps the most important part of the Drake Equation for one simple reason: The Milky Way is massive. We possess nowhere near the technology needed to travel to other planets outside of our solar system and check them out ourselves. Even a journey to the nearest star would take thousands and thousands of years. This means we are limited to long-range observational methods of exploring our galaxy, at least for the foreseeable future.

Put plainly: Unless intelligent life is detectable, we will never know they are there. In fact, when you consider that our civilization has only been sending signals into space for a bit over a hundred years, it’s certainly possible there are other intelligent civilizations out there looking for people like us, but we remain undetectable to them.

L = Time

The final variable concerns the passage of time. Since (as far as we know) any signal emanating from an alien world can’t travel faster than the speed of light, the chances of our detection of any such alien life are directly related to how long they have been around. In other words: The older a civilization is, the further its transmissions would have traveled into space. Young civilizations that far away remain undetectable until their emissions reach us.

Furthermore, we can assume as a civilization ages its technology will progress, and logically it stands to reason that their methods of communication and travel, and even their weapons, would have improved to where they are perhaps easier to spot.

That is, of course, if they don’t destroy themselves first.

The Milky Way is vast. If there is intelligent life our there, will we ever find it?

The Milky Way is vast. If there is intelligent life our there, will we ever find it?

Drake Equation Estimates

If you are the optimistic sort, by this point you may be convinced that, at least according to the Drake Equation, the existence of intelligent alien life in the Milky Way is an absolute certainty. If you are a little more pragmatic you likely realize that if the correct value for any variable in the equation is zero then the whole thing falls apart.

We know for sure that some of those variables aren’t zero, but once we get about halfway through the equation we are forced to make guesses. Some scholars (and bloggers) have put out some very interesting ideas about how we may arrive at reasonable values for these unknown variables.

Ultimately, we can’t know what we don’t know.

Drake himself guess there could be 10,000,000 intelligent civilizations capable of contact5. Bear in mind this was based on the knowledge of our galaxy fifty years ago.

Estimates have varied ever since, with numbers two to three times higher, down to the ultimate low: Nil. If you find yourself bored at work or in class you may try plugging in your own values and seeing what you come up with.

Is There Intelligent Alien Life in the Milky Way Galaxy?

Consider our own Earth and how it relates to the Drake Equation. We have our star. We have our planet, which is capable of sustaining life. Intelligent life has developed and it is capable of sending detectable emissions into space. We check all the boxes up to this point.

But, even though life on Earth has existed for billions of years, we so-called intelligent humanoids have only been around for about 200,000 of those years. We’ve only been living in arrangements that could be called civilization for a few thousand years, and, as we’ve seen, we’ve only had the technology to reach out into space for the last hundred of those years.

And already we seem to be on the brink of destroying ourselves and our planet. So, what are the chances that an intelligent alien civilization that sends out a signal which travels over ten thousand light-years is still around by the time we detect it?

Using conservative numbers, the Drake Equation gives us a logical reason to conclude there is a probability that alien life is (or was) out there, somewhere. Even in cases where the ultimate solution is zero, there is still a strong case for intelligent life we can’t detect, either because their technology is insufficient or because they haven't been around long enough.

Until not very long ago, that was us. We are just beginning our outreach into the Milky Way and a decade from now it is very likely that even more of the variables in the Drake Equation can be filled with scientifically plausible values.

Alien Life: Do You Believe it is Out There?


  1. How Many Stars in the Milky Way?,
  2. Nearly Every Star Hosts at Least One Alien Planet,
  3. 160 Billion Alien Planets May Exist in Our Milky Way Galaxy,
  4. Goldilocks Zone,
  5. Drake Equation: 55 Years Old,

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.


cryptid (author) from USA on November 13, 2015:

Good point, jackclee. Possibly moons and other natural satellite objects should be considered as part of the ne variable, that is whether or not a planet is capable of sustaining life.

Jack Lee from Yorktown NY on November 13, 2015:

I think the drake equation is too optimistic. One cannot underestimate the importance of our moon for life on earth. The goldilock zone is necessary for a planet to support life as we know it. However, the moon is crutial to various life cycles including the monthly menstral cycle. What percent of the planets have a moon comparable to ours? In the latest effort for SETI, it would be wise to look for planets with moons of comparable size to earth.

cryptid (author) from USA on November 13, 2015:

Thanks Glenn! I agree about the time thing too. It's quite a conundrum, because logically it seems very likely that some kind of life must exist out there. But the distance/time thing, when you think about it, makes it appear fairly unlikely and any two civilizations would ever know about each other.

Glenn Stok from Long Island, NY on November 12, 2015:

Great review of all the elements of Drake's Equation. I enjoyed reading you hub very much.

Of all the elements of Drake's Equation, time creates the biggest problem. The farther away any intelligent life is in space, the longer it takes for radio waves to travel. Therefore the greater the chances are that both will miss each other's transmission while one or the other is still alive.

cryptid (author) from USA on August 24, 2015:

Thanks for all the positive comments over the weekend everyone!

Certified Noob on August 24, 2015:

Interesting hub.IMHO there are only two possibilities . 1. Alien life more intelligent than humans does not exist. 2 . If it does exist and is more advanced than humans then they don't really like us and don't want to have any contact with humans.

kjforce from Florida on August 22, 2015:

Enjoyed your very well researched and informative pretentious of us as Humans to think or presume that we are the only intelligent life form in the planetary system.there maybe others, however perhaps at different levels. more or less advanced..hmmm just my thoughts..

thanks for sharing..

Buildreps from Europe on August 21, 2015:

The universe must be teeming of life, that's for sure, but why did no one ever captured an UFO properly? People tend to think that life must look like us, and that they must fly around in things like we do. People are able to capture almost anything on cam, up to the most weird car accidents. Even meteors that come down to earth are captured. So, why no UFO? Because there aren't any. This seems to prove that humanity is still one of the most developed species in their kind. Nice Hub. Well done.