The Drake Equation is an attempt to put together all the variables that would be relevant to establishing the number of intelligent civilizations that existed in the Milky Way galaxy and which were broadcasting radio signals at this particular point in time. The Drake Equation consists of seven terms.

Back in 1950 a very smart and talented physicist Enrico Fermi was taking a lunch break away from making nuclear bombs at the Los Alanos National Laboratory , when he asked a question to his companions 

“Where are they?” ,



alien image 1

What he meant by this was, considering the age of the universe and the mind boggling number of stars and planets that could possibly sustain life , shouldn’t by now the universe be blooming with life? much of it being intelligent ? and even some on it being able to travel among the stars ?

What is more shocking to you:

  1. That there are thousands of alien civilizations in the universe?
  2. Or that we are alone ?

In September 1959, physicists Giuseppe Cocconi and Philip Morrison published an article in the journal Nature with the provocative title “Searching for Interstellar Communications“. Cocconi and Morrison argued that radio telescopes had become sensitive enough to pick up transmissions that might be broadcast into space by civilizations orbiting other stars. Such messages, they suggested, might be transmitted at a wavelength of 21 cm (1,420.4 MHz). This is the wavelength of radio emission by neutral hydrogen, the most common element in the universe, and they reasoned that other intelligences might see this as a logical landmark in the radio spectrum.

Two months later, Harvard University astronomy professor Harlow Shapley speculated on the number of inhabited planets in the universe, saying “The universe has 10 million, million, million suns (10 followed by 18 zeros) similar to our own.

One in a million has planets around it.

Only one in a million million has the right combination of chemicals, temperature, water, days and nights to support planetary life as we know it. This calculation arrives at the estimated figure of 100 million worlds where life has been forged by evolution.

There are more stars in the universe than there are individual grains of sand here on Earth!

star image

Just think about this fact for a while and let it sink in.

Well if that was true, we should have been visited by intelligent species time and time again. But instead in our decades of search for extra terrestrial species we till now haven’t even come close to find one yet.

This famous question of Fermi came to be known as the Fermi Paradox.

fermi paradox

And ever since the 1950 ,people have tried to explain it. And probably the most famous attempt to explain it was something called the “Drake Equation”.

Back in 1961 an astronomer named Frank Drake


tried to solve this astronomical conundrum. He listed all the questions that scientists agreed to answer in order to predict detectable alien civilization in our galaxy. He realized that if we multiplied all these terms and factors together we would be able to find an estimate for the same. He conveyed his equation officially in the SETI Institute which stands for Search for Extraterrestrial Intelligence where Carl Sagan was also also present.

seti institute

The Drake Equation is surprisingly simple and Drake intended it to be that way.

It is a probabilistic formula that uses at this point random guesses and wild speculations as inputs as we don’t know a lot about the universe or life at this moment .But don’t get me wrong, the logic behind the equation is pretty sound!

The Drake Equation goes as ,

drake equation

Lets explore what each of these terms mean individually :

n factor

It is the number of civilizations , whose communications we would possibly detect within the milky way galaxy or in other words its the number of alien civilizations we would communicate with, which is the whole point of this equation.

r* factor

This is the rate of star formation. (find more about how stars are formed)

star formation

Astrophysicists have determined that in the Milky Way about 3 solar masses worth of material forms into stars every year. Now solar mass is basically the mass of our sun and 3 times that doesn’t mean that 3 suns are made, it could be a massive sun or combination of many small stars. Latest calculations from NASA and the European Space Agency indicate that the current rate of star formation in our galaxy is about 0.68–1.45 M (solar mass) of material per year. To get the number of stars per year, this must account for the initial mass function (IMF) for stars, where the average new star mass is about 0.5 M. This gives a star formation rate of about 1.5–3 stars per year.

So we do have a value for it = 3 , so the equation becomes ,

drake equation 1

Next we have,

fp factor

It stands for the fraction of the stars that have planetsRecent analysis of microlensing surveys has found that fp may approach 1—that is, stars are orbited by planets as a rule, rather than the exception; and that there are one or more bound planets per Milky Way star.



ne factor

Number of planets capable of supporting life and planets that orbit the star in its Goldilocks zone,

goldilocks zone

where things are not to hot or too cold , but just right. We call these exo-planets and we have been discovering them like crazy lately. There could be possibly billions of such planets in the milky way aloneIn November 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of sun-like stars and red dwarf stars within the Milky Way Galaxy. 11 billion of these estimated planets may be orbiting sun-like stars. The nearest planet in the habitable zone may be as close as 12 light-years away, according to the scientists. 

Till now atleast 3400 such exo-planets capable of supporting life within the milky way galaxy have been found.

∴ ne = 3400 and the equation becomes :

drake equation 3


From here onwards the equation keeps getting unclear,

fl factor

Stands for the fraction of those exoplanets where life evolves. So far we can say with absolute certainty that life has only evolved once. And since we are talking about ourselves we cant really put that in the equation, because we are not trying to communicate with ourselves. There has been a lot of exoplanets that are beinh discovered now due to the modern techniques in astronomy.


Geological evidence from the Earth suggests that fl may be high; life on Earth appears to have begun around the same time as favorable conditions arose, suggesting that abiogenesis may be relatively common once conditions are right. However, this evidence only looks at the Earth (a single model planet), and contains anthropic bias, as the planet of study was not chosen randomly, but by the living organisms that already inhabit it (ourselves). From a classical hypothesis testing standpoint, there are zero degrees of freedom, permitting no valid estimates to be made.

If life were to be on MarsEuropaEnceladus or Titan that developed independently from life on Earth it would imply a value for fclose to 1. While this would raise the degrees of freedom from zero to one, there would remain a great deal of uncertainty on any estimate due to the small sample size, and the chance they are not really independent.

fi factor

It is the fraction of life that has developed intelligence. 

alien intelligence

This value remains particularly controversial. Those who favor a low value, such as the biologist Ernst Mayr, point out that of the billions of species that have existed on Earth, only one has become intelligent and from this, infer a tiny value for fi. Likewise, the Rare Earth hypothesis, notwithstanding their low value for ne above, also think a low value for fdominates the analysis. 

Those who favor higher values note the generally increasing complexity of life over time, concluding that the appearance of intelligence is almost inevitable, implying an fi approaching 1. Skeptics point out that the large spread of values in this factor and others make all estimates unreliable.

fc factor

Fraction of that intelligent life that develops detectable communication , like radio waves. Drake himself believed that the extra terrestrial life form would use radio signals to communicate , just like we do here on earth.

alien communication

For deliberate communication, the one example we have (the Earth) does not do much explicit communication, though there are some efforts covering only a tiny fraction of the stars that might look for our presence to the alien civilization.

There is considerable speculation why an extraterrestrial civilization might exist but choose not to communicate. However, deliberate communication is not required, and calculations indicate that current or near-future Earth-level technology might well be detectable to civilizations not too much more advanced than our own. By this standard, the Earth is a communicating civilization.

We have been telegraphing evidence of our technological civilization for nearly 80 years now. There are hundreds of stars in 80 light year from earth.

Another question is what percentage of civilizations in the galaxy are close enough for us to detect, assuming that they send out signals. For example, existing Earth radio telescopes could only detect Earth radio transmissions from roughly a light year away.

l factor

This is the final term of the Drake Equation and honestly it is a little bit depressing  It stands for the average length of time a technological civilization stays detectable, which simply means the longevity of communication.

radio telescope

Drake himself was worried that an advanced civilization might not be detectable for very long, its possible that many civilizations are detectable for only a short period of time , even though they continue to thrive for 1000s of years.

So whats the answer?

As many skeptics have pointed out, the Drake equation can give a very wide range of values, depending on the assumptions, and the values used in portions of the Drake equation are not well-established. In particular, the result can be 

  1. N ≪ 1, meaning we are likely alone in the galaxy,
  2. or N ≫ 1, implying there are many civilizations we might contact.

One of the few points of wide agreement is that the presence of humanity implies a probability of intelligence arising of greater than zero.

As an example of a low estimate, combining NASA’s star formation rates, the rare Earth hypothesis value of fp · ne · fl = 10−5,  Drake’s values of  :

R = 1.5–3 yr−1fp · ne · fl = 10−5,  fi = 10−9, fc = 0.2  and L = 304 years


N = 1.5 × 10−5 × 10−9 × 0.2 × 304 = 9.1 × 10−11
i.e., suggesting that we are probably alone in this galaxy, and possibly in the observable universe.


On the other hand, with larger values for each of the parameters above, values of N can be derived that are greater than 1. The following higher values that have been proposed for each of the parameters:

R = 1.5–3 yr−1 fp = 1ne = 0.2 fl = 0.13 fi = 1 fc = 0.2 , and L = 109 years.

Use of these parameters gives:

N = 3 × 1 × 0.2 × 0.13 × 1 × 0.2 × 109 = 15,600,000
i.e it suggests that our universe might be filled with alien species yet to be discovered.
earth found

Monte Carlo simulations of estimates of the Drake equation factors based on a stellar and planetary model of the Milky Way have resulted in the number of civilizations varying by a factor of 100.

Conclusion :

Drake himself maintains an air of optimism. He has a strong conviction that extra-terrestrial civilizations exists in our galaxy today. He estimates that there are about 10,000 of them in the milky way right now. Which sounds like a lot right?

But even if all of them were detectable it would still take a long time to find even one. If our galaxy had a minimum of a 100 billion stars than you need to search 10 million stars before you start to worry that there arent so common .

The major problem with this equation that i believe is the answer can be big 0, if any one of the factors ever turns out to be 0 . And this is a problem. So astrobiologists ,  astrophysicists and astronomers will keep searching the universe and our milky way galaxy and the data they come back with , will be plugged by some people into the Drake equation to see if it will eventually spit out a verifiable answer.

But in the mean time it is serving as a great tool for getting people to think about the possibility of alien lives and not just that , but about the future of our own human civilization as well.

This equation is like a funnel that keeps straining out the various odds at various stages and keep fine tuning itself as we progress with our knowledge of the cosmos. And who knows oneday we might make our first contact.

More articles on ALIENs will be coming soon.

please leave a comment below or ask any question you want to regarding this article

I would highly recommend few books that would really help you to know  in depth regarding the cosmos:

  4. A Brief History of the Universe: From Ancient Babylon to the Big Bang (Brief Histories)
  5. The Physics Book: From the Big Bang to Quantum Resurrection, 250 Milestones in the History of Physics (Sterling Milestones)
  7. Cosmos
  8. Relativity: The Special and the General Theory (Routledge Classics)
  9. Black Holes: The Reith Lectures
  10. The Oxford Companion to Cosmology (Oxford Quick Reference)



I am Mayukh Bagchi. 4th year Undergraduate Engineering student at SRM University , physics and in general Astrophysics is my passion and I want to pursue the same. This blog is dedicated to serve the community with honest and clear insights to the cosmos.


J Bagchi · June 29, 2018 at 5:23 pm

Great job!

    Mayukh · June 30, 2018 at 4:33 am

    Thank you

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