Are we alone in the universe?

As a child I was obsessed with films and TV shows where aliens came to visit Earth, or humans ventured off into space to find new lifeforms. I loved E.T. the Extra Terrestrial, the Ewoks in Star Wars and Widget the World Watcher. But as I grew older, just like learning that Santa Claus isn’t real, I had to face the sad reality that my favourite characters were just that, characters. But what if the universe was actually teeming with life? What if there really were strange and mysterious creatures out there? And If they did exist, how would we know?

Could there be other civilisations out there like ours? Image credit: Alex Iby via Unsplash


In 1959 physicists Guiseppe Cocconi and Philip Morrison published an article “Searching for Interstellar Communications” in the journal Nature. The paper argued that technology had advanced to the point that radio telescopes were now sensitive enough to pick up signals transmitted by civilisations orbiting other stars. This was an exciting realisation and several months later astronomer and astrophysicist Frank Drake began the first systematic search to try and find signals from communicative extraterrestrial civilisations. Though well designed, Drake’s research found no signals, but this didn’t discourage him.

Shortly afterwards Drake hosted a “search for extraterrestrial intelligence”, gathering with other great minds of the time, like Carl Sagan, to discuss detecting alien radio signals. In Drake’s words, “As I planned the meeting, I realised a few days 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 civilisations in our galaxy. This was aimed at the radio search, and not to search for primordial or primitive life forms.” And thus the famous drake equation was born.

The Drake equation

N =R∗ x fp x ne x fl x fi x fc x L


But what does this all mean?

N = the number of civilisations in our galaxy with which communication might be possible

R = the average rate of star formation in our galaxy

fp = the fraction of those stars that have planets

ne = the average number of planets that can potentially support life per star that has planets

fl = the fraction of planets that could support life that actually develop life at some point

fi = the fraction of planets with life that actually go on to develop intelligent life (civilisations)

fc = the fraction of civilisations that develop a technology that releases detectable signs of their existence into space

L = the length of time for which such civilisations release detectable signals into space


With 100 billion planets in the Milky Way, it’s hard to imagine we are alone. Image credit: Joshua Earle via Unsplash


Some of these factors we know to a high level of certainty. But others are very hard (if not impossible) to know, and even more difficult to estimate. This means that predictions made using the Drake equation can vary wildly.

The rare Earth hypothesis

The rare Earth hypothesis argues that the origin of life and the evolution of complex biological organisms on Earth required an improbable combination of astrophysical and geological events and circumstances. Viewing the Drake equation from this perspective and using the lowest values for the factors in the equation yields a probability that there are 0.000000000091 civilisations with which communication might be possible. In this case, sadly it is very likely that we are all alone in the galaxy and probably the universe.

The principle of mediocrity

Carl Sagan and Frank Drake disagreed with the rare Earth hypothesis and instead supported the principle of mediocrity, a claim that the Earth is a typical rocky planet, in a typical planetary system, located in a non-exceptional region of a common barred-spiral galaxy. Using larger values for the factors in the Drake equation yields a probability of there being 15,600,000 civilisations that we might be able to communicate with in our galaxy alone. In this case the Milky Way is teeming with intelligent life. This prediction led to physicist Enrico Fermi pointing out a paradox between the contradiction that the universe has such a high predicted number of intelligent civilisations, however we are yet to discover any evidence of such civilisations. Fermi famously asked a question we’d all like to know, “Where is everybody?”

There are a number of important criticisms of the Drake equation. Several of the terms are based entirely on conjecture, and many of the problems stem from our lack of scientific understanding of abiogenesis, how life is initially formed. Without a strong grasp of what gives rise to life in its very early stages, we are limited in our approximations of the abundance of life across the universe.This means that the error margin for the equation is huge, making the results of its predictions relatively meaningless and unscientific. And furthermore, Drake’s equation only gives us variables which can show us estimates about lifeforms that evolve to the point of being capable of developing communication technologies as advanced as our own. This tells us nothing about the abundance of less intelligent lifeforms in our galaxy and the universe. 

So, is there a real life Ewok floating somewhere out there in space? I guess I’ll never know, but at least the Drake equation has given me a new hope.