**Are we alone in the universe? A question that has occupied and fascinated the human mind for generations. Our understanding of the universe, although still relatively small in the grand scheme of things, is still increasing every year. The existence of up to 4,300 exoplanets has already been confirmed by NASA, and scientists are estimating there are approximately 100 billion to 400 billion planets in our own Milky Way (The Milky Way being just one of an estimated 200 billion galaxies in the universe!). Therefore even if the probability of life developing on other planets might be extremely small, why haven’t we yet encountered any concrete evidence of extraterrestrial life?**

### The Drake equation

A simple way to give this a bit of a better perspective is the Drake Equation, named after its writer Frank Drake. Frank Drake wrote his equation in 1961 and his equation has since proven a valid basis for searching for extraterrestrial life. The Drake Equation specifies the main criteria one must look into when searching for potential signs of other life. The equation is:

N = R_{*} ⋅ f_{p} ⋅ n_{e} ⋅ f_{1} ⋅ f_{i} ⋅ f_{c}⋅ L

where:

N = the number of civilizations with whom communication is possible

R_{* }= the average rate of star formation

f_{p}_{ }= the fraction of stars that have planets

n_{e}_{ }= the average number of planets that could support life per stars that have planets

f_{1} = the fraction of habitable planets that develop life at some point

f_{i} = the fraction of planets with life that develop intelligent life

f_{c}= the fraction of intelligent life that develop enough to release detectable sign into space

L = the length of time for which such intelligent life releases detectable signs.

Sadly though, this equation doesn’t give us any concrete answers. Although scientists agree with the argumentation behind Drake’s equation, many state that some of the values of the equation, such as the fraction of planets that might develop life at some point, are (very) hard to estimate or just unknown. For instance, back then using very optimistic values in the formula can give a result of around 50,000 civilizations, while very pessimistic values return a number smaller than one, suggesting we are indeed alone in the universe. Despite the inaccuracy of Drake’s equation, it still offers us a perspective of which criteria we have to look into when searching for signs of extraterrestrial life. Instead of giving humanity a concrete answer, it has helped us think about how we can best organize our search for extraterrestrial life.

### A different perspective

However, with the amount of knowledge about the universe we have obtained since the introduction of the Drake Equation, a study from 2016 took another look at the Drake equation with modern data. Instead of asking themselves how many civilizations there may be in the universe, this study reversed the question: Are we the only intelligent life form ever formed? Although the fraction of intelligent life forming still is unknown, by making some smart changes to the formula they were able to limit the formula to just 3 terms:

A = N_{ast} ⋅ f_{bt}

where:

A = the number of technological species to ever have formed

N_{ast }= the number of habitable planets in a given volume of our universe

f_{bt }= the probability of a technological species developing on such a planet

Since R_{* }, f_{p }and n_{e} are relatively well known today, we are able to put them together in just the single term N_{ast}. Furthermore, by rephrasing the question to ‘Has intelligent life ever formed’, the scientists involved in the study were able to eliminate the L term. Lastly, they combined the terms f_{1} , f_{i} and f_{c} into a single term: f_{bt}.

With this newly constructed equation, one can estimate the probability that us humans are the only intelligent life to have ever formed in the universe so far. We can set A equal to one, since we assume we humans on earth are the only intelligent life form to have developed so far. N_{ast} can also be calculated, with new data suggesting there are 2*10^{22} stars, with on average half of them hosting a habitable planet. Inserting these numbers into the equation results in a probability of only 1 x 10^{-22}, or one in 10 billion trillion! An incredibly small number which would suggest that we are not unique in the universe, especially considering earlier pessimistic probabilities like one in a quadrillion (1 in 10^{15}) would still leave us with an estimated 10^{7}civilizations.

### Fermi Paradox

Despite estimations suggesting a high number of other civilizations, we are yet to find any conclusive evidence of their existence. This leads us to the following question: if the estimated number of other civilizations is so high, why have we not seen any sign of them yet? This contradiction is also more commonly known as the Fermi Paradox.

One possible explanation for the Fermi Paradox is that such civilizations live for just a very short time, causing them to only broadcast for a brief period of time. For instance, our civilization hasn’t even been able to broadcast and receive radio signals for more than 100 years, while our earth formed over 4.5 billion years ago. Some signal from a distant civilization might have reached us a long time ago, but without our current technology we would have missed it. Or with the great distance a signal might need to travel, it could take centuries for such a signal to be received on earth.

Despite all the explanations we might have, they all just remain speculations for now. Even with all the knowledge we have obtained over the years so far, we are limited by the technology at the moment to search more precisely. Despite these limitations, with the enormous size of the universe and the number of habitable planets it contains, the odds seem to be in our favor. It would be rather grim however if we truly are the only ones out there.

*This article was written by **Patrick Jans*