If there are "zillions" of planets in the universe, then where are the aliens?

Have you ever, while enjoying the night sky, ever wondered about the size of the universe? To give you an idea, according to Tim Urban from Wait But Why, when you look at the sky on a particularly clear night, if you have the patience to count dots, you can pick out about 2, 500 stars.

And all these bright dots we see are just our closest neighbors - and make up only a tiny fraction of our galaxy. In fact, almost all of these stars are less than a thousand light years away from Earth, which means that what we really see when we look at the sky is something like this:

Did you feel small? Calm down ... you will feel even smaller! It is estimated that the Milky Way alone contains between 100 and 400 billion stars, and that is also the estimated number of galaxies in the observable universe, which is 90 billion light years in diameter! For all this adds up to a result of around 10 22 to 10 24 stars in the cosmos - or about 10, 000 stars for every grain of sand on the face of the earth.


According to Tim, there is a consensus among scientists that between 5% and 20% of all these stars are similar to our Sun. For, by conservative calculation - considering 5% of suns and 10 22 as our total number. stars - we estimate that there are 500 quintillion solar stars out there.

Scientists also believe that between 20% and 50% of these stars could have Earth-like planets orbiting around them, that is, worlds with temperatures and conditions that allow liquid water to exist and potentially host life forms. . Thus, once again considering the most conservative data, we can assume that there is a habitable planet orbiting about 1% of the stars in the universe.

That means there may be 100 quintillion worlds like ours out there - or, going back to the sand grain analogy, 100 planets for every grain on Earth! With this scenario in mind, it is virtually impossible not to think about the possibility that we are not alone in the universe, don't you think? Well let's continue with the speculations.


So imagine that after billions of years of existence, 1% of these planets similar to ours - or the equivalent of the earth's grains of sand - develops some kind of life, and of these, only 1% of these lifeforms advance. until we reach a level of intelligence like ours.

Thus, we should have 10 quadrillions of intelligent civilizations in the observable universe. And considering conservatively that the Milky Way has 100 billion stars and one billion habitable planets, then our galaxy should be home to some 100, 000 intelligent civilizations.

The odd thing, according to Tim, is that compared to the age of the universe, our sun is just a boy, and there are much older stars, orbited by planets like ours - only much older. That should have given plenty of time for much more advanced civilizations than ours to have developed, right?


Let's imagine the following: comparing the Earth, 4.5 billion years old, to a hypothetical planet of 8 billion, if this world has a history similar to ours, from the Big Bang so far, see at what point in its development civilization would be:

Now imagine this: If time travel were possible, think of the shock a medieval person would suffer if he were transported to our time. If so much has changed in a thousand years of history, have you ever wondered what it would be like to come across an intelligent civilization with millions of years ahead of ours? It would not be much to suppose that this society would be able to make interplanetary trips or even to colonize our galaxy.

For, according to Tim, they invented something called the Kardashev Scale - which brings together intelligent civilizations in basically three categories (I, II, and III) to make our lives easier. Thus, a type I civilization would be capable of using all the available energy on its planet, and we earthlings have not even reached this level yet.

A type II civilization would not only be able to harness all the energy on the planet, but would have the ability to harness the energy of its star. A type III civilization would be capable of controlling the entire galaxy and annihilating the other two if it wanted to. In other words, no world would like to have the misfortune to stumble with these people there!

Fermi's paradox

But back to speculation, if only 1% of intelligent life survived the time needed to reach the level of development of type III, in our galaxy alone there should be a thousand such societies. And considering the power these civilizations would have, their presence would surely be noted.

However, while everyone has heard stories of alleged contacts between Earthlings and aliens, intriguing as they may be, the truth is that none of these encounters has ever been proven. Even SETI personnel - an organization that looks for signs of extraterrestrial life - have not detected any signals with their various radio telescopes, whether radio, laser or otherwise. Never. So where's everyone?

We at Mega Curious have already talked briefly about Fermi's Paradox, which tries to explain why we haven't contacted alien beings yet, despite the countless number of potentially habitable planets in the universe. For the possible answers to this paradox are fascinating.

The big filter

One group of explanations is based on the theory that we find no evidence of the presence of type II and III civilizations simply because they do not exist. After all, estimates indicate that there are thousands or millions of over-advanced societies, so at least one should have made contact with us. And if that hasn't happened yet, there must be a reason - and scientists refer to that "reason" as the great filter .

The great filter establishes that at some point between the origin of life and the development of type III, there is a kind of obstacle that holds back the evolutionary process, making it virtually impossible for life to advance. And if this theory is correct, the key would be to find out at what point in the timeline the great filter arises.

Unique, pioneering or chipped?

In the case of humanity, depending on where the great filter is on the timeline, we have three possibilities: we are exceptionally rare, we are the first, or we are chipped. Since we are unique, we can assume that we have already passed the great filter - and that other life forms are incredibly unlikely to have reached our level of intelligence, which will say the level of Type III civilizations.

And if this is our case, then it is possible that the great filter is just in the beginning of the timeline, which means that it was practically a miracle that mankind emerged in the first place. This explanation is very valid, because if we consider that life took billions of years to appear on Earth, and that no one has ever been able to replicate this event in the laboratory, then there may be no other kind of life out there.

Another possibility, even if we are unique, is that only our civilization has managed to overcome the great filter - while there are thousands of life forms in the universe that can still take that step. On the other hand, if we are the first to go through the big filter, it may be that, for the first time since the Big Bang, the universe began to provide the necessary conditions for intelligent life to develop.

Think of Earth at the beginning of its existence and how it was bombarded by asteroids, and its surface was shaken by earthquakes and volcanic eruptions. For things had to settle in our world until the first sparks of life began to emerge. Now think of the extraordinary gamma-ray bursts that astronomers have been seeing lately in distant galaxies.

For, according to Tim, just as with our planet, it may be that only recently things have begun to soften in the Universe, ushering in a new astrobiological phase that, in turn, is allowing the evolution of life. Thus, both us and other life forms could be moving towards the level of development of Type III civilizations - and they would not have been detected because none exist yet.

We are chipped ...

Since we're not exceptionally rare, nor are we the first, so, according to Tim, we can only conclude that the big filter is yet to come - and that's bad news. Having this “hurdle” in our future suggests that life evolves periodically until it reaches a certain point in the timeline, and that sooner or later something keeps it from moving forward.

This obstacle could come in the form of catastrophic natural disasters - such as a gamma ray burst, for example - that occur from time to time. And the development of humanity is happening precisely during one of these intervals. It may even be that almost all civilizations eventually self-destruct after reaching a certain technological level. Who knows...

In fact, although everyone is hoping that Curiosity will find complex life forms on Mars, if that happens, instead of being incredible, the finding would be devastating to our species, as it would indicate that quite possibly the great filter. is before us - and that would mean we are chipped.

More Explanations

Another set of explanations for Fermi's Paradox dismiss the idea that there is a great filter of life - or that humanity is exceptionally rare or a pioneer. The theories here revolve around the assumption that the level of development of humans is nothing special, and the assumption that type II and type III civilizations do exist, but there are obvious reasons why we have not found traces of them yet.

Thus, according to Tim, it may be that advanced civilizations visited the earth, but before humans appeared about 50, 000 years ago. For if the ETs visited us before then, they probably scared a flock of prehistoric birds and that's it. In addition, written history has existed for some 5, 500 years, so who guarantees that cavemen did not have “immediate contacts” but left no record for us to discover?

One more possibility is that there is a lot of activity in space, but our technology is not advanced enough to detect any signal - or that we are focusing on the wrong thing. Like when you walk into a building with a walkie talkie, and after trying to communicate with someone and getting no answer (because everyone is using WhatsApp, obviously!), You conclude that there is no one there.

It may also be that the earth is in a kind of desert zone of the galaxy, where intelligent civilizations think there is no one. Another idea is that these advanced societies have reached such an absurd level of development, and created such a perfect environment for their very existence, that they simply have no interest in going around searching the Universe for life.

It may also be that advanced civilizations are out there, but we are too primitive to perceive their presence - just as ants from an anthill located next to an airport would probably not realize the presence of the entire structure around them, nor understand the because it was built. Or, the higher civilizations are around, but only watching us and avoiding contact because we are inferior.

Beyond those possibilities, who guarantees that there are no bloodthirsty civilizations out there, but our neighbors know it, and are quiet enough not to get their attention - while we fools are sending signals through the Universe to make contact? Or perhaps there is a single murderous civilization prepared to annihilate anyone beyond a certain level of development.

Another possibility is that governments know that aliens exist, and even exchange ideas with them from time to time, but they are hiding everything from us. Or maybe the universe is not what we think, and our reality is completely different from what we believe in - and we actually live in a hologram or inside a Matrix!


Regardless of what the correct answer to Fermi's Paradox is - any of them would be surreal - the truth is that humanity will continue to look for signs of intelligent life off Earth, and find that we have company or, on the contrary, that we are completely alone in the world. Universe, would be equally scary.

In addition, the Paradox also leads us to evaluate our own existence, and to face the fact that deep down we know very, very little about the cosmos, ourselves, and everything around us.

And it may be that within a few centuries, when our descendants look back, they see us just as we see our ancestors today - who believed that the earth was flat and that it was the center of everything - and find that we had no idea what reality was.

* Originally posted on 16/07/2015.