When will they come? Because they will come, for sure

The human being has spent centuries wondering about the existence of intelligent life in other places of our Galaxy.

This interest, perhaps desire, to know that we are not alone is evident in the large number of successful cinematographic works that revolve around the subject.

In fact, half of the twenty highest grossing films of all time are based on the premise that aliens exist.

It is logical to think that we are not alone in the universe.

It is enough to look at the sky on a dark night to see, with the naked eye, thousands of stars like ours that, in addition, only represent a small fraction of the approximately 250,000 million stars that the Milky Way contains.

Just 25 years ago we could still wonder if ours, for some reason, was special, the only one hosting planets.

However, since the discovery of the first extrasolar planet in 1995, the existence of 4,296 planets and 3,188 planetary systems has been confirmed.

In fact, today we know that special stars are those that do not host planets.

Most of the stars in our Galaxy are considerably older than our Sun, which, with an age of 4.5 billion years, is crossing the meridian of its life.

Our planet formed around the same time, but was not trampled on by our species until much later.

The oldest remains of

are only 350,000 years old and, in this short period of time, humans have gone from chasing fire to building space probes that, like New Horizons, are capable of transcending the limits of our System Solar.

This leads us to ask ourselves: if despite the relative youth of our planet, our species has developed technology capable of traveling a distance of 5.5 billion kilometers, such as the one that separates Pluto from Earth, there should be civilizations with sufficient capacity, not only to escape their planetary systems, but to colonize the entire galaxy.

Knowing that there are very old planets, we are left with the alternative that civilizations cannot survive on them for long.

However, we do not have any evidence for the existence of other extraterrestrial civilizations.

We have been looking for signs for half a century to no avail.

This tension between the high probability of not being alone in the universe and the absence of any trace of life is known as the Fermi paradox, in honor of Enrico Fermi who, already in 1950, posed this question.

One possible solution to this paradox is that the premises are wrong.

Specifically, the premise that there are civilizations much older than ours.

Knowing that there are very old planets, we are left with the alternative that civilizations cannot survive on them for long.

This, in principle, could be due to self-destruction (development could be inevitably linked to a destruction of natural resources), but it can also be linked to astrophysical phenomena.

A first external threat to our civilization is that the Earth is hit by an asteroid or comet.

In 1994 we observed how the collision of Comet Shoemaker-Levi with Jupiter produced a series of huge fireballs.

It is believed that the collision of a much smaller body with the Earth, about 65 million years ago, triggered the extinction of the dinosaurs.

Some small primitive mammals survived, but any organism the size of a human being would almost certainly have been wiped out.

Most comets that approach Earth have their origin in the Oort cloud, a gigantic sphere that could host between one and one hundred billion objects with masses that can reach up to five times that of Earth.

The objects of the outer Oort cloud are very little gravitationally bound to the Sun and this means that disturbances due to the passage of other stars or gas clouds in our galaxy can affect them and cause them to be thrown towards the inner solar system.

Right now the passage of a star near the Solar System is unlikely, since our planet is in a region with few stars, on the outside of the galaxy, right between two spiral arms, which are regions with higher density.

However, our position in this quiet part of the galaxy has not always been and will not always be.

In particular, it is estimated that our Solar System encounters a spiral arm every 100 million years or so.

When this happens, we will have many stars passing at relatively close distances, which will "push" hundreds of thousands of objects from the Oort cloud towards the center of the Solar System.

It is enough that one of them hits our planet to end our civilization.

On the other hand, in the spiral arms is where most of the stars in our galaxy are born and die.

The highest mass stars die in impressive explosions, called supernovae, releasing enormous amounts of energy.

A single supernova explosion releases trillions of trillions of trillions of times more energy than the most powerful nuclear weapon ever built by man.

Well, in our Galaxy one of these explosions takes place, on average, every ten years.

True, most readers have been on the planet for more than ten years, but of course, our Galaxy is very large and when a supernova explodes at the other end we do not feel its effects.

With some simple calculations, we can estimate that the distance a supernova would have to explode to destroy us is approximately 30 light-years.

We can count up to 100 stars that are currently at this distance, although, breathe easy, none of them will end their days as supernovae, since only the stars of more mass do.

However, when the Earth approaches a spiral arm, this number will increase enormously.

These events are inevitable and we know they will happen.

In fact, about 10 million years ago a supernova exploded close to Earth, about 100 light-years away.

We can see radioactive isotopes on the ocean floor that clearly come from this explosion.

If this supernova had been only three times closer to our planet, it would have destroyed its atmosphere and, along with it, the life in it.

These are just some of the dangers to which any planet in our galaxy is exposed, so when they arrive, let's not remind them that they are late and give them a big welcome instead.

Pablo G. Pérez González

Patricia Sánchez Blázquez

Cosmic Void

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