How many aliens in our galaxy? Explanations of two researchers

ALIENS – Are we alone? The universe is so vast that it is likely that other forms of life exist. But if so, why haven’t we heard of them? These are the mysteries at the heart of the Fermi paradox, which questions the lack of extraterrestrial discoveries.

Published in the magazine The Astrophysical Journal on April 4, a new paper is interested in this question and asks the following: how many possible CETIs are there in our galaxy and what is the probability of successfully communicating with them?

The two Chinese authors of the study, Wenjie Song and He Gao, members of the astronomy department at Peking University, estimate that there are at least 111 communicating extraterrestrial intelligent civilizations (or CETIs) in our galaxy.

Up to 42,000 intelligent civilizations in our galaxy?

It is difficult to study other civilizations because we only have one point of reference: humans on Earth. Nevertheless, many researchers have addressed the problem. For example, a 2020 study concluded that there are likely 36 CETIs in the Milky Way. The scientific method that had been used is taken up in this new research, focusing on two parameters that have been hypothetical until now.

The first parameter concerns the number of habitable planets and the frequency with which life on these planets evolves towards an intelligent civilization (CETI). The second is how old its host star is when such a life form appears. According to their results, the number of CETIs that exist or have existed in the Milky Way varies from more than 42,000 in the most optimistic scenario to 111 in the most pessimistic scenario.

The Astrophysical Journal/Wenjie Song/He Gao/2022 Table of simulation results. The percentage F at the top represents the evolutionary stage of the host star necessary for the development of a CETI. The percentage fc is the percentage of terrestrial planets that can accommodate a CETI.

The Astrophysical Journal/Wenjie Song/He Gao/2022

For scientists, there would therefore be at worst more than a hundred life forms to discover within the Milky Way. But to find them, humanity will probably have to be patient.

Going back to their scenarios, the two researchers estimate that it will take another 2000 at least to survive to establish two-way communication with us. This duration can also be much longer if we follow the pessimistic scenario and push up to 400,000 years.

On the scale of our species, it is very (very) long. Our species has actually only existed for a few hundred thousand years and only discovered agriculture 12,000 years ago. It is short.

A paradox, hypotheses

The search for other forms of life, many scientists have tackled it. This is the case with Enrico Fermi, one of the creators of the atomic bomb. During a lunch in the 1950s with other physicists, the question of extraterrestrial life came up.

The Italian researcher then points out this: given the vastness and age of the universe, if an extraterrestrial civilization existed, it would certainly have had time to spread over thousands of worlds and visit us. So where are they?

This is the “Fermi Paradox”. Since then, thousands of people have tried to find an answer to this problem. In 1961, Franck Drake, an American astronomer, set up an equation to calculate the number of extraterrestrial civilizations in our galaxy. The problem is that this equation has many unknowns. “But we are beginning to know more of the concepts: the number of stars on planets, the type of planets”, specifies Gabriel Chardin, physicist at the CNRS.

While waiting to solve the “Drake equation”, there are many theories to explain the Fermi paradox. Some not very serious, others are based on real scientific foundations.

The big filter problem

At this point the big filter comes into play. Defined in 1998 by Robin Hanson, it is a series of barriers that hinder the emergence of an advanced extraterrestrial civilization. The authors approach this problem as follows: “The lifespan of civilizations is most likely self-limited due to many potential perturbations, such as population problems, nuclear annihilation, sudden climate changes, comet riots, ecological changes, etc.”

But will humanity ever meet another civilization? As with all questions regarding this topic, the mystery remains as there are so many unknowns. Therefore, it is not impossible to imagine a scenario where “humans may not receive any signals from other CETIs before their extinction.”

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