Are we truly alone in the Milky Way?

In his book 'On Natura' (400 BC), the Greek philosopher Metrodorus of Chios doubts the existence of a single habitable world in the universe. And there we have it. The first documented belief of extraterrestrial life. Fast forward to today, what’s our stance? July 2nd,  International UFO Day, we take a look at extraterrestrial life.

Space is full of strange and unexplained objects. On June 24, 1947, Kenneth Arnold observed a boomerang-like object in the sky; he described its trajectory like “a saucer skipped across water". From there, a journalist headlined "Pilot saw objects like saucers flying at 2000 km/h in Oregon", confusing the trajectory and the shape of the object. One thing led to another, or rather, from error to error, the shape of a flying saucer filled the minds, and the sky.


Image: Kenneth Arnold poses next to his "flying saucer" representation

Intelligent life on other planets, on the Milky Way, or in another galaxy, is a subject that has fascinated people for several hundred years. For evidence of life, researchers do not look up at the sky and hope to see something. Although...

Direct evidence of extraterrestrial life?


For years, evidence in favor of extraterrestrial life has been appearing. 

Say we skip the often controversial flying saucers in the sky, perhaps you have heard of Martian canals? From the end of the nineteenth century until the beginning of the twentieth century, large rectilinear tracks on the surface of Mars were interpreted as irrigation canals, probably created by intelligent life. What a disappointment it must have been when it turned out that these "Martian canals" were just optical illusions. They are in fact only rectilinear traces of geological origin...


Some researchers, like the physicist Enrico Fermi, do not believe at all in the existence of extraterrestrial life. He explains that "if there were extraterrestrial civilizations, their representatives should already be with us. So where are they?"

Astrophysicist Michael Hart has explored this question through a more rigorous analysis. According to him, intelligent life may have evolved but chosen not to contact us. He also argues that civilizations have evolved only recently and have not had time to reach us. Finally, he concludes that the presence of intelligent life in our galaxy is highly unlikely.

More recently, in 2020, phosphine has been observed in the atmosphere of Venus, by Greaves and his colleagues. It is a gas that, on Earth, can be created by anaerobic bacteria. In the abstract of their paper, the authors write, " Phosphine could originate from unknown photochemistry or geochemistry, or, by analogy with biological production of phosphine on Earth, from the presence of life." So? Could this be the ultimate and long-awaited proof of the presence of life on another planet? The proof of the existence of a microscopic life form on the surface of Venus? In any case, media seized it. EarthSky wrote: "Possible life signs in the clouds of Venus". We have finally found evidence of extraterrestrial life.


But the study is in fact much less equivocal... In their article, Greaves and his colleagues specify that the presence of anaerobic bacteria to explain this phosphine is only one explanation among many others. And still, not the most parsimonious. Does the fact that some organisms on earth produce this gas imply that in totally different physico-chemical conditions, they also produce it? If phosphine were found in a clearly uninhabitable environment on Earth (for example, in a magma chamber), no biologist would seriously consider the hypothesis that it could be the product of organisms living in that environment. Since an organism able to synthesize molecules (here phosphine) is a very complex process, it is much more parsimonious to consider that it is an abiotic phenomenon which emits phosphine on Venus. Finally, in a more sociological way, there is such an infatuation with the idea of extraterrestrial life that one tends to over interpret the evidence. New disappointment, the phosphine present on Venus is probably not synonymous with an extraterrestrial life form...

These failures to find life are not necessarily tangible proof that it does not exist.


Finally, there is no direct evidence for the existence of life outside the earth at the moment. But does an absence of proof mean an absence altogether? No reason to think so, because nobody, not even science, has the methodological tools to prove the non-existence of something. Other hypotheses are proposed to explain the silence of intelligent life, for example human technical limitations that prevent us from picking up a signal. Meanwhile, in a less glamorous, but more effective way, researchers are trying to find out what are the probabilities of the appearance of intelligent life capable of communicating with us, using the Drake equation:


N = R·fp ·ne ·fli·fi ·fc ·L 


This simple equation allows us to calculate the number of intelligent civilizations in the Milky Way capable of communicating with us now, taking into account several factors, such as the rate of star formation in the Milky Way, those that have planets that can support life, the fraction of these planets where life is evolving, the fraction that is evolving into so-called intelligent beings, and finally the fraction of these intelligent lives that are developing a means of radio communication. In the 1960s, this N was estimated to be between 20 and 50,000, which means that there would be between 20 and 50,000 intelligent civilizations capable of communicating with us in the Milky Way. Dr. Zion Elani estimates that when his paper "Space, the final frontier: In the scientific pursuit of extraterrestrial life away from Earth" was released in 2020, the Drake equation estimates predict between 0 and 21 extraterrestrial civilizations intelligent enough to develop radio in the Milky Way. But perhaps their means of communication are different from ours, which would prevent us from understanding each other...


Dr. Xiang and his colleagues add that the probability of life in a pre-biotic state, the different potential time scales for biological evolution, and the probability of self-annihilation of complex life are also factored into the equation. However, the model omits unexpected catastrophes, such as meteorites crashing on Earth. The calculations in their paper estimate a time and place where it was most likely that life formed within a galaxy. The time depends on the age of the stars: as Sun-like stars age, the probability of life appearing decreases, implying that the amount of life does not increase with time: there is a peak where the probability of intelligent life appearing is highest, and then the probability decreases after that peak. The authors also note that our location is not where intelligent life is most likely to form, which would imply that our location may be too far from more complex life forms. In other words, it's not that there is no other intelligent life, it's just that it's too far away, or that it may be too young for the moment, in its early stages of evolution.

Are WE the proof of extraterrestrial life?


The big drawback of the Drake equation comes from a still unsolvable question: how to estimate the possibility of the appearance of life? How does it originate in the form of simple biomolecules, and how do these biomolecules form a basis for life? Where does this primitive life come from?


The question is still relevant today: after an internship at the origin of life laboratory at Mcmaster university in Hamilton, Ontario, Alix Dujardin continues her research with a thesis to understand the origin of life on Earth or on Earth-like planets. Since she was a child, her intense curiosity has led her to investigate where living things come from. The doctoral student seeks to create RNA chains from nucleotides already formed. In particular, she works with a machine that allows her to simulate different environmental conditions, changing temperature, pressure, humidity, radiation,... by changing different parameters, Alix simulates small warm ponds, conducive to the formation of these RNA chains.


Her thesis is related to the appearance of extraterrestrial life and she has participated in many conferences on the question "how life began on Earth-like planets". With more knowledge on the appearance of life on earth, we will be better able to understand the appearance of extraterrestrial life.

"I find it super interesting to wonder where we came from, where everything starts. But I find it even more fascinating to think that we'll never have the answer."

Alix Dujardin


In conclusion


Dr. Elani synthesizes the current research on intelligent life in his article. And what the estimates presented clearly indicate is that there is most likely an extraterrestrial civilization somewhere capable of communicating with us. However, the vastness of the galaxy, indeed of the universe, strongly negates the attractive possibility of past or future extraterrestrial visits.

References : 


Cai, Xiang, et al. "A Statistical Estimation of the Occurrence of Extraterrestrial Intelligence in the Milky Way Galaxy." Galaxies 9.1 (2021): 5.


Cockell, Charles S., Sean McMahon, and Jennifer F. Biddle. "When is life a viable hypothesis? The case of venusian phosphine." Astrobiology 21.3 (2021): 261-264.


Elani, Zion. "Space, the final frontier: In the scientific pursuit of extraterrestrial life away from Earth." Physics Essays 33.4 (2020): 367-379.

Greaves, Jane S., et al. "Phosphine gas in the cloud decks of Venus." Nature Astronomy (2020): 1-10.