The famous British astrophysicist Stephen Hawking, who died today from Charcot’s disease, was sure of it: life exists elsewhere and we will have proof of it by the end of the century. For several months now, alien hunters have been on deck again: NASA announced last February the discovery of another solar system located 40 light-years from Earth. Will we ever find living beings there? If the American Space Agency is to believe, three of the seven planets spotted around the star Trappist-1 could indeed present conditions favorable to life. After abundantly feeding science fiction films, the hypothesis of extraterrestrial life is now taken very seriously by many scientists. Because, in our galaxy alone, there would be hundreds of billions of habitable planets: a number so dizzying that it makes statistically unlikely the fact that life only developed on our planet. Astrophysicist Stephen Hawking declares himself “more convinced than ever that we are not alone in the Universe”. Seth Shostak, a researcher at Seti ( Search for ExtraTerrestrial Intelligence Institute), says that we will find life in space by the end of the century. Really? For the past thirty years, hundreds of millions of dollars have been invested in this American research institute which relentlessly scans the sky for signals from another planet. Despite everything, the little green men remain untraceable. If they do exist, why have we not yet detected their trace? Here are six hypotheses to answer this question.
We are not looking for them in the right place
Spontaneously, the scientists first directed their research towards planets located around stars similar to our Sun. However, the latter represents only 10% of the 200,000 million stars in our galaxy … Researchers are now turning to another category of stars: red dwarfs, stars of lower mass and less hot than the Sun. Not only do these represent 70% of the stars in the Universe, but most of them have one or two planets in the habitable zone – that is to say at a distance compatible with the presence of liquid water. This is why Seti launched in March 2016 an ambitious program which aims to observe 20,000 red dwarfs for two years, in the hope of detecting signals from extraterrestrial origin.
They are too different from us
If we trust what we know about Earth, the emergence of life requires two ingredients: water and organic matter (molecules containing carbon). Water is a medium in which molecules dissolve easily and where they can come into contact, which promotes chemical reactions. Carbon, on the other hand, is able to bind to several other atoms such as hydrogen, nitrogen or oxygen, allowing the production of molecules with various structures – like sugars or amino acids, the basic building blocks of life. “Most of the molecules identified in interstellar space are organic,” observes biochemist Robert Pascal of the Max Mousseron Institute of Biomolecules. “However, other living chemistries could exist,” says Dirk Schulze-Makuch, an astrobiologist at the Technical University of Berlin. On Titan, a moon of Saturn sheltering oceans of methane, silicon could, for example, play the role of carbon. But if other living chemistries existed, how to detect them when we do not know their biological functioning? To get around the obstacle, a Swiss research team has developed a system capable of detecting the movement of microorganisms on the scale of a billionth of a meter. “Movement is a universal signature of life,” say the scientists. Such a detector could ultimately be taken on board a space mission. how to detect them when we do not know their biological functioning? To get around the obstacle, a Swiss research team has developed a system capable of detecting the movement of microorganisms on the scale of a billionth of a meter. “Movement is a universal signature of life,” say the scientists. Such a detector could ultimately be taken on board a space mission. how to detect them when we do not know their biological functioning? To get around the obstacle, a Swiss research team has developed a system capable of detecting the movement of microorganisms on the scale of a billionth of a meter. “Movement is a universal signature of life,” say the scientists. Such a detector could ultimately be taken on board a space mission.
We didn’t recognize them
What if we already have evidence of extraterrestrial life in front of us? In 1996, NASA researchers announced the discovery of fossil bacteria in a meteorite from Mars. Other teams have since announced similar discoveries. But these results are controversial within the scientific community. “Once fossilized, the bacteria look like simple bubbles in the rock,” said Didier Néraudeau, geoscience researcher at the University of Rennes. To find out whether these are indeed microorganisms, it would be necessary to find in these structures organic residues of the bacteria itself. However, if we succeeded, we would still have to demonstrate that the origin of the micro-organism is indeed Martian. Clues proving the existence of extraterrestrial life could also hide in the data collected by the telescopes which record the light coming from distant stars. Computer programs designed to study this data could miss signals sent through space by intelligent beings. “If we find more efficient methods for processing data, it may be interesting to revisit past observations,” said Laurent Jorda, an astronomer at the Astrophysics Laboratory in Marseille. Perhaps there is a message hiding there from another planet.
They are too far
If we are now able to send spacecraft through the Solar System, we are still far from being able to reach the nearest star – Alpha Centauri, 4 light-years away from Earth – let alone the 3,600 exoplanets discovered to date. To study them from a distance, astronomers use the transit method: “When a planet passes between us and its star, its light crosses the planetary atmosphere”, explains astrophysicist Isabelle Boisse of the Astrophysics Laboratory from Marseille. “Atmospheric molecules each absorb very specific wavelengths. So much so that the analysis of the light that reaches us allows us to deduce the composition of this atmosphere. To detect the presence of extraterrestrial life, scientists hope to discover oxygen, methane – a gas notably from biological sources – or even molecules resulting from possible pollution produced by aliens, such as Freon, used in aerosols! But telescopes can only see up to a distance of a few hundred light-years. A range that does not yet allow us to take a look at the whole of our galaxy, which spans at least 100,000 light-years.
They are not yet born
With 14 billion years on the clock, the Universe is still young. Astrophysicists estimate that stars could still form there for 100,000 billion years. And in the future, there could still appear 10 times more planets in the Universe than there exist today. In total, 92% of Earth-like planets – rocky worlds of similar size and on which liquid water could exist – have not yet seen the light of day. So many possibilities for the emergence of a future extraterrestrial life.
They are already dead
If life ever existed elsewhere in the Universe, it may not have survived long. Because the conditions necessary for its emergence may very well appear and then disappear. Take the example of Mars. “At the start, the red planet must have looked like ours: we find the same types of minerals on its surface, it also experienced a period of volcanism and therefore could have, like Earth, an atmosphere of carbon dioxide sufficiently dense to that the climate is favorable to the presence of liquid water, ”explains Michel Cabane of the Atmospheres, environments, and space observations laboratory in Paris. We also observe meanders dug by the flow of water, which is confirmed by the hydrated minerals discovered on the spot by the Curiosity robot. Mars was, therefore, a favorable environment for life.
But that did not last: while cooling, the core of the planet froze 3.5 billion years ago. However, the movements of this metallic core were at the origin of a powerful magnetic field, a real shield protecting the planet from the solar wind. In the absence of this protective field, the solar particles have ceased to be deflected and have chased molecules from the Martian atmosphere by striking them. The atmospheric pressure has decreased, causing the liquid water to evaporate. “The lakes and rivers disappeared from Mars at least 2 billion years ago,” says Michel Cabane. Today, atmospheric pressure is more than 150 times lower than on Earth and, in the absence of a greenhouse effect, the temperature is permanently below 0 ° C. If there was a day of life on Mars