The belief in the existence of extraterrestrial life beyond bacteria and other unicellular organisms is well implanted in our unconscious. Whether thanks to science fiction movies or the tireless propaganda carried out by numerous scientists, the truth is that it is an almost immovable dogma. “If we were alone… what a waste of space!” said Jodie Foster in the movie Contact, based on the homonymous novel by the well-known Carl Sagan. Leaving aside the puerile teleology that hides such a statement -it seems as if the cosmos cared something that only one intelligent species existed-, what is hidden behind that grandiloquent phrase is a peculiar philosophical blow, the recognition of mediocrity , which Sagan and the Russian astrophysicist Iosif S. Shklovskii cleverly expounded in their classic 1966 book Intelligent Life in the Universe .
The underlying idea is that the human being is not special . According to them, neither the human being, nor our planet, nor the Sun are something strange, a cosmic anomaly. The region of the cosmos in which we live is typical, it has nothing special, therefore all the processes that have given rise to life and intelligence can be repeated in any other place if the right conditions are met. That is, we just need to let a reasonable time pass for life (and intelligence) to appear naturally .
In the last decades of the last century, two discoveries gave wings to this way of thinking that the Nobel Prize for Medicine Christian de Duve summed up with the phrase “life as a cosmic imperative”. The first was the discovery of extrasolar planets, a sine qua non condition for the appearance of life. The second, to discover that life is robust, capable of surviving in unthinkable conditions: from highly acidic environments -such as the Rio Tinto-, at temperatures above 100º C, at pressures equivalent to 11,000 meters under the sea… and even in space. This seems to indicate that life does not require very specific conditions to flourish . And seeing the millions of galaxies in the universe, each with more than a hundred billion stars, it is difficult to think that the Earth is the only planet with living beings on its surface. The conclusion? If we couple these considerations with the ‘loss of space hypothesis’, there is no doubt that the universe must be teeming with life. Or not?
There are a few scientists who row against the current and are not so optimistic. On the one hand, they say, no matter how many galaxies there are in the sky, not all of them are valid for life . For example, in active galaxies, such as the Seyferts, their cores emit fluxes of radiation capable of sterilizing the surface of any planet found there. Nor are valid those that, like elliptical galaxies, have a low amount of elements such as iron, carbon, phosphorus, sodium… (in astronomical language they are said to be galaxies of low metallicity). Without raw material, the appearance of planets and life forms is unfeasible. And even if we find a galaxy friendly to life, not every place in it is appropriate.
Let’s look at the Milky Way: in the regions near the central super black hole, the levels of radiation that reach the neighboring regions are so high (250,000 times higher than that received by our planet) that they make the appearance of complex molecules impossible. Similarly, areas with a high density of stars are also prohibited by the threat posed by supernovae, which when exploding are capable of seriously affecting life on planets located less than 30 light-years away. This creates a barren sphere around the galactic center at least 10,000 light-years in radius . In contrast, in the outermost regions the same thing happens as in elliptical galaxies: there is a significant shortage of heavy elements essential for the formation of rocky planets. The universe may not be that teeming with life despite how much space there is.
Geologist Peter D. Ward and astronomer Donald Brownlee have elaborated on this idea which they have called the Rare Earth Hypothesis, a shot to the waterline of the most optimistic astrobiology. The idea is simple: life is common in the universe but only in its simplest forms , which have proven to be able to survive in the most adverse conditions, but we cannot say the same about higher life, animal and plant, much more sensitive to environmental conditions. In addition, it not only has to appear but it must be able to survive in time, which means that the planet must provide a stable environment for eons. Thus, a star is needed that provides light in a more or less stable way, without explosions or sudden changes in brightness, and without producing large amounts of lethal ultraviolet radiation (which massive stars do), for at least 5,000 million years. years. A certain climatic tranquility is also necessary -something that on Earth is provided by the Moon, which by stabilizing the orientation of the Earth’s axis prevents us from suffering chaotic variations in the climate-. And that without speaking, for example, of the need for a contained greenhouse effect that provides a pleasant temperature and does not run amok, as is the case with Venus.
Put simply: unlike in the case of microbial life, the existence of higher living beings requires such finesse in the adjustment of environmental conditions -whether astronomical, planetary or ecological- that makes their appearance in the Earth very difficult. surface of a planet .
Now, what about intelligent life? To think, as the famous Carl Sagan defended, that we are in a Milky Way with more than a million civilizations waiting to contact us, is a long live Cartagena: even Carl Sagan himself admitted that using this principle was “essentially an act of faith”.
The Swedish-American cosmologist Max Tegmark in his book Our mathematical universe also discusses Sagan’s ‘loss of space’ hypothesis. Tegmark assumes that there is no reason to believe that two intelligent civilizations should be within a certain distance, but that they are equally likely to be a thousand, a million, a billion, or a trillion light-years apart. Bearing this in mind and assuming that (1) interstellar colonization is technologically possible for a civilization a million years more advanced than our own; (2) that there are billions of habitable planets in the Milky Way, which may have formed billions of years before Earth; and (3) that there are civilizations that, being able to colonize space, choose to do so, Tegmark’s calculation tells us that the minimum distance at which we will find the closest civilization is greater than the size of our galaxy .
From another point of view, the famous biologist Enrst Mayr puts his grain of sand to the absence of extraterrestrial intelligences. How many species have there been on Earth? It is difficult to know but many more than the 8 million species that exist today. Of all those millions, how many have been able to develop technology? A. The ‘waste of space’ argument is blurred.
