The cosmos is a depositary of great many mind blowing notions, be the dark energy, the dark matter, the notion of infinity, the billions of galaxies each with billions of stars, the big-bang narrative and the breath-taking space ship journeys... But the cosmos is also a source of great gifts and surprises. The last comes from the last Kepler space mission data, with the information, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones within the Milky Way, with many of them possibly orbiting Sun-like stars.

The notion of habitable zone is already a first non-trivial point. Generally, according to the NASA indications, surface planetary habitability is thought to require orbiting at the right distance from the host star for liquid surface water to be present, in addition of various other geophysical and geodynamical aspects we cannot discuss here.

The extraordinary high number of planets in the so-called habitable zones makes immediately think, that in some of them there is life. And then immediately, however, comes to mind the famous “Fermi paradox”. The Fermi paradox is also based on the high number of potentially habitable planets, asking however this question: “yes, but where is everybody?” – pointing to the fact that we have not received any sign of alien life – ever.

There are a lot of considerations about this question and the Fermi paradox in general, and in this brief article we will deal with some of them. Let us instead first make the acquaintance with some of the planets which are considered habitable. An arbitrary short list (eliminating the planets which are more than 50 light years away) is given below – see also the artistic rendering in Figure 1:

Proxima Centauri b or just Proxima b, a rocky planet which is “only” 4.2 light-years away, a mass 1.3 times that of the Earth, and it orbits Proxima Centauri every 11.2 days. According to our astronomers, it may allow for the presence of liquid water.
Wolf 1061c. This exoplanet is located about 13.8 light-years from Earth in the constellation Ophiuchus, making it the second-closest known potentially habitable planet to Earth, after Proxima b.
Gliese 832c. Gliese 832c is located just 16 light-years from Earth and lies within the habitable zone of a red dwarf star called Gliese 832.
Gliese 581g This more controversial planet, in the constellation Libra, is a rocky world. It is about 20 light-years away from Earth, and two to three times as massive as Earth.
Gliese 667Cc is about 22 light-years away in the constellation Scorpius. The planet is at least 4.5 times bigger than Earth. GJ 667C – the parent star – actually part of a triple-star system.
Trappist-1d, discovered in 2016, orbits around an ultracool dwarf star known as Trappist-1, which lies approximately 40 light-years away from Earth in the constellation of Aquarius.
HD 40307g lies about 42 light-years away from Earth in the constellation Pictor.
HD 85512b, discovered in 2011, is about 3.6 times more massive than Earth. It is about 35 light-years away from Earth’s sun, in the constellation Vela.

The Fermi paradox

On the basis of these data, let’s go back to the Fermi paradox.

The question “where are they?” implies communication between at least two different civilizations, and this in turn implies two conditions: the condition of “quasi simultaneity”, namely that the worlds in question were born and evolved temporarily close to each other, so as to possibly permit similar technical evolution, and that they are close enough in space to pay a visit to each other or at least to send/receive mutual signals. Indeed, it does not make much sense to consider the communication between worlds which are, say, one hundred million years apart with origin and evolution.

The first of these two conditions is born out from the consideration that if alien visitors would have come to our Earth only ten million years ago, they would have not found any intelligent life; and it is very doubtful that they would find intelligent life if they would come in one million years from now. In other words, intelligent life is most probably restricted to a modest window of time (in cosmic geological terms). I believe that the condition of the “quasi simultaneity” of two different worlds already accounts for the Fermi paradox. If then we add the other condition, namely that the two worlds should be close enough to each other to permit communicability, then we really get into a non-winning ticket.

In particular, concerning the “visit”, we learn from the previous list of habitable exoplanets that there is nothing in our vicinity closer than 4 light-years. This means, that even if their space ship would fly at the speed of 1/100 of the light velocity (which is orders of magnitude faster than the Apollo 10, which was 31 Km/s), it would take ca. 400 years to arrive to us.

This does not exclude the possibility of mutual signals at the speed of light, and to that, we can only answer that until now we have not received any signal from an alien world.

Of course, all these arguments to explain or rule out the Fermi paradox are based on our present science knowledge. It is usual, on discussing these themes, to rapidly arrive at science fiction, and to invoke tele-transportation, ships or signals that go faster than light – or unknown forms of intelligent life. All this makes perhaps the dialogue more amusing, but then, it is no longer a scientific discourse. It is not science fiction to accept the idea of a possible form of intelligent life which developed independently in one or a few of those billions of planets discovered by the Kepler space mission. But there is no way to know.

Bacterial life

The arguments above, centered around the Fermi paradox, are about intelligent life. A different question is whether some form of much simpler life, for example bacterial life, can exist in those exoplanets.

We have to consider at this regard, that in our own planet, where life exists since ca. 4 billion years, there have been only microbes (see Figure 6) – unicellular bacterial life – for the first two billion years. Only ca. 700 million years ago multicellular life began to develop, and mankind appeared only 7 million years ago (see more details in Figure 3). In fact, when we talk about the origin of life on Earth, we should think in terms of three qualitatively different origins – and there is no logical follow up from one to the other. But let us stay with the simple question: could at least bacterial life exist in the other planets?

But even that, does not warrant a clear positive answer. Those astronomical data, relating to the habitable zones, concerns in fact only one aspect of the problem: the necessary conditions for life. Certainly, we need the right set of temperature, pressure, atmosphere, liquid water, and so on. Is this enough to make sure about life? Well, what about the necessary conditions?

Again, let us move within the frame of science, leaving aside science fiction. Accordingly, life on earth originated from non-life, namely from simple molecular structures which underwent a process of molecular evolution, creating spontaneously more and more complex structure/functions, till the arising of structures which could reproduce themselves – the first living cells.

This view, completely based on natural forces, eliminates any divine intervention in the creation of life – and of course if you change this premise, and have faith in a Creator, all changes. But then is no longer science, but faith.

On the basis of this premise, commonly shared by the vast majority of scientists in the field, our scientists have been studying the origin of life since 1950 circa, really some of the best minds in chemistry, biology, molecular biology: and nobody has discovered how life started on earth, and how we can create life in the laboratory Actually, two generations of chemists have tried hard to find the conditions for making life, even in the simplest possible forms, in the laboratory, and failed – even forcing somehow conditions, for example starting from already formed membranes, or even from some magically formed RNA molecules. But actually, we do not even know how ordered macromolecules like proteins or nucleic acids have originated on our Earth. There are of course speculations and hypotheses, like the story of the prebiotic RNA world, a nice speculation without any sound chemical basis – as I have argued in my last 2016 book.

Generally, this failing of our science to understand how life stared on earth, and/or how to make simple life in the laboratory, unequivocally shows that bacterial life is not a trivial outcome of the necessary conditions. Life does not arise spontaneously and therefore to have a set of necessary conditions in another planet – temperature and pressure and chemicals and environmental conditions like ours – does not demonstrate anything about the possible presence of life in that planet. Contingency, namely the fortuitous juxtaposition of per se independent parameters has been – for modern science – the creative driving force for the origin of life. An event then of incommensurably small probability, certainly not a thermodynamic drive towards an energy minimum. This, again, at the stand of our present science. It is possible that tomorrow a new bio-Einstein will discover how life started in our planet. Actually, let me add something important at this regard: let us suppose that in the near future we discover the existence of bacterial life in many planets in the cosmos. Well, this discovery would probably force us to abandon the notion of contingency, in favor of some deterministic drive to make life. But for the moment, we are still facing a mystery. In conclusion: are we perhaps alone in the universe?

Mass psychology and Jungian archetypes, are such, which the majority of people, emotionally, do not like to accept this view. For most uneducated people in science, feelings, emotions, are more important than science truth, and this since primordial times, and probably for the next centuries to come. I am a skeptical, as you have noticed from these few pages – but I have to acknowledge that when in the night, all alone, I look at the many stars in the sky, the question: “is it possible that nobody is there?” come immediately to my mind.

Accepting science may be hard and, in this sense, let me close with a quotation of Jacque Monod, who interpreted – I believe correctly – this relation between science and feelings (from his 1971 book, Chance and Necessity):

Article written in collaboration with Prof. Angelo Merante

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