For a much more detailed discussion of this subject I refer the interested reader to my Covid period essay Are We Alone? Where Are They? Here I will merely summarize my present thinking about life beyond the boundaries of our planet.
To set the stage, we must distinguish between two fundamentally different categories: basic microbial life and advanced intelligent life. The former of these existed on Earth in the form of prokaryotic life for the first billion and a half years, or so. The latter, however, has existed only for a few hundred thousand years as Homo sapiens, and has achieved the technological level of electromagnetic communication within barely the last century of that period.
Basic Life
At the outset, it must be stated that we don’t even know how basic life was engendered on our planet. We barely know when. That gap in our knowledge represents a major obstacle to being capable of estimating the prevalence of microbial life elsewhere in the universe. We know that prokaryotic life was based on carbon and its unique chemical bonding properties but we have not been able to reproduce the underlying natural
processes leading to the assemblage of the fundamental molecules of life, RNA and DNA.
At the same time, we have no evidence of such basic life outside the Earth. We have found the organic building blocks of such life in interstellar molecular clouds and in meteorites but not a trace of fossil life, so far. The search for extraterrestrial microbial life is being undertaken in two principal directions, using different methodologies: within our Solar system and on exoplanets.
The search in the Solar system is and will be concentrated on finding fossil life on Mars and on detecting extant life in the subsurface of moons of Jupiter and Saturn. It is generally believed that there are no active life forms on Mars which has been explored quite extensively over the last few decades by means of surface rovers. It is expected that a sample of Martian soil will be retrieved in the near future whose detailed laboratories analysis on Earth will confirm the latter and determine if any life has ever existed leaving identifiable fossils.
The most promising life abodes within our Solar system are the sub-crustal seas of some of the larger moons of the planets Jupiter (e.g., Io and Europa) and Saturn (e.g., Titan and Enceladus). The interior of these moons are believed to be potentially compatible with simple forms of life because of the composition and temperature of their subsurface seas. Their exploration will require direct sampling of vents, eruptions or the surfaces of those moons. Plans are underway to carry out such probing in the near future.
The discovery of life on any of those moons would be of momentous importance. We are facing, in my view, a crucially binary outcome. Should any form of primitive life be detected on those moons or of fossil life on Mars, we can safely conclude that such life is common in our galaxy and most probably on the universe at large. Should, however, no life be detected in our Solar system, a very different conclusion becomes justified: extraterrestrial life is uncommon. I consider the resolution of that dichotomy to be essential, and I hope that it will be actively pursued by NASA and the European Space Agency by means of interplanetary probes. These missions should take total priority over any manned space explorations such as a trip to Mars, an extremely expensive and quixotic enterprise.
Now, the other realm to be searched for basic extraterrestrial life, as mentioned above, is that of the surface of exoplanets. That pursuit is being based, principally at this time, by spectroscopic analysis of the chemical composition of their atmospheres. That technique consists of detecting and identifying characteristic absorption lines in the spectrum obtained by observing the electromagnetic radiation from the host star as it grazes through the exoplanet’s atmosphere, if it possesses one.
It is assumed that certain elements and compounds are produced by biological processes and thus constitute indicants of the presence of life. It is believed that the presence of, for example, oxygen and methane in more than trace amounts should be interpreted as byproducts of life. Oxygen is an inherently unstable, i.e., reactive gas that, on Earth, is replenished continuously by plant photosynthesis.
One of the problems of this method of detection of the presence of life within the atmospheres of exoplanets is that of specificity, i.e., are there other processes than those associated with life that can generate those gases that, here on Earth, we associate uniquely with the presence of life?
The solution of that ambiguity will require, among other approaches, increasing the sensitivity and resolution of the exoplanet’s spectroscopy.
Concurrently, we need to discover the processes that lead to abiogenesis, the appearance of microbial life on Earth. That determination will help in determining the likelihood of the appearance of extraterrestrial life and the conditions required for that to occur elsewhere.
Advanced Intelligent Life
The existence of extraterrestrial technologically advanced life capable of electromagnetic communication is an altogether different question than that of the existence of basic, microbial life. The detection/identification of such advanced intelligent life requires an entirely different methodology.
The presence of such intelligent life can either be gleaned from the actual physical presence of “aliens” in the form of spacecraft, Von Neumann machines[1], or other unequivocal artificial objects originating outside our planet.
Alternatively, in view of the absence of such direct physical evidence of advanced intelligence, the search is, at present, entirely concentrated on SETI (Search for Extra Terrestrial Intelligence). This pursuit consists, in essence, of looking for electromagnetic signals generated by alien civilizations. This search has been underway since the 1960s utilizing increasingly sophisticated technologies, principally based on radioastronomy.
To date (2024), no artificial signals of extraterrestrial origin have been detected. This has been labelled as the Great Silence and has elicited a multitude of explanations, ranging from the so-called Great Filter — short survival of advanced civilizations — to humanity’s uniqueness in the universe or, at least, within our galaxy.
The overall questions raised by the null result of, heretofore, performed SETI has been characterized as the so-called Fermi Paradox, i.e., the absence of any evidence of extraterrestrial intelligence contrasted with the number of potentially favorable abodes for such life throughout the universe.
Notwithstanding the abundance of alternative explanations to the Fermi Paradox, my own view, detailed and justified in my long essay referred to above, is that we are, indeed alone at least within our Milky Way galaxy, and even possibly in the observable universe.
This latter hypothesis rests not only on the observation of the Great Silence but on the prevalent opinion of a majority of evolutionary biologists as opposed to the position taken by many astronomers and astrophysicists. The latter base their view mainly on statistical considerations, i.e., given the staggering number of exoplanets in the observable universe, extrapolating from the prevalence of exoplanets in our galactic vicinity, the probability of the existence of numerous abodes of advanced civilizations becomes more than significant.
The opposing position of the evolutionary cohort, in turn, is based on several considerations derived principally from the history of life on our planet. As Stephen J. Gould has stated, that evolutionary sequence would not be replayed if started anew. The convoluted evolutionary path that life has followed over nearly 4 billion years, modulated by a series of improbable and contingent events, some of which were of catastrophic magnitude, render the Homo Sapiens outcome rather improbable. Suffice it to cite as an example of the improbable contingency of that sequence is the resolution of the evolutionary bottleneck of mammals through the demise of the dinosaurs by meteoric impact 66 million years ago.
In conclusion, I believe that there is no Fermi Paradox, i.e., the answer to the famous physicist’s question: “Where are they” is simply: They are nowhere. I tend to subscribe to this latter solipsistic position. We are likely to be alone.
If there is a true paradox, it is the very existence of Homo sapiens sapiens.
Our appearance in the universe was utterly improbable and unlikely to have occurred anywhere else.
The probability for advanced intelligent life existing in the universe would become even more negligible if we are unable to find even the most basic life elsewhere in our solar system or on exoplanets, a distinct possibility.
[1] A von Neumann probe is a spacecraft capable of replicating itself.
Pedro Lilienfeld 
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