First of all, I must say that when I was asked to write this article, I thought about whether or not to accept. As a planetary geologist and astrobiologist —and one of the promoters of astrobiology in Spain and internationally—, I have always considered that we are the first to act with the utmost scientific rigor and not give rise to pseudosciences , with assessments or considerations even without evidence. . For this reason, I have insistently refused to participate in debates where the ruse of putting the expert researcher and the non-scientist on the same level as the spectator is usually used. On the subject of extraterrestrial life — and even more so on that of intelligent life — there are hypotheses, inferences and extrapolations that require solid scientific evidence to back them up. It could be said that, in the field of astrobiological research and space missions, we are still immersed in this search and investigation process. To this we must add three aspects, in my opinion, relevant that I will address later: at present there is no specific definition of the concept of life; just different approaches; the concepts of life and planetary habitability are usually confused (sometimes self-servingly); and, regarding the second part of the title of the article, there is no scientific agreement about what intelligence is, for which, as with life, there is still no unitary definition.
For this reason, I will try to offer succinctly the current situation of the subject and the open perspectives, as an essay, bearing in mind that the conclusion or answer to the two questions raised in the title is: we do not know yet and, therefore, we continue to investigate. And I say succinctly, because if we put (as of January 7, 2022), for example in Google, the terms “extraterrestrial life” appear 1,040,000 results, which becomes 2,260,000 when we perform this search in English (“extraterrestrial life»). I am aware that this search only indicates a very global panorama, where many things are mixed in an almost chaotic way. If we observe, using Google Trends, the worldwide trend, from 2004 to 2022, a decrease in general interest in this topic is observed from December 2017 to the present and a maintenance of low interest in searches, from the peak reached in December 2010.
Going to a more detailed analysis, if we focus on the scientific field, a search in the Web of Science using these same terms (extraterrestrial life) as keywords, results in a total of 5425 publications, distributed by thematic areas, in which some of them are obviously combined for the same publication: thus, Astronomy/Astrophysics is the one with the highest results (4,115), followed by Engineering: 2,337, Environmental Sciences: 1,717, Geology: 1,265, Life Sciences and Biomedicine: 1,235, Physiology: 957, Instrumentation: 918, Biochemistry/Molecular Biology: 856, Science, Technology and other subjects: 822 and, finally, Physics: 686. Of all of them, the country where the largest number of published articles is located is the USA. , with 2761, followed at a considerable distance by the United Kingdom, with 550, and France and Germany, equaled with 480 registrations. Spain appears with 229 publications, above countries such as Canada and Japan, both with 224.
Lastly, regarding the scientific publications in which the terms “extraterrestrial life” appear, the first is Astrobiology, with 504 entries, and other journals such as Icarus and the Astrophysical Journal , with 220, Advances in Space Research , with 193, Planetary and Space Science , 182, Acta Astronautica , 152 and Nature , 144.
With this, we can already get an idea about the current panorama of publications on this subject and that, despite the ambiguity and the absence of evidence of life outside the Earth , there is information for debate and an undoubted interest in its investigation. which, how could it be otherwise, covers a wide spectrum of disciplines and approaches.
astrobiological research
From the field of astrobiological research, if we want to compare the operation of physical or chemical laws between the Earth, the Moon, Mars, Pluto or an asteroid or comet, we can do it and we see that they are universal. We also see it with geological principles and the study of materials (rocks and minerals) and the processes that generate and modify them. This, for example, is being of great help in the interpretation and identification, characterization and modeling of planetary paleoenvironments and their habitability conditions, as seen on Mars through the missions. With terrestrial analogs, we can compare rocks, minerals and unique areas of our planet, a basalt, a jarosite, a gypsum, fluvial or wind erosion, geomorphological features, volcanic processes, etc., between the Earth and the red planet or any other planetary body. But, when we approach any attempt to establish analogies with life, for its search outside the Earth, we are limited because we cannot make any type of comparison. There are no biological analogs and we can only make hypotheses or inferences, taking into account that the only life known to date is that of our planet , with all its biodiversity, including extreme environments and extremophilic organisms, which are being of great use. and astrobiological importance.
With regard to extraterrestrial life, the first lines of the description that appears on Wikipedia itself (in English) are really revealing and summarize the issue very well. I transcribe it here, with slight modifications: «Extraterrestrial life is a hypothetical life that may exist outside the Earth and that did not originate on our planet. Such life could range from simple prokaryotes (or similar life forms) to intelligent beings, perhaps even civilizations more advanced than humanity.” In other words, the definition contemplates the two possibilities referred to in the title of this article.
From the point of view of astrobiology, especially in its modern conception at the end of the 20th century and the creation of the NASA Astrobiology Institute with its corresponding roadmap (which already has several versions), if it is assumed that life is a consequence of evolution of the universe and the conditions that have occurred on Earth will be replicated on any other planet or moon, life could perfectly emerge and evolve just as it happened on ours .
That is, our famous LUCA (an acronym for Last Universal Common Ancestor) could emerge or have emerged on another planet or moon, from abiotic to biotic, if the site context were appropriate for it. The contribution of organic compounds from carbonaceous chondrites or comets also seems to have played a very important role in the emergence of life on the primitive Earth. And I refer specifically to meteorites and comets as carriers of organic compounds and not of life, as postulated by panspermia. This hypothesis has so far no confirmation or evidence to support it and, furthermore, it does not solve the origin of life, but simply moves the problem to another site.
Although several top-tier astrobiological targets exist in our own solar system, such as Mars, Enceladus, Europa, Titan, or even recently, the atmosphere of Venus, no microorganisms or traces (biosignatures, biomarkers) of their existence have yet been found in none of them, nor in any meteor or comet. Abiotic organic compounds yes, but nothing related to life. Obviously the universe is huge and there are billions of galaxies and billions or trillions of planets where life could have emerged . In this sense, as of January 1, 2022, 4,905 confirmed exoplanets (extrasolar planets) have already been discovered, in 3,629 planetary systems, with 808 systems having more than one planet. The main potentially habitable exoplanets, classified according to the Index of Similarity to Earth.
What is life?
As previously indicated, one of the difficulties is that there is no consensus on the definition of what is life and what is not . It would be pretentious to enter here into all the scientific, and even philosophical, debate that exists on the subject, but it does seem appropriate to record it, since it is an important conditioning factor when trying to detect possible extraterrestrial life. A popular definition of life is that organisms are open systems that maintain homeostasis (regulation of their internal environment to maintain a constant state), that are composed of cells, that have a life cycle, that metabolize, that can grow and adapt to their environment, that respond to stimuli, that reproduce and evolve. Here, for example, the topic of viruses and viroids would come under discussion. In fact, in addition to the different approaches to the definition of life (so far there are more than 100 definitions), from physics, chemistry, geology, biology or philosophy, there is the consideration, according to some authors, that life it should be considered as a property of ecosystems or even as a process rather than a substance. The debate about what is life and, therefore, what is extraterrestrial life, is still open.
Hence, another of the fundamental aspects related to its detection outside the Earth is to determine exactly and rigorouslywhat is a biomarker, since the term is being used, by communicators, media and also some scientists, in an ambiguous way, which can lead to confusion. The assessment and consideration of this concept is closely related to the aforementioned connection between life and habitability. As indicated by Simoneit —who proposed the concept of biomarker— only those organic compounds that unequivocally come from the metabolic activity of an organism should be called that. everything else would begeomarkers, a term that I myself proposed a few years ago to talk about environmental and habitability markers. In this way, a chemical compound or an isotopic ratio related, perhaps, to biological activity would be geomarkers. Water would also be an environmental geomarker, but in no way would it be a biomarker because it would not correspond to organic compounds unequivocally related to life. A mineral, for example, magnetite, would also be a geomarker but not a biomarker. This terminological confusion means that, even from the scientific field, misunderstandings towards society are promoted, subsequently triggering, in certain media, sensational headlines about life on exoplanets, on Venus, Mars, Pluto, Europa, meteorites, Titan or on objects extrasolar (this would link to the last part of the article), without any foundation or evidence to confirm it. This unethical approach of the scientist to launch approaches close to pseudoscience is clearly counterproductive for anyone who tries to approach his research with the maximum detail and rigor and discredits us all, in addition to being just interested noise for popularity or other reasons.
Study of terrestrial analogs
The investigation of the so-called terrestrial analogs (or planetary analogs) in which geologists, microbial ecologists, geomicrobiologists and other specialists are working side by side, is being of great help to alleviate the impossibility of carrying out comparative planetology from the point of view biological/astrobiological. Thus, an attempt is made to cover the wide range of places, many of them extreme, in which the unique existing ecosystems are studied and characterized, to infer from this what they could be like if they occurred in other planetary bodies.
Spain has great biodiversity and geodiversity and, in this sense, we have internationally recognized terrestrial analogs, such as Río Tinto, El Jaroso, the Gulf of Cádiz, Bujaraloz, the Calatrava volcanic area or the Canary Islands, especially Tenerife and Lanzarote. . Will we find unusual bacterial consortia on Mars like those seen in Rio Tinto, Antarctica, or the Atacama? Will there be microorganisms under the icy crust of Europa or Enceladus similar to those found in the clathrates, crusts, nodules and chimneys of the Gulf of Cadiz? Could we perhaps find hydrothermal ecosystems on volcanic planets similar to those we see in areas such as the Teide tiles or in other areas of the Canary Islands? Did they perhaps exist in the past and left some trace? This is an indirect form of research, but no less valid for that, thanks to which, from Earth, we can work astrobiologically, bridging the gap, as if we were on the red planet or on an icy moon. But, above all, without confusing the habitability conditions (at different scales) of a planetary body with the existence of life.
Unanswered
But, once we have seen the most relevant aspects related to possible extraterrestrial life (focused above all on microbial life and its detection and identification), can we go further? Can we rigorously analyze scientifically what is related to the possible existence of intelligent life? At the moment, as indicated above, the answers are: “We don’t know and we must analyze and establish what intelligence is ” and, from here, make a series of postulates and inferences. Everything published so far is speculative and without sufficient evidence , from the Wow! signal, detected on August 15, 1977 and coming from the Sagittarius constellation, to the BLC1 signal, detected and observed during April and May 2019 , probably coming from Proxima Centauri, or to the recent hypotheses lacking scientific evidence about the interstellar object Oumuamua, discovered by Robert Weryk on October 19, 2017. There are several SETI projects (acronym for Search for ExtraTerrestrial Intelligence) that try to detect intelligent extraterrestrial life by analyzing electromagnetic signals captured in radio telescopes or even sending messages into space. So far, except as noted above, there have been no positive results or responses.
This is not the time to discuss what intelligence is or how to define it, including all the debate about its concept, individual, social, emotional, and whether or not it is quantifiable not only in humans, but also in other animals, plants, computers ( artificial intelligence), etc For this reason, talking about extraterrestrial intelligence also entails a problem similar to that of talking about extraterrestrial life: there is no agreed definition of what life is, nor is there one of what intelligence is. In any case, it seems inevitable, when talking about extraterrestrial (intelligent) life, to cite the Drake equation , proposed in 1961 by the astronomer and astrophysicist Frank Drake, more as an approximation than as a mathematical equation. This is a highly contested and speculative equation with considerable scientific disagreement about the values and the rigor with which its parameters are used.
Basically, it indicates that:
N = R* fp ne fl fi fc L
Where:
N = number of civilizations in our galaxy, with which we could communicate.
R *= stars capable of hosting planets.
fp = number of those stars that have planetary systems.
ne = number of those planets orbiting in the habitable zone with the possibility of life that there would be in those systems.
fl = fraction of those planets in the habitable zone on which life would have developed.
fi = number of planets on which intelligent life actually develops.
fc = number of planets with intelligent life and that, in addition, could communicate with others.
L = moment of life of the planet in which that civilization develops.
It goes without saying that, in these last 60 years, there have been all kinds of re-evaluations, with scientific debates and the proposal of new hypothetical parameters on the Drake equation. The latest corresponds to the study, using the Kepler space telescope, published in 2020 in The Astronomical Journal , in a collaboration between scientists from NASA, the SETI Institute and other institutions led by Steve Bryson. In this study they suggest that, in our galaxy alone, there could be about 300 million habitable planets (remember that habitability does not imply life).
“We do not know yet and, therefore, we continue to investigate”
It is not easy to write a popular science article on this subject, in which a solid ethical component must be maintained when transferring existing knowledge without sensationalism to society. For this reason, as I have already indicated, I consider that the best answer is: “We don’t know yet and, therefore, we are continuing to investigate.” Obviously, explaining the current state of the subject and future perspectives. Personally, I have been fortunate to investigate and promote many of these astrobiological topics and studies, including what astrobiology entails as an inter- and trans-disciplinary body of knowledge. And I have done it, among other activities, by participating in a NASA flight to understand the role of comets (Leonids) in the origin of life, by being part of the science teams of rovers that are on Mars (Curiosity or Perseverance ), trying to experimentally address lithopanspermia directly in space or in contact with ESA astronauts, as an instructor in planetary geology and astrobiology courses, in analogs such as Lanzarote for future manned missions to the Moon or Mars. I believe that the study of terrestrial analogs is fundamental and I have been able to address it with campaigns in places as diverse as Costa Rica, Spain, Iceland, Mauritania or Antarctica.
If we want to be able to identify and detect extraterrestrial life, we must carry out a parallel study that involves understanding how life originated on our planet , something that has not yet been resolved. So far, we are still investigating. As Carl Sagan said: “The universe is a huge place. If it’s just us, it would seem like a real waste of space to me.”
Jesús Martínez Frías is president of the Spanish Network of Planetology and Astrobiology (REDESPA) and of the Planetary Geology Commission of the Geological Society of Spain.
