Despite decades of studies, we have no evidence of the existence of life in the solar system or on extrasolar planets. This could change in the next decade with observations from the James Webb Telescope or the ARIEL (Atmospheric Remote-Sensing Infrared Exoplanet Large-survey) mission, the project of a space telescope for the study of extrasolar planets, with their physical conditions and chemical compositions, which is expected to be launched in 2029. It will be very unlikely that anything new will come from the SETI, OSETI, and METI projects for all the reasons discussed in the previous chapter. Therefore, the following question remains: are we alone still standing? Obviously, there are studies that have given a probabilistic answer to the question by trying to overcome the limits on knowledge of the parameters of the Drake equation. Returning to the Drake equation, we have three astrophysical parameters that are quite well measured today (R* fp ne). The factors fl, fi, and fc that have to do with the emergence of life, intelligence, and technology are not known, as is the duration of civilization, the L factor. In 2016, Adam Frank and Woodruff Sullivan published a paper in the journal Astrobiology, reviewing the Drake equation in light of Kepler’s discoveries. The two scientists reformulated the starting question in such a way as not being interested in the average duration of a civilization, the L factor, or whether this civilization still exists to be able to receive the possible messages sent by it. Their choice distances their study from the formulation of the Drake equation, which aims to calculate the number of existing technological species. The question we ask ourselves is therefore: what are the chances that ours is the only technologically advanced civilization that has ever existed? This change in perspective reduces the uncertainty terms present in the Drake equation. As already mentioned, the 3 astrophysical parameters are well estimated, and the three terms fl, fi, and fc remain in the equation. By eliminating the L factor and taking into account the entire Universe, not just our galaxy, the Drake equation transforms into the product of the factors N = (N* fp ne) (fl fi fc) = Nastrophysics fbt. In other words, the number of civilizations that existed in any epoch in the Universe are given by the product of astrophysical factors, NAstrophysics, with N* the total number of stars in the Universe, which is on the order of 2 × 1022, fp approximately 1 and ne approximately 0.2, as in Chap. 10 , and from fbt, which collects the factors related to the birth of life, intelligence and technology. The two scientists worked in statistical terms and estimated the lower limit of the probability that one or more technological civilizations evolved at some place and time in the observable Universe. In terms of probability, if N was equal to 0.01, this would mean that if the history of the Universe was repeated 100 times, only one technological civilization would appear. The important result they obtained is that unless the probability that an alien civilization has developed on a habitable planet is less than one in a million billion billion, 10−24 (i.e., 0.00000000000000000000001), which is a truly small number, humans are not the first technologically advanced life form to have inhabited the observable Universe. Repeating the calculations for a galaxy such as ours, we obtain that the probability is equal to 1.7 × 10−11; that is, we are sure that a technological species has developed in the history of our galaxy if the probability that a technological species appears on a habitable planet is greater than 1 in 60 billion. In the past, when the Drake equation was used, more or less pessimistic hypotheses have been formulated about the formation of civilizations on other planets. One of the most negative claims is that the probability of a civilization forming is 1 in 10 billion for each planet. Taking into account this pessimistic estimate and Frank and Woodruff’s result for the entire Universe, trillions of technological civilizations would have existed in the history of the Universe. Obviously, the study does not refer only to the past but is valid for future eras; therefore, we must expect that civilizations have existed before us and will exist after us. Before Frank and Woodruff, Amir D. Aczel, a few years after the discovery of the first exoplanet, published a book, Probability 1, in which he calculated the probability of existence of a planet with life in the Universe via statistics. Unlike Frank and Woodruff, Aczel wanted to answer the following question: are we alone? not if civilizations existed throughout the whole history of the Universe. He assumed fp = 0.5 in the Drake equation; today, we know that it is approximately 1, and he assumed that among the exoplanets known at the time (only 9), there was at least one in the habitable zone. He assumed that the probability of life originating is very low: 1 in 1012, that the number of stars in our galaxy is 300 billion, that there are 100 billion galaxies, and that the number of stars in the universe is on the order of 3 × 1022 and used an elementary statistical rule, that of the union of independent elements, to find the probability of life around a star in the Universe, finding that the probability is P = 1−(0.999999999999995)30 000 000 000 000 000 000 000 000, i.e., a probability indistinguishable from 1, i.e., 100%. The same result would be achieved even if there were 10 billion stars in our galaxy and if a billion galaxies existed. Even though the probability on a single planet is very low, the compound probability that life exists on at least one planet increases steadily due to the large number of stars and planets. Another thing to add is that today, we know that our universe has a flat geometry; therefore, it could be infinite, and this further increases the probability. In 2020, another statistical study by Amedeo Balbi and Claudio Grimaldi evaluated the impact of a discovery of life on a single planet on the number of planets on which there is life. According to the study, if a planet with life was found, there should be at least 100,000 in our galaxy. To confirm these studies directly, we must wait for the next few decades to study the atmospheres of habitable extrasolar planets. These studies could also provide more precise numbers via the Seager equation, for example. One way to overcome the parameter problems of the Drake equation is the equation introduced by Sara Seager. If one needs to know the number of planets with detectable signs of life, one needs to know the number of stars observed, the number of stable stars, the fraction of stars with rocky planets in the habitable zone, the fraction of those planets that can be observed, the fraction with life present, and the fraction on which life produces gaseous biosignatures detectable. The equation focuses on finding planets with biosignature gases, which are gases produced by life that can accumulate in a planet’s atmosphere to levels that can be detected with remote space telescopes rather than aliens equipped with radio technology. In other words, this equation, with the observation of powerful space telescopes, makes it possible to reveal whether life exists on a planet. In summary, compared with the times in which Drake began his studies, today, we have much more information on the astrophysical parameters that serve to answer the question of whether there is or has been life in the Universe. Using this new knowledge and statistics, we can answer our question, and it seems that there have been civilizations in the history of the Universe. Answers to our question using methods such as SETI may never be present, but in a decade or two with the study of the atmospheres of nearby habitable exoplanets, we will be able to say with certainty whether there is life on them. Therefore, the answer to the question is as follows: have there been and will there be civilizations in our Universe? is almost certainly positive. However, it is very likely that we will not have direct contacts, as in the film Contact or Close Encounters of the Third Kind, or indirect contacts with exchanges of electromagnetic signals.

We Are Not Alone

章节
发表日期：

2025年

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Antonino Del Popolo

摘要

Despite decades of studies, we have no evidence of the existence of life in the solar system or on extrasolar planets. This could change in the next decade with observations from the James Webb Telescope or the ARIEL (Atmospheric Remote-Sensing Infrared Exoplanet Large-survey) mission, the project of a space telescope for the study of extrasolar planets, with their physical conditions and chemical compositions, which is expected to be launched in 2029. It will be very unlikely that anything new will come from the SETI, OSETI, and METI projects for all the reasons discussed in the previous chapter. Therefore, the following question remains: are we alone still standing? Obviously, there are studies that have given a probabilistic answer to the question by trying to overcome the limits on knowledge of the parameters of the Drake equation. Returning to the Drake equation, we have three astrophysical parameters that are quite well measured today (R* fp ne). The factors fl, fi, and fc that have to do with the emergence of life, intelligence, and technology are not known, as is the duration of civilization, the L factor. In 2016, Adam Frank and Woodruff Sullivan published a paper in the journal Astrobiology, reviewing the Drake equation in light of Kepler’s discoveries. The two scientists reformulated the starting question in such a way as not being interested in the average duration of a civilization, the L factor, or whether this civilization still exists to be able to receive the possible messages sent by it. Their choice distances their study from the formulation of the Drake equation, which aims to calculate the number of existing technological species. The question we ask ourselves is therefore: what are the chances that ours is the only technologically advanced civilization that has ever existed? This change in perspective reduces the uncertainty terms present in the Drake equation. As already mentioned, the 3 astrophysical parameters are well estimated, and the three terms fl, fi, and fc remain in the equation. By eliminating the L factor and taking into account the entire Universe, not just our galaxy, the Drake equation transforms into the product of the factors N = (N* fp ne) (fl fi fc) = Nastrophysics fbt. In other words, the number of civilizations that existed in any epoch in the Universe are given by the product of astrophysical factors, NAstrophysics, with N* the total number of stars in the Universe, which is on the order of 2 × 1022, fp approximately 1 and ne approximately 0.2, as in Chap. 10 , and from fbt, which collects the factors related to the birth of life, intelligence and technology. The two scientists worked in statistical terms and estimated the lower limit of the probability that one or more technological civilizations evolved at some place and time in the observable Universe. In terms of probability, if N was equal to 0.01, this would mean that if the history of the Universe was repeated 100 times, only one technological civilization would appear. The important result they obtained is that unless the probability that an alien civilization has developed on a habitable planet is less than one in a million billion billion, 10−24 (i.e., 0.00000000000000000000001), which is a truly small number, humans are not the first technologically advanced life form to have inhabited the observable Universe. Repeating the calculations for a galaxy such as ours, we obtain that the probability is equal to 1.7 × 10−11; that is, we are sure that a technological species has developed in the history of our galaxy if the probability that a technological species appears on a habitable planet is greater than 1 in 60 billion. In the past, when the Drake equation was used, more or less pessimistic hypotheses have been formulated about the formation of civilizations on other planets. One of the most negative claims is that the probability of a civilization forming is 1 in 10 billion for each planet. Taking into account this pessimistic estimate and Frank and Woodruff’s result for the entire Universe, trillions of technological civilizations would have existed in the history of the Universe. Obviously, the study does not refer only to the past but is valid for future eras; therefore, we must expect that civilizations have existed before us and will exist after us. Before Frank and Woodruff, Amir D. Aczel, a few years after the discovery of the first exoplanet, published a book, Probability 1, in which he calculated the probability of existence of a planet with life in the Universe via statistics. Unlike Frank and Woodruff, Aczel wanted to answer the following question: are we alone? not if civilizations existed throughout the whole history of the Universe. He assumed fp = 0.5 in the Drake equation; today, we know that it is approximately 1, and he assumed that among the exoplanets known at the time (only 9), there was at least one in the habitable zone. He assumed that the probability of life originating is very low: 1 in 1012, that the number of stars in our galaxy is 300 billion, that there are 100 billion galaxies, and that the number of stars in the universe is on the order of 3 × 1022 and used an elementary statistical rule, that of the union of independent elements, to find the probability of life around a star in the Universe, finding that the probability is P = 1−(0.999999999999995)30 000 000 000 000 000 000 000 000, i.e., a probability indistinguishable from 1, i.e., 100%. The same result would be achieved even if there were 10 billion stars in our galaxy and if a billion galaxies existed. Even though the probability on a single planet is very low, the compound probability that life exists on at least one planet increases steadily due to the large number of stars and planets. Another thing to add is that today, we know that our universe has a flat geometry; therefore, it could be infinite, and this further increases the probability. In 2020, another statistical study by Amedeo Balbi and Claudio Grimaldi evaluated the impact of a discovery of life on a single planet on the number of planets on which there is life. According to the study, if a planet with life was found, there should be at least 100,000 in our galaxy. To confirm these studies directly, we must wait for the next few decades to study the atmospheres of habitable extrasolar planets. These studies could also provide more precise numbers via the Seager equation, for example. One way to overcome the parameter problems of the Drake equation is the equation introduced by Sara Seager. If one needs to know the number of planets with detectable signs of life, one needs to know the number of stars observed, the number of stable stars, the fraction of stars with rocky planets in the habitable zone, the fraction of those planets that can be observed, the fraction with life present, and the fraction on which life produces gaseous biosignatures detectable. The equation focuses on finding planets with biosignature gases, which are gases produced by life that can accumulate in a planet’s atmosphere to levels that can be detected with remote space telescopes rather than aliens equipped with radio technology. In other words, this equation, with the observation of powerful space telescopes, makes it possible to reveal whether life exists on a planet. In summary, compared with the times in which Drake began his studies, today, we have much more information on the astrophysical parameters that serve to answer the question of whether there is or has been life in the Universe. Using this new knowledge and statistics, we can answer our question, and it seems that there have been civilizations in the history of the Universe. Answers to our question using methods such as SETI may never be present, but in a decade or two with the study of the atmospheres of nearby habitable exoplanets, we will be able to say with certainty whether there is life on them. Therefore, the answer to the question is as follows: have there been and will there be civilizations in our Universe? is almost certainly positive. However, it is very likely that we will not have direct contacts, as in the film Contact or Close Encounters of the Third Kind, or indirect contacts with exchanges of electromagnetic signals.