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Exoplanet Discoveries and the Fermi Paradox J 65th International Astronautical Congress, Toronto, Canada. Copyright ©2014 by Jerome Pearson Published by the IAF with permission, and released to the IAF to publish in all forms. IAC-14.A4.1.8 Exoplanet Discoveries and the Fermi Paradox J. Pearson* Star Technology and Research, Inc. USA, [email protected] Abstract Frank Drake examined the probability of extraterrestrial intelligence by developing his famous equation in the 1960s, which stated that the number of intelligence species in the galaxy is the product of 7 factors, from the rate of star formation through the probabilities of planets, habitability, life arising, communicating civilizations developing, and their expected lifetimes. Drake concluded that the number of intelligent species in the galaxy was on the order of one thousand to 100 million, with the nearest being perhaps 80-800 light years away. With Drake’s numbers, it appeared that we should hear from the extraterrestrials rather soon; but after half a century of watching and listening, we have no evidence of them. The recent astronomical evidence for thousands of planets in other solar systems enhances the Fermi paradox between the increasing number of potentially habitable planets in the Galaxy and the lack of any evidence of extraterrestrial civilizations. This prompts a new look at several potential answers to the paradox, covering some aspects not often addressed by space scientists and SETI researchers. These answers involve space science, life science and social science. The astronomical probability of habitable planets is reviewed, emphasizing the potential rarity of Earth-like planets, and the orbital wandering of “hot Jupiters.” The probability of the evolution of complex life is reviewed from a biological standpoint, from unicellular organisms to more complex life forms. The likelihood of the survival of intelligent species is reviewed from a physical anthropology standpoint, including past instances of near-extinction of the human species. The potential causes of the decline of societies are discussed, along with the expected lifetime of civilizations based on Earth experience. Finally, all these factors are combined into a new look at the Drake Equation to develop a revised estimate of the number of intelligent species in the galaxy. I. INTRODUCTION fp = the fraction of those stars that have planets, In 1959, Cornell physicists Giuseppi Cocconi and ne = the average number of planets that can Philip Morrison published an article in Nature in potentially support life per star that has planets, which they pointed out the potential for using fl = the fraction of planets that could support life microwave radio to communicate between the stars. that actually develop life at some point, In 1960, Frank Drake at the National Radio Astronomy Observatory in Green Bank, West fi = the fraction of planets with life that actually Virginia, first used the 85-foot radio telescope to go on to develop intelligent life (civilizations), listen for extraterrestrial signals from the nearby Sun- fc = the fraction of civilizations that develop a like stars Tau Ceti and Epsilon Eridani at the 21-cm technological civilization, and hydrogen line. Although it had negative results, it pioneered a new branch of science, the Search for L = the length of time for which such civilizations Extraterrestrial Intelligence (SETI). Drake called it release detectable signals into space. 1 Project Ozma . As a part of this effort, Frank Drake introduced Table 1 summarizes these parameters and the his famous equation predicting the number of initial estimates of their values in the 1960’s. Using communicating civilizations in the galaxy. The their guesses, they expected the number of Drake Equation in its original form is: civilizations in the galaxy to be as few as 20 or as high as 50 million. The nearest one could range from N = R*∙ fp∙ ne∙ fl∙ fi∙ fc∙ L 80 to 8000 light years away. Drake said that where: considering the uncertainties, he expected N ~ L, N = the number of civilizations in our galaxy with giving N of 1000 to 100,000,000 societies, and the which radio-communication might be possible, nearest could be as close as 40 light years away. R* = the average rate of star formation in our galaxy, 1 65th International Astronautical Congress, Toronto, Canada. Copyright ©2014 by Jerome Pearson Published by the IAF with permission, and released to the IAF to publish in all forms. Table 1. Initial Estimates of the Number of all detection methods, such as astrometry and direct Civilizations in the Galaxy imaging, the number of confirmed and candidate 5 Drake Parameter Original exoplanets now totals more than 4200 . Estimate The exoplanet search results seem to indicate that Rate of star formation in the Galaxy 1/year on the average, there is about one planet per star in Fraction of stars with planets 0.2-0.5 the Galaxy, and we would expect that many of these Number of habitable planets per star 1-5 are apparently in the habitable zone of their parent Fraction of those with life 1 stars. The sheer number of exoplanets is surprising; it Fraction of those with intelligent life 1 appears that almost every star in our galaxy has Fraction with technological 0.1-0.2 planets and there may be on the average one civilization habitable planet per star. In 1950 Enrico Fermi was Lifetime of technological 103-108 years lunching with colleagues at Los Alamos, and the civilizations discussion led to extraterrestrial intelligence and life Number of civilizations in the 20 to 5x107 on other planets. Fermi pondered for a moment and 6 Galaxy then suddenly said “Where is everybody?” This has come to be known as the Fermi Paradox. With so many planets, and the apparent technical possibilities Encouraged by these estimates, the Soviet Union of interstellar travel described by Mole7 and performed some all-sky searches2 during the 1960’s, Woodcock,8 it is puzzling that we see no and NASA Ames and JPL established SETI programs extraterrestrials on Earth. 3 in the 1970’s . In the late 1980’s, Ames examined 9 1,000 Sun-like stars in a Targeted Search, capable of Michael Hart reviewed this paradox , classified detecting weak or sporadic signals, and JPL the potential explanations as physical (astronomical, systematically swept all directions in a Sky Survey; biological or technical problems), sociological but the program was terminated by Congress in 1992. (interest, motivation, political problems), and More recently, the SETI Institute and UC Berkeley temporal (we are the first). He concluded that the formed a joint project to build a SETI-dedicated array likely answer is that extraterrestrials do not exist. of telescopes that will equal a 100-meter radio Hart was writing in 1975, before the discoveries of telescope, the Allen Telescope Array. SRI exoplanets, but this only intensifies the paradox. We International replaced Berkeley, and under funding will review the current observational and theoretical from Paul Allen of Microsoft, the initial 42 dishes of evidence for the parameters in the Drake Equation, the planned 350 have been completed. and try to determine a rational explanation for the lack of evidence of extraterrestrials. We will review SETI has examined about 1000 stars, without each of the potential parameters for an explanation, detecting any artificial signals, but a related search from stars and planets to life, intelligence, and technique, looking for extraterrestrial planets, or civilization. exoplanets, has borne fruit4. There are 3 main methods for exoplanet detection. Most have been detected by the transit method, which detects the II. GALAXIES AND THE MILKY WAY slight dimming of the star’s light as a planet moves in front of it. This method detects only planets whose The Sun is in the Milky Way galaxy, a barred orbital plane is in our line of sight, but the NASA spiral about 100,000 light years across the spiral Kepler space telescope mission has discovered over arms, containing about 200 billion solar masses. The 1100 such exoplanets. Milky Way is brighter and has higher metallicity (presence of elements heavier than hydrogen and Next is the radial velocity method, in which a helium) than 98% of all galaxies10. The presence of star’s motion in the direction toward and away from heavier elements allows stellar nebulas to condense Earth can reveal the periods, distances, and sizes of and form solar systems with planets, along with planets. This method has detected more than 500 asteroids, comets, and meteors, and is necessary for exoplanets. The third method is even more the development of life. The Milky Way’s oldest serendipitous, depending on Einstein’s general theory stars are about 13 billion years old, almost as old as of relativity and its prediction that light from a distant the universe. The Milky Way includes dark matter star can be gravitationally lensed by the mass of a and other mass beyond the spiral arms but let us foreground star and planet. Only 26 planets have leave out stars in globular clusters in the halo of the been discovered by this microlensing method, and galaxy, and also stars in the galaxy’s companions the follow-up observations are very difficult. Including Magellanic Clouds, and just consider the galaxy 2 65th International Astronautical Congress, Toronto, Canada. Copyright ©2014 by Jerome Pearson Published by the IAF with permission, and released to the IAF to publish in all forms. population across the diameter of the spiral arms. nebulas could not form rocky planets. When that The central bulge has about 40 billion stars, and the first generation of massive stars became supernovas, disk has about 120 billion stars. they created the heavier elements by nuclear synthesis, and spewed them out when they exploded. Just as a star has a habitable zone over which a New stars forming from those clouds had rocky planet can have liquid water, the galaxy also has a minerals and could form planetary systems.
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