By Walter E. Meyers

Copyright (c) 1983 by Willis E. McNelly All rights reserved.

While not bearing directly on the question of the existence of God, the likelihood of the presence of intelligent life can be determined by arguments thousands of years old. These arguments have been sharpened by the exploration of thousands of worlds, strengthening some, disproving others. They lead, as we shall see, to one of the most baffling questions mankind has ever faced.

Through all of this essay, we shall be discussing intelligent life of the kind represented by humanity. We shall not concern our-selves with speculation on what other kinds of intelligence may be possible, but rather limit ourselves to the kind we know most about--ourselves. And the first area we shall consider is that of the stars and their planetary systems which are hospitable to life.

The best theoretical considerations, supported by primary investigation, demonstrate that something over 90% of stars have planetary systems of one kind or another. For our galaxy that would give about 280 billion planetary systems. Of this vast number, about a quarter circle stars between spectral classes F2 and M2--that is, stars that range between 1-1/2 times the mass of Sol or Wake to 1/2 half their mass. Above the top of this range, the lifetime of the star is too short to allow intelligent life to evolve, while below the bottom end, the tidal effects of the star on her planets make the evolution of life unlikely. Eliminating those stars larger or smaller than this range leaves 70 billion planetary systems circling stars with the right mass, luminosity, and lifespan for the evolution of intelligent life.

Many of those stars, however, are binary or multiple systems: somewhere around 65% of them. The existence of such systems does not necessarily rule out conditions for the maintenance of life, except in those binaries where one member is a giant. Such systems are unsuitable because the giant would explode as a supernova long before intelligent life could evolve. But in many other cases, one of the stars in the binary system possesses planets, one of which falls within the ecosphere that supports life: Ecaz, fourth planet of Alpha Centauri Beta, is an example of such. Moreover, in some binary systems, both stars have planetary systems and suitable ecospheres: thus, Yorba, the third planet of Cygni Alpha, is hardly affected by the existence of its mother star's companion, while Diana, second planet of Cygni Beta, is similarly unharmed by the existence of Cygni Alpha. Subtracting those stars without suitable conditions because of their membership in a multiple system, we are left with about fifty billion appropriate stars or star systems with ecospheres that will support life of our kind.

Of course, there may be no suitable planet within the ecosphere of the star, or the chemical composition of the star may be wrong. Those stars, principally toward the center of the galaxy, that are called Population II stars, were formed almost entirely from hydrogen and helium, and hence their planetary systems must also be composed of those elements. Any planets they possess would be gas giants. They would lack the carbon, oxygen, nitrogen, and so on necessary for the life familiar to us. Moreover, the enormous amount of radiation at galactic centers is inimical to life.

Of the Population I stars, on the outskirts of the galaxy, many would have been enriched by heavy elements in the gas clouds formed by the exploding giants of the region. Stars formed from these clouds, so-called second generation" Population I stars--and only these--would have planetary systems whose chemical composition includes those elements needed for life. About 10% of our former total are second generation Population I stars, or 5 billion.

Second, consider the ecosphere itself; "ecosphere" is the term used for the range of distance from the star within which life of our kind is possible: closer to the star, conditions are too hot; farther from it, conditions are too cold. Thus, the star must not only have a suitable ecosphere, but the planets in its system must move in almost circular, widely separated orbits, and most important, one of the planets must be within the ecosphere. Now, how "deep" is the ecosphere? Theories here have never been exact, ranging from 100 million kilometers thick (large enough for two or three planets) to 10 million kilometers (barely enough for one). Exploration has disclosed ecospheres of varying depth, but in general only about half of the planetary systems under discussion here include a planet which falls within the ecosphere. Systems such as that of Lalande, in which two planets (Carillon, the second, and Libermann, the third) fall within the ecosphere are very rare. Therefore our total is further reduced: stars with planets within a suitable ecosphere in our galaxy number about 2 and a half billion.

Third, consider the composition of those suitable planets. The planet must have enough mass to possess a gravity sufficient to hold a suitable atmosphere. Using the mass of Giedi Prime as the norm, planets with less than .4 of its mass will be unable to retain a dense enough atmosphere for free surface liquid to exist. Just about 40% of the planets within suitable ecospheres are in the right range of mass, and have surface water--about 1 billion planets. Yet conditions hostile to the evolution of intelligent life may still occur. Planets with eccentric orbits will produce extreme variation in surface conditions, as may planets with slow rotations. Planets on the massive side of the range may collect so much water that the ocean covers, or nearly covers, their surfaces (Gamont is such a world; it is unlikely that intelligent life could have evolved on its scattered archipelagos); smaller planets may collect too little water, turning the planet into a desert (Arrakis is no counterexample here, since the desert conditions there were not those prevailing in the relatively recent geological past).

Other things may be wrong: too high a degree of vulcanism, or too intense a meteoric bombardment (both conditions have been advanced as the cause of the disappearance of Harmonthep in historical times). Planets not subject to any of these disqualifications, ones like Caladan, Finally, Refuge, Topaz, Wallach, or the many other human-inhabited worlds, represent about one-half the total number: hence, we are led to the figure of about 500 million habitable worlds.

To measure the likelihood of life arising on one of these planets, we must now turn to chemistry and biology. In the thousands of years that mankind has studied this problem, hundred of theories have been advanced about how, when, or whether chemical reactions would produce complex proteins and nucleic acids in the combinations needed for life. These theories of the occurrence of life have swung from "impossible" to "inevitable" and back again with frustrating regularity. In the worst possible case--that the generation of life depends on complexities with a vanishingly small chance of happening, I would not have written this essay, nor you be reading it. Our intention here is to examine the best possible case: that in the conditions suitable for the preservation of life, life will arise, indeed must arise, after an elapsed time of about one billion years. That span of time comprises 8% of the lifetime of a habitable planet; hence, of the total of such planets, about 40,000,000 (8%) have had insufficient time for life to arise. But that means that 92% have had enough time: 460 million planets should be life-bearing. But what kind of life? Using the fossil records of those planets which have been explored, we find that multi-cellular organisms take, on the average, four billion years to evolve. Therefore one-third of habitable planets bearing life have nothing more complex than unicellular organisms.

Subtracting these, we are left with something like 307 million planets with higher forms of life. What percentage of these, then, should have intelligent life? Explorers have often approached a world that seemed to have all the right features, even one with sea life of complexity, yet the land masses were sterile--devoid of life (one such is Vereitelung). It appears that 36% of a planet's lifetime is necessary for the conquest of the land--an environment sufficiently protective allow the existence of life and sufficiently challenging to give an advantage to intelligence. By this measure, 294 million worlds have had enough time for intelligent life to arise. Ten thousand years should certainly suffice for the development of space flight, and if human civilization is taken as the measure of the progress of all civilizations (given a planetary lifespan of some 12 billion years), then the civilizations of almost all of these inhabited worlds should be more advanced than we are. By the arguments here advanced (none of which are novel), we are led to the conclusion that there should be an enormous number of intelligent species in the universe, each of which has evolved independently. Even if these arguments are mistaken by a factor of 100, there should still be 2,800,000 civilizations in our galaxy more advanced than we are. Thus we confront the question which twenty thousand years of human thought has failed to answer:? Where are they?


It is useless to argue that intelligence may necessarily imply a violence that so intense that it inherently limits the development of civilizations. If this were the case--if civilizations inevitably destroyed themselves after a shorter or longer time, we should have found some trace by now of those destroyed societies. None has been found. (See THE CROMPTON RUINS)