Extraterrestrial Civilizations - Isaac Asimov [70]
New stars, forming out of gas clouds containing elements produced in an older star that had distributed those elements in its death throes, are called second-generation stars. Our Sun, which formed only 5 billion years ago, when the Galaxy was already 10 billion years old and after hundreds of millions of stars had already died, is a second-generation star.
The cloud out of which second-generation stars are formed contain the elements out of which ices, rocks, and metals are formed, and therefore can produce planetary systems similar to our own Solar system.
If we look for Sunlike stars that are capable of incubating life, therefore, we must eliminate Population II stars and even many of the Population I stars. We can only consider second-generation Population I stars.
Population II stars are confined to only a small portion of the total volume of a galaxy, to its compact central regions and to the almost as compact globular clusters. All the open vastness of the outer regions is the domain of Population I stars.
That is not, however, as impressive as it sounds. Some 80 percent of the stars of a galaxy are to be found in the compact central regions and in the globular clusters.
We might argue, too, that only half of the 20 percent of stars that are in the Population I regions are second-generation stars. That means that only 10 percent of all the Sunlike stars with effective ecospheres are second-generation Population I stars, and can conceivably have Earthlike planets revolving about them.
That gives us our fifth number:
5—The number of second-generation, Population I, Sunlike stars in our Galaxy with a useful ecosphere = 5,200,000,000.
THE ECOSPHERE
Even if a star is a perfect incubator, if it is the precise duplicate of our Sun in every respect, that is still not enough. What is needed is not only an incubator, but something to be incubated as well. In short, there must be a planet on which life can develop in the beneficent radiation of the star it circles.
To be sure, we have already decided that virtually every star has its planetary system, so that there are 5,200,000,000 second-generation, Population I, Sunlike stars in our Galaxy with planets—but where are those planets located?
A given star might be a perfect incubator, but some of its planets may be too close to it and therefore too hot to bear life, while others might be too far and therefore too cold to bear life. There might be no planet at all within the star’s ecosphere on which water could exist as a liquid.
What are the chances, then, that a given star has a planet, at least one, within its ecosphere?
In trying to make a judgment here, we are badly hampered by the fact that we know only one planetary system in detail—our own. What’s more, we have no way at all at present of possibly learning any appropriate details about any other planetary system. The few planets we may possibly have detected circling nearby stars are all the size of Jupiter or larger.
Such giant planets are the only ones we can possibly detect at the moment, and that only with great difficulty and considerable uncertainty. Whether there are any planets actually within the ecosphere of such stars, planets that lie closer to the star and that are small enough to be Earthlike, it is impossible to tell.
We are forced to fall back on the only thing we have, our own planetary system. It may conceivably be a very atypical, freakish structure that simply can’t be used as a guide, but we have no reason to think so. The temptation is to follow the principle of mediocrity and to suppose that the planetary system in which we find ourselves is a typical one and that it can be used as a guide.
There is some hope that this is not just prejudice on our part, or wishful thinking. The American astronomer Stephen H. Dole has checked this, as well as one can, by computer. Beginning with a cloud of dust and gas of the mass and density thought to have served as the origin of the Solar system, he set up the requirements for random motion, for