Extraterrestrial Civilizations - Isaac Asimov [40]
The Earth-sized substar would be composed of a much larger percentage of volatiles than Earth itself, since there would have been no nearby hot star to raise the temperature in surrounding space and make the collection of volatiles impossible. Therefore, again as on Ganymede and Callisto, we might imagine a world-girdling ocean, probably of water, kept liquid by internal heat, but covered by a thick crust of ice.
Substars still smaller than the Earth would have less internal heat and would be even more likely to be icy, have less in the way of sporadic sources of appreciable energy, have smaller internal oceans or none at all.
If a body were small enough to attract little or no volatile matter even at the low temperatures that would exist in the absence of a nearby star, it would be an asteroidal body of rock or metal or both.
What about substars that are larger than Earth and therefore possess greater and more intense reservoirs of internal heat? Such a larger body is bound to be Jupiterlike. A large substar is certain to be made up largely of volatile matter, particularly hydrogen and helium; and high internal heat will make the planet entirely liquid.
Heat can circulate much more freely through liquid by convection than through solids by slow conduction. We can expect ample heat at or near the surface in such large substars and the heat may remain ample for billions of years. However, again the most we can expect on a large substar is intelligent life of the dolphin variety—and no technological civilization.
In short, the formation of substars would rather resemble the formation of bodies in the outer Solar system, and we may expect no more of the former than of the latter.
For a technological civilization, we need a solid planet with both oceans and dry land, so that life as we know it can develop in the former and emerge on the latter. To form such a world there must be a nearby star to supply the heat that would drive away most of the volatile matter, but not all. The nearby star would also supply the necessary energy for the formation and maintenance of life in a copious and steady manner.
In that case, we must concentrate our attention on the stars. These, at least, we can see. We know they exist and need not simply assume the probability of their existence as in the case of the substars.
THE MILKY WAY
If we turn to the stars and consider them as energy sources in the neighborhood of which we may find life, possibly intelligence, and possibly even technological civilizations, our first impression may be heartening, for there seem to be a great many of them. Therefore, if we fail to find life in connection with one, we may do so in connection with another.
In fact, the stars may well have impressed the early, less sophisticated watchers of the sky as innumerable. Thus, according to the Biblical story, when the Lord wished to assure the patriarch Abraham that, despite his childlessness, he would be the ancestor of many people, this is how it is described:
“And he [God] brought him [Abraham] forth abroad, and said, ‘Look now toward heaven, and tell the stars, if thou be able to number them’; and he [God] said unto him [Abraham], ‘So shall thy seed be.’ ”
Yet if God were promising Abraham that he would ultimately have as many descendants as there were stars in the sky that he could see, God was not promising as much as might be assumed.
The stars have been counted by later generations of astronomers who were less impressed with their innumerability. It turns out the number of stars that can be seen with the unaided eye (assuming excellent vision) is, in total, about 6,000.
At any one time, of course, half the stars are below the horizon, and others, while present above the horizon, are so near it as to be blotted out through light absorption by an unusually great thickness of even clear air. It follows that on a cloudless, moonless night,