Extraterrestrial Civilizations - Isaac Asimov [74]
We can imagine life might develop that could withstand such extremes, but it seems reasonable to suppose that the chances are it would not.
Again, extremes would lower the chances of life’s coming into being if a planet’s axis of rotation were inclined so steeply to the vertical (relative to its plane of revolution about its star) that the major portion of the planet would be in sunlight for half a year and in the dark for half a year.
And yet again, if a planet rotates very slowly, the days and nights are each long enough to allow undesirable temperature extremes.
If a planet is a little on the massive side, it may just happen to collect enough water to make its ocean a planetary one, with little or no dry land. Even if life then develops, it is not likely that technology will, and we are looking not for life alone, but technology as well.
In reverse, if a planet is a little on the nonmassive side and little water is collected, the world may be mostly desert, and life may at best form to only a limited extent and reach insufficient levels of complexity.
The atmosphere may not be quite right in some ways, and block off too much of the sunlight, or too little of the ultraviolet radiation. Or else the crust may not be quite right and there may be too much volcanic action or earthquakes. Or else the surroundings in near space may not be quite right and meteoric bombardments may be too intense for life to maintain itself.
None of these imperfections is very likely, perhaps. After all, among the planets of our Solar system, only two (Mercury and Pluto) have orbits that are significantly elliptical; only one (Uranus) has an enormous axial tilt; only two (Mercury and Venus) have very slow periods of rotation, and so on.
Yet although each one of the imperfections is unlikely in itself and may affect only one out of ten Earthlike planets, or fewer, all the various imperfections mount up.
Again, we might suppose (intuitively) that only one out of every two Earthlike planets is Earthlike in every important particular; that it has a day and night of reasonable length, seasons that do not go to unreasonable extremes, oceans that are neither too extensive nor too restricted, a crust that is neither too unsettled nor too geologically inert, and so on.
We might say that such planets are “completely Earthlike” or, better, simply “habitable.” In fact, we no longer have to specify that we are speaking of Sunlike stars, or of second-generation Population I stars, or of ecospheres. The term habitable would imply all that out of necessity.
If, then, one out of every two Earthlike planets are habitable, we have our eighth figure:
8—The number of habitable planets in our Galaxy = 650,000,000
This sounds like a large number and, of course, it is, but it represents a measure of our conservatism also. This number means that in our Galaxy, only one star out of 460 can boast a habitable planet. What’s more, it is a more conservative figure than some astronomers would suggest. Carl Sagan, who is one of the leading investigators of the possibility of extraterrestrial intelligence, suggests there may be as many as one billion habitable planets in the Galaxy.
*To be sure, if the Earth were as far from either 61 Cygni star as it is from the Sun, Earth would be frozen into a permanent ice age. On the other hand, if it were imagined to be at the distance from either star than Venus is from the Sun, Earth might do very well.
* It is because the stars of our own region of the Galaxy are of this type that they got the “I” classification.
* We judge the habitability of a world by the fact that life can originate on it and be maintained on it independently of other worlds. If human beings eventually establish a base on the Moon, that should be credited not to the Moon’s habitability but to human ingenuity and technology.
CHAPTER 9
Life
SPONTANEOUS GENERATION
It