Extraterrestrial Civilizations - Isaac Asimov [46]
Just as the Sun is a star of intermediate size, the Milky Way Galaxy is one of intermediate size. There are galaxies with masses 100 times larger than that of the Milky Way Galaxy, and tiny galaxies with only a hundred-thousandth the mass of the Milky Way Galaxy.
Again, the small objects of a particular class greatly outnumber the large objects, and we might estimate rather roughly that there are on the average 10 billion stars to a galaxy, so that the average galaxy is of the size of the Large Magellanic Cloud.
That would mean that in the observable universe, there are as many as 1,000,000,000,000,000,000,000 (a billion trillion) stars.
This one consideration alone makes it almost certain extraterrestrial intelligence exists. After all, the existence of intelligence is not a zero-probability matter, since we exist. And if it is merely a near-zero probability, considering that near-zero probability for each of a billion trillion stars makes it almost certain that somewhere among them intelligence and even technological civilizations exist.
If, for instance, the probability were only one in a billion that near a given star there existed a technological civilization, that would mean that in the Universe as a whole, a trillion different such civilizations would exist.
Let us move on, though, and see if there is any way we can put actual figures to the estimates; or, at least, the best figures we can.
In doing so, let us concentrate on our own Galaxy. If there are extraterrestrial civilizations in the Universe, those in our own Galaxy are clearly of greatest interest to us since they would be far closer to us than any others. And any figures we arrive at that are of interest in connection with our own Galaxy can always be easily converted into figures of significance for the others.
Begin with a figure that deals with our Galaxy and divide it by 30 and you will have the analogous figure for the average galaxy. Begin with a figure that deals with our Galaxy and multiply it by 3.3 billion and you have the analogous figure for the entire Universe.
We start then with a figure we have already mentioned:
1 —The number of stars in our Galaxy = 300,000,000,000.
* Our Sun, it is perhaps needless to say, is a star, and seems so different from all the rest only because it is so much closer to us
* Since light travels at the rate of 299,792 kilometers (186,282 miles) per second, a light-year is 9,460,000,000,000 kilometers (5,878,500,000,000 miles) long. The distance of Sirius is therefore 82 trillion kilometers (50 trillion miles). It is simpler to use light-years.
CHAPTER 6
Planetary Systems
NEBULAR HYPOTHESIS
The existence of the stars themselves, in no matter how huge a number, does not guarantee the existence of civilizations, or even of life, if only stars exist. The stars supply the necessary energy, but life must develop at a temperature compatible with the existence of the complex organic compounds that are the chemical basis of it.
This means that there must be a planet existing in the neighborhood of the star. On that planet, warmed and, in general, energized by that star, life might conceivably exist.
We must therefore not consider stars, but planetary systems—of which our own Solar system is the only example that we know definitely and in detail.
Unfortunately, we cannot observe the neighborhood of any star other than that of our own Sun with sufficient minuteness to be able to detect, directly, the presence of planets circling them.* Does this defeat us at the start and make it impossible to come to any further conclusions as to the existence of extraterrestrial intelligence?
Not necessarily. If we can determine how our own Solar system was formed, we might be able to draw conclusions as to the probability of the formation of other planetary systems.
For instance, the first theory of Solar system formation