Extraterrestrial Civilizations - Isaac Asimov [58]
Intelligent life, however, did not come upon the Earth at its very beginning, but only as the result of a long course of evolution. If our Sun had only shone as it does now for 500,000 years after the formation of the Earth, and had then left the main sequence, it is highly doubtful if there would have been time for even the simplest protolife to form in Earth’s oceans.
In fact, judging from the experience of Earth, it takes some 5 billion years of planetary existence for life to develop to the point of complexity where a civilization can be established.
We can’t, of course, be sure how typical Earth’s case is of the Universe as a whole. It may be that evolution has, for some trivial reason or other, been extraordinarily slow on Earth, and that on other planets much less time has been required for the evolution of intelligence. It may, on the other hand, be that evolution on Earth has, for some trivial reason or other, been extraordinarily rapid, and that on other planets much more time is required for the evolution of intelligence.
There is no way, at the moment, in which we can tell whether either alternative is true. We have no recourse but to adhere to “the principle of mediocrity” and to assume that this one case we know of—that of Earth—is not atypical, but is about average in its nature.
We must, therefore, cling to a 5-billion-year lifetime on the main sequence as an essential minimum for the development of civilization.
A star that is 1.4 times as massive as the Sun and is of spectral class F2 remains on the main sequence for 5 billion years, and we can therefore come to the conclusion that any star more massive than 1.4 times the mass of the Sun will not serve as an appropriate incubator for life. There may indeed be life on a planet circling such a too-massive star, but the chance that it will exist long enough to reach the appropriate pitch of complexity to produce an extraterrestrial civilization is small enough to ignore.
This means that the bright stars we see in the sky, which are (at least, most of them are) considerably more massive than the Sun, are unsuitable incubators. Sirius, for instance, will remain on the main sequence for 500 million years altogether, Rigel for only 400 million years. We can ignore such stars.
As it happens, however, it is precisely these massive short-lived stars that are fast-rotators and were therefore not included by me in the number of stars possessing a planetary system. Their exclusion from further consideration is thus doubly justified.
MIDGET STARS
Let’s try the other extreme, now, and consider a star with 1/16 the mass of the Sun and one-millionth the luminosity. (Any object less massive than that would probably not be massive enough to ignite the nuclear fires at the center and would not, therefore, be a true star.)
A midget star with 1/16 the mass of the Sun would be 65 times as massive as the planet Jupiter, but would surely be much more dense and might not be much larger than Jupiter in size. It might perhaps be 150,000 kilometers (93,000 miles) in diameter.
Next, suppose that Earth were 300,000 kilometers (186,000 miles) from the center of such a star and therefore circling it at a height of 150,000 kilometers (93,000 miles) above its surface. Earth would circle that star every 1.1 hours.
Earth would receive as much total energy from that very nearby midget star as the Earth now does from the Sun. The fact that the midget star would be barely red hot would be made up for by the fact that from the distance of the planet its apparent size would be 3,000 times that of the Sun as we see it from Earth.
To be sure, the nature of the energy received from the midget star would be different from that of the Sun. The midget star would deliver virtually no ultraviolet radiation and, in fact, very little visible light. Most of its energy would be in the form of infrared light.
This would be very inconvenient from our standpoint. To our own eyes,