Extraterrestrial Civilizations - Isaac Asimov [71]
The computer worked our different random happenings, and in every case a planetary system very much like ours resulted. There were from seven to fourteen planets, with small planets near the Sun, large planets farther out, and small planets again still farther out. In almost every case, there was a planet rather like the Earth in mass, at rather Earth’s distance from the Sun, and planets much like Jupiter in mass at much like Jupiter’s distance from the Sun, and so on.
In fact, if a diagram of the real Solar system is mixed in with the various computer simulations, it is not at all easy to separate the real from the simulated.
It is hard to say how much importance we can lend to such computer simulations, but for what they are worth, they do give a color of truth to the principle of mediocrity, at least in this respect.
If we now study our own planetary system on the assumption that it is typical, we can see that the planets move in nearly circular orbits that are widely spaced, and that the orbit of one does not overlap the orbit of the planet within or the one without.
This tends to make sense, since orbits that are too closely spaced would, in the long run, prove unstable. Between collisions and gravitational interactions, the worlds are bound to nudge themselves apart early in the history of the planetary system.
This means that it is completely unlikely that there will be very many worlds crammed into the ecosphere of a Sunlike star. The ecosphere is not likely to be wide enough for that. In fact, we might suspect intuitively that once the planets are done nudging themselves apart, not more than one planet is likely to find itself within the ecosphere; or two, if we find ourselves dealing with a double planet on the order of the Earth and the Moon.
How does this check with our own planetary system?
Here, for instance, Earth is clearly within the Sun’s ecosphere, or you and I would not exist to question the matter.
Even as late as a generation ago, the ecosphere would have seemed to be some 100 million kilometers (62 million miles) deep at least, since it was generally supposed that while Venus might be uncomfortably warm and Mars uncomfortably cool, both had environments not so extreme as to preclude the presence of life.
Not so. Venus has suffered a runaway greenhouse effect and is far too hot for life. Mars may be in a permanent ice age and be far too cold for life. The trigger leading in either direction may be a minor one.
If this is so, the Sun’s ecosphere may be shallower than we think. Indeed, in 1978, Michael Hart of NASA simulated Earth’s past history by computer and if his starting assumptions are correct, and his computer programming likewise, then it would seem that Earth, at one stage in its history, escaped a runaway greenhouse effect by a narrow margin and at another stage escaped a runaway ice age by a narrow margin. A little nearer the Sun or a little farther from it, and Earth would have fallen prey to one or the other. It may be, from Hart’s figures, that the Sun’s ecosphere is only 10 million kilometers (6,200,000 miles) thick and it is only a most fortunate coincidence that Earth happens to be in it.
Well, then, what can we say? If the ecosphere is quite wide (even if not wide enough to include either Venus or Mars), then from Dole’s computer simulation of planetary systems, a planet is virtually certain to form within it somewhere. The probability would be roughly 1.0.
On the other hand, if Hart’s computer simulation of Earth’s past history is accurate, then it is very likely that no planet at all will form within the ecosphere, and that all the planets near the star will be Venuslike or Marslike, and only on quite rare occasions Earthlike. The probability of a planet within the ecosphere would then be close to 0.0.
The results of computer simulation are still too recent and, perhaps, too crude to allow us to lean too certainly in either the optimistic or the pessimistic direction.