Extraterrestrial Civilizations - Isaac Asimov [72]
That would give us our sixth number:
6—The number of second-generation Population I stars in our Galaxy with a useful ecosphere and a planet circling it within that ecosphere = 2,600,000,000.
HABITABILITY
The mere fact that a planet is in the ecosphere does not mean that it is a suitable abode for life; that it is habitable, in other words.
For proof of that we need look no farther than our own Solar system. The Earth itself is the only planet in the Solar system that is clearly within the ecosphere of the star it circles. Our definition of the word planet, however, obscures the fact that there are two worlds in the ecosphere just the same.
The Moon, strictly speaking, is not a planet, because it circles the Earth (or rather the Earth-Moon center of gravity, which the Earth also circles), but it is a world. What’s more, it is a world that is just as firmly within the ecosphere as the Earth and yet the Moon is not a habitable world.* The Moon clearly has too little mass to be habitable, since because of its small mass it cannot retain an atmosphere or liquid water. What, then, can we say about the masses of planets?
As I have said in the case of Population II stars where the only materials for planetary structure are hydrogen and helium, the only possible planets would seem to be giants with the mass of Uranus or more. Nothing less would possess the gravitational intensity that would make it possible to hold on to hydrogen and helium.
In the case of Population I stars, which are the only ones we are considering as suitable incubators for life, we have metals, rocks, and ices in addition to hydrogen and helium for uses as structural materials. Again here, only giant planets can make use of the hydro-gen and helium, and it is precisely because they can that they are giant planets.
On the other hand, where Population I stars are concerned, smaller worlds of all sizes can be built up of metals, rocks, and ices, since these will hold together through forces other than gravitational.
How large can these smaller worlds be?
Not very large, for even among Population I stars of the second generation, the quantity of materials other than hydrogen and helium is rather small, and cannot be used to build a large world. And if these stars could, they would gather hydrogen and helium and become giant worlds.
Dole’s computer simulations of planetary formation make it seem pretty clear that within the ecosphere of Sunlike stars those planets that are not giants are quite small.
How large and massive can a nongiant planet be?
If we exclude the four giant planets of the Solar system (and the Sun itself, of course), then the largest body in the Solar system is none other than the Earth itself.
Earth is, therefore, very likely to be near the top limit of mass for nongiant, nonhydrogen planets.
A planet somewhat larger than Earth, but not much larger, would, if all other factors were suitable, surely be habitable. The one unavoidable consequence of the greater mass would be a more intense gravitational field, which might manifest itself as a somewhat higher surface gravity. There is no reason to think that life could not adapt itself to a somewhat higher surface gravity.
After all, life on Earth evolved in the ocean where, thanks to buoyancy, the influence of gravity is minor. Living organisms invaded the dry land, where the influence of gravity is major, yet not only coped with it but even evolved ways of moving rapidly despite gravity. A somewhat greater surface gravity would surely not defeat life when it has shown such amazing adaptability on the one world where we can study it in detail.
Then, too, if a world is somewhat more massive than Earth, but also somewhat less dense, so that its surface is farther from the center than one would expect under Earthlike conditions, the surface gravity may be no higher than that of Earth, or even a bit lower.
We might reasonably conclude, then,