Extraterrestrial Civilizations (22 page)

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, that in the ecosphere where a star’s heat will be great enough to preclude the gathering of
hydrogen and helium, planets will not form that are too massive for life.

Worlds that are not massive enough can certainly form, as for instance the Moon, but how massive is not massive enough?

To support life, a world must be massive enough to generate a sufficiently large gravitational field to hold a substantial atmosphere—not so much for the sake of the atmosphere, as because that alone would make it possible to have free liquid on the surface.

In the Solar system there are exactly four of the nongiant worlds with substantial atmospheres: Earth, Venus, Mars, and Titan.

Venus, with a mass 0.82 that of the Earth, has a considerably denser atmosphere than Earth (but is nonhabitable for other reasons). Mars, which has 0.11 times the mass of the Earth, has a very thin atmosphere; one that, while substantial, is clearly not sufficient to support anything but, just possibly, the simplest forms of life. Titan, which has a mass 0.02 that of Earth, has an atmosphere that may be somewhat more substantial than that of Mars, but which exists at all only because Titan is far beyond the outermost reach of the ecosphere.

Within the ecosphere, a world can maintain an adequate atmosphere if it is not as massive as Earth, but it should certainly be more massive than Mars. If, let us say, its mass were 0.4 times that of Earth, that might be sufficient.

In or near the Sun’s ecosphere, there are four worlds of considerable size: Earth, Venus, Mars, and the Moon. (There are also bodies of trifling size, such as the two satellites of Mars, and periodic entries of asteroids or comets, but these may all be ignored as not significant.) Of these four, Earth and Venus are higher in mass than the 0.4 mark, while Mars and the Moon are lower.

If we use the principle of mediocrity and consider this as a fair sample of the situation in the Universe as a whole, we could conclude that of all the worlds in or near appropriate ecospheres surrounding appropriate stars, only half have masses suitable for habitability.

If a world of the proper mass is present in the ecosphere, many of its characteristics would automatically be like those of the Earth. For instance, it would be too warm for substantial quantities of the icy materials to be in the solid state; and in liquid or gaseous state, the gravitational field of the world would not be intense enough to hold them. Therefore, a world of the proper mass in the ecosphere would
be built up primarily of rock, or of rock and metal, as are all the worlds of the inner Solar system.

Water, as the icy material that melts and boils at the highest temperature, that is the most common, and that most readily combines with rocky substances, is on all three counts the most likely of the ices to be retained to some degree. Therefore, worlds of the proper mass in the ecosphere are very likely to have quantities of surface water in gaseous, liquid, and solid form. They would have oceans that would cover at least part of the surface.

In short, a world in the ecosphere that is of the proper mass would be “Earthlike” in character.

If one out of every two worlds in the ecosphere is Earthlike, we have our seventh figure:

Even an Earthlike planet, in terms of temperature and structure, might be nonhabitable for any of a variety of minor reasons. It could not very well support life if it were subjected, for instance, to great extremes of environmental conditions.

Suppose a planet had an average distance from the Sun that was right in the middle of the ecosphere, but suppose it also had a particularly eccentric orbit. At one end of its orbit it might swoop so far toward the Sun as to be well inside the inner border of the ecosphere, while on the other side it would recede so far from the Sun as to be well outside the outer border. Such a planet would have a short, unbelievably torrid summer that might briefly bring the oceans to a boil; and a long, unbelievably frigid winter, during which the oceans may begin to freeze.

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:

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.

It is rather breathtaking to decide on the basis of (we hope) strict logic and the best evidence we can find that there are 650 million habitable planets in our Galaxy alone, and therefore over 2 billion billion in the Universe as a whole. And yet, from the standpoint of the subject matter of this book, of what value are habitable planets in themselves? If they lack life, their habitability comes to nothing.

Our calculations concerning extraterrestrial intelligence must therefore come to a halt right here, unless we can say something reasonable about the chance that a habitable planet actually has life on it.

In order to do that, we must again turn to something that is known, and that is the one habitable planet that we
know
to have life on it—Earth itself. In other words, before we can say anything sensible about life on habitable planets in general, we must be able to say something sensible about how life came to exist on the Earth.

Early speculations about the existence of life on Earth invariably assumed it to have been created through some nonnatural agency,
usually through the action of some god or demigod. The best-known story in our Western tradition is that humanity was created in the same series of divine acts that created the Universe generally.

In six days of creation the job was done. God created light on the first day; the land and sea on the second; plant life on the third; the heavenly bodies on the fourth; animal life of the sea and air on the fifth; and animal life on land on the sixth. As the last creative act on the sixth day, humanity was brought into being.

Life, created on three different days, was considered as having come into being in separate species (“after his kind” it says in the King James Bible). Presumably, these were the species that continued to exist into contemporary times. As some believed, no species were added to the first creation and none subtracted.

As to the date of this Divine creation, the Bible is not specific, for the habit of dating with compulsive precision is a rather late development in historical writing. Deductions based on various statements in the Bible, however, place the date of creation only a few thousand years in the past. The precise date usually found in the headings of the King James Bible is 4004
B.C
., this date having been worked out by the Irish theologian James Ussher (1581–1656).

Although the creation of the world (or of different worlds) was assumed to be a once-for-all act, it was common in early times to assume that this was not necessarily true for life.

Actually, this is a reasonable attitude. After all, while there was no visual evidence of any creation of worlds in the course of human history, there did seem to be visual evidence for the creation of living things without the intervention of earlier living things.

Field mice may make their nests in holes burrowed into stores of wheat, and these nests may be lined with scraps of scavenged wool. The farmer, coming across nests from which the mother mouse has had to flee, and finding only tiny, naked, blind infant mice, may come to the most natural conclusion in the world: he has interrupted a process in which mice were being formed from musty wheat and rotting wool.

Let meat decay and small wormlike maggots will appear in it. Frogs can seem to arise out of river mud.

If the notion were true for various species of vermin, it might be true for all species of organisms, though perhaps less common for the
larger and more complex species such as horses, eagles, lions, and human beings.

In fact, if one were sufficiently daring, one might suppose that the tale in Genesis was a fable; that this sort of “spontaneous generation” of living things from nonliving antecedents might account for the
original
beginning of life. Little by little each species might have formed, first the simple ones and later the more complex ones, with human beings, naturally enough, last of all.

And in that case, if we were to apply this to habitable planets generally, we would see that they, too, would naturally form life. All of them would bear life.