Extraterrestrial Civilizations (21 page)

Under such packed conditions, collisions and near-collisions may not be very rare. Transfer and capture of mass may serve to build up stars of great mass that quickly explode with a force that leads to a veritable chain reaction of explosions and to the formation of “black holes.” These are the ultimate in star condensations (see my book,
The Collapsing Universe
).

A black hole is matter at its ultimate density, and has a gravitational field so intense at its surface that nothing can escape it, not even light.

If a black hole is formed under conditions in which matter of all kinds surrounds it (as in galactic centers), such matter is constantly spiraling into the black hole, releasing x-rays and other energetic radiation in the process. (This radiation is released before the matter actually enters the black hole, so that it can escape into outer space.)
The black hole gains in mass and may eventually be large enough to swallow stars whole.

There is a strong radiation source at the very center of our own Galaxy, and it may well be that a black hole is present there, one that has a mass of 100 million stars. The giant galaxy M87 was reported in 1978 to have a black hole in its center in all likelihood, one that has a mass as high as that of 10 billion stars. It may even be that every galaxy and every globular cluster has a black hole at its core.

Such violent events at the centers of galaxies may produce the massive atoms of complex elements and spread them through space, but of what use would that be? Those violent events are the sites of emission of enormous quantities of energetic radiation, and for many light-years in every direction, life (as we know it) might for that reason be impossible.

The Population II regions are therefore, considering chemical constitution or energetic radiation, doubly unsuitable for life.

Suppose we pass on to the outskirts now, regions where the violence and radiation of the center does not reach.

Here, the primordial gas was relatively thin and was distributed irregularly. For that reason, stars were formed irregularly, and giant stars were routinely formed in numbers that could not possibly have existed in the center. (Of course, many medium and small stars were also formed.)

The stars in the outskirts of a galaxy, rich in giants and spread out irregularly over much vaster volumes of space than exist in the central regions, are referred to as Population I stars.
^*
What’s more, there were places in the outskirts where the gas was too thin to condense readily. To this day, therefore, the outer Population I regions of the galaxies are rich in clouds of gas and dust.

The original Population I stars were as entirely hydrogen-helium in constitution as were the Population II stars. There was this difference, however:

The giant stars that formed in the galactic outskirts didn’t remain on the main sequence long. A few hundred thousand years only, for the real monsters; a few million years for the mere titans; as
much as a billion years for those that were simply giant.

And when they left the main sequence, expanded and finally collapsed, they exploded into supernovas of unimaginable violence. Vast volumes of gas, containing significant quantities of complex elements rolled out into space, adding themselves to the clouds of uncondensed gas that were already present.

Such explosions take place repeatedly in the outer regions of a galaxy, but so widely separated are the stars in those vast outer regions that supernovas do not seriously affect any stars other than (at most) their immediate neighbors.

As many as 500 million supernova explosions may have taken place in the outskirts of our own Galaxy since it came into being. The 500 million have enriched space enormously with complex elements, and have added to the density of the clouds of gas and dust that existed from the beginning. The outward force of the explosion may even have served as an initiation of swirls and compressions in nearby gas clouds that led to the formation of a new star, or whole groups of new stars.

New stars, forming out of gas clouds containing elements produced in an older star that had distributed those elements in its death throes, are called second-generation stars. Our Sun, which formed only 5 billion years ago, when the Galaxy was already 10 billion years old and after hundreds of millions of stars had already died, is a second-generation star.

The cloud out of which second-generation stars are formed contain the elements out of which ices, rocks, and metals are formed, and therefore can produce planetary systems similar to our own Solar system.

If we look for Sunlike stars that are capable of incubating life, therefore, we must eliminate Population II stars and even many of the Population I stars. We can only consider second-generation Population I stars.

Population II stars are confined to only a small portion of the total volume of a galaxy, to its compact central regions and to the almost as compact globular clusters. All the open vastness of the outer regions is the domain of Population I stars.

That is not, however, as impressive as it sounds. Some 80 percent of the stars of a galaxy are to be found in the compact central regions and in the globular clusters.

We might argue, too, that only half of the 20 percent of stars that are in the Population I regions are second-generation stars. That means that only 10 percent of all the Sunlike stars with effective ecospheres are second-generation Population I stars, and can conceivably have Earthlike planets revolving about them.

That gives us our fifth number:

Even if a star is a perfect incubator, if it is the precise duplicate of our Sun in every respect, that is still not enough. What is needed is not only an incubator, but something to be incubated as well. In short, there must be a planet on which life can develop in the beneficent radiation of the star it circles.

To be sure, we have already decided that virtually every star has its planetary system, so that there are 5,200,000,000 second-generation, Population I, Sunlike stars in our Galaxy with planets—but where are those planets located?

A given star might be a perfect incubator, but some of its planets may be too close to it and therefore too hot to bear life, while others might be too far and therefore too cold to bear life. There might be no planet at all within the star’s ecosphere on which water could exist as a liquid.

What are the chances, then, that a given star has a planet, at least one, within its ecosphere?

In trying to make a judgment here, we are badly hampered by the fact that we know only one planetary system in detail—our own. What’s more, we have no way at all at present of possibly learning any appropriate details about any other planetary system. The few planets we may possibly have detected circling nearby stars are all the size of Jupiter or larger.

Such giant planets are the only ones we can possibly detect at the moment, and that only with great difficulty and considerable uncertainty. Whether there are any planets actually within the ecosphere of such stars, planets that lie closer to the star and that are small enough to be Earthlike, it is impossible to tell.

We are forced to fall back on the only thing we have, our own planetary system. It may conceivably be a very atypical, freakish structure that simply can’t be used as a guide, but we have no reason to think so. The temptation is to follow the principle of mediocrity and to suppose that the planetary system in which we find ourselves is a typical one and that it can be used as a guide.

There is some hope that this is not just prejudice on our part, or wishful thinking. The American astronomer Stephen H. Dole has checked this, as well as one can, by computer. Beginning with a cloud of dust and gas of the mass and density thought to have served as the origin of the Solar system, he set up the requirements for random motion, for coalescence on collision, for gravitational effects, and so on. The computer calculated the results.

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. It might be best to split the
difference and to suppose that the probability of a planet within the ecosphere is close to 0.5, or 1 in 2.

That would give us our sixth number:

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.