It appears that biogenic planets, and therefore planets capable of giving rise to civilizations, should be found primarily near the corotational circle of the Galaxy.

If we accept this reconstruction of the history of our system, we will be forced to revise our previous notions regarding the psychozoic density of the Universe.

We are fairly sure that none of the stars in the Sun’s vicinity — within a radius of fifty light-years — is home to any civilization that possesses a communications technology at least equal to ours.

The radius of the corotational circle is about 105 parsecs — that is, 34,000 light-years. The whole Galaxy has more than 150 billion stars. Assuming that a third of the stars are located in the nucleus and the thick bases of the spiral arms, we obtain — for the arms themselves — a total of 100 billion stars. We do not know how thick to make the torus, a figure in the shape of an automobile tire, which, if drawn around the corotational circle, will contain the whole zone favoring the emergence of life-bearing planets. Let us assume that in the zone that makes up the biogenic torus lie a hundred-thousandth of all the stars of the galactic spiral — that means millions. The entire circumference of the corotational circle is about 215,000 light-years. If each of the stars there produced one civilization, the average distance between two inhabited planets would equal 5 light-years. But the stars near the corotational circle are not spread out evenly in space. Moreover, planet-bearing stars are more likely to be found within the spiral arms, and stars with a planet on which life can evolve without fatal disturbances are more likely to be found in the space between the arms, where there is less exposure to stellar upheavals. However, most of the stars are inside the arms, where stars are most densely concentrated.

Therefore one should seek signals of extraterrestrial intelligence along the corotational arc ahead of the Sun and behind the Sun on the galactic plane — that is, between the stellar clouds of Perseus and Sagittarius, because the stars there, like our Sun, have the galactic passage behind them and are now moving, like our solar system, in the empty space between the arms.

But these simplified statistical reflections have little value. Let us return to our reconstructed history of the Sun and its planets.

Where the corotational circle intersects the spiral arms, their thickness is about three hundred parsecs. The protosolar gas cloud, moving along an orbit at an angle of seven or eight degrees to the plane of the Galaxy, entered the arm for the first time about 4.9 billion years ago. For three hundred million years the cloud underwent the stormy conditions of passage through the entire width of the arm; since it left the arm, it has been traveling through calm space. That trip has lasted much longer than the passage through the arm, because the corotational circle, along which the Sun moves, intersects the spiral arms at a sharp angle, making the arc of the solar orbit between the arms longer than the arc within the arm.

The diagram (after L. S. Marochkin, Priroda [Nature], no. 6, 1982) shows our Galaxy, the radius of the corotational circle, and the orbit of the solar system around the galactic nucleus. The speed with which the Sun and the planets move relative to the spiral arms is a subject of controversy. The diagram shows our system having passed through both arms. If that was the case, then the first passage was made by a cloud of gas and dust, which began condensing noticeably only when it crossed the second galactic arm. Whether we have behind us one or two passages is not important, because it has to do with the age of the cloud — that is, with when it first formed — and not with when it began the fragmentation that was the first stage of astrogenesis. Stars are born in a similar way even now.

An isolated cloud cannot contract gravitationally into a star because it would preserve its angular momentum (in accordance with the laws of motion) and rotate faster the smaller its radius. If eventually a star formed, it would rotate at the equator with a speed exceeding light, but this is impossible: the centrifugal forces would tear it apart much earlier. So stars emerge in great numbers from separate fragments of a cloud, in the course of processes that are gradual at first but become increasingly violent. Moving apart during condensation, the fragments of cloud take away part of the young stars’ angular momentum. If one speaks of “the yield of astrogenesis” as the ratio between the mass of the original cloud and the combined mass of the stars formed from it, that yield is not large. The Galaxy is therefore a “producer” that squanders the initial capital of matter. But the scattered parts of the star-bearing clouds eventually begin to contract gravitationally again, and the process repeats itself.

When the cloud contracts, not every cloud fragment behaves the same. When the great collapse begins that leads to the formation of a star, the center of the cloud is denser than its periphery. For this reason, the star- creating fragments vary in size. In the center, they are two to four times the Sun’s size; on the circumference, ten to twenty times. From inner condensates there can form small, long-lived stars that burn with more or less the same brightness for billions of years. The Sun is one of these. On the other hand, the large, peripheral stars can generate supernovas, which, after an astronomically short life, are blown apart by powerful explosions.

How our particular cloud began to condense we do not know; all we can re-create is the fate of that local fragment in which the Sun and the planets had their origin. When the process began, the nearby supernovas contaminated the protosolar cloud with radioactive particles. At least two such contaminations occurred. The first time, the protosolar cloud was contaminated with isotopes of iodine and plutonium, probably near the inner edge of the spiral arm; the second time (three hundred million years later), deep inside the spiral, another supernova bombarded it with the radioactive isotope of aluminum.

From the degree to which these isotopes have been transformed, by radioactive decay, into other elements, one can tell when each contamination occurred. The short-lived isotopes of iodine and plutonium went to a stable isotope of xenon, and the radioactive isotope of aluminum became magnesium. The xenon and the magnesium have been found in meteors of our solar system. Comparing these data with the age of the Earth’s crust (using as a yardstick the times of decay of uranium and thorium, the long-lived isotopes contained in it), one can reconstruct approximate if not exact “scenarios” of the solar cosmogony.

The diagram shows the scenario by which a gaseous cloud first passed through the spiral nine and a half billion years ago. Its density was still subcritical then, so there was no fragmentation and formation of condensates. That occurred only after its entrance into the next arm of the Galaxy, 4.6 billion years ago. On the outer edge of the condensates, conditions favored the rise of supernovas; on the inside, of smaller stars, like the Sun. Subjected to compression and the explosions of the supernovas, the protosolar fragment changed into the young Sun and the planets, comets, and meteors. This cosmogonic scenario is simplified: the fragmentation of the gaseous clouds is random; through the enormous expanse of the arms run shock fronts, produced by various cataclysms; erupting supernovas can take part in generating such fronts.

Galaxies continue to give birth to stars, because the Universe in which we live, while certainly not young, is not yet old. Computer simulations reaching far into the future show that in the end all the star-generating material will be depleted, the stars will be extinguished, and whole galaxies will “vaporize” into radiation and particles.

From this “thermodynamic death” we are separated by some 10100 years. Long before that — in 1015 years — all the stars will lose their planets from having other stars pass close to them. The planets, whether lifeless or inhabited, torn from their orbits by strong perturbations, will be swallowed in endless darkness and a temperature close to absolute zero. Paradoxically, it is easier to describe what will become of the Universe in 1015 or in 10100 years, or what took place in the first few minutes of its existence, than to reconstruct the different stages of solar and terrestrial history. It is even more difficult to foresee what will become of our system when it leaves the calm space that stretches between Perseus and Sagittarius, between the stellar clouds of the two galactic arms. Assuming that the difference between the speed of the Sun and the spiral equals one kilometer per second, we shall reach the next spiral in five hundred million years.

In dealing with cosmogony, astrophysics proceeds like a detective gathering circumstantial evidence: there are only a few “footprints and exhibits,” from which, like the scattered pieces of a jigsaw puzzle (and many of the pieces are lost), one must put together a consistent whole. What is worse, it appears that from these bits of evidence one can build a number of unidentical models. Not all the data, especially in the case that interests us, are numerically determinable (for example, the difference between the orbital speed of the Sun and that of the galactic spiral). In addition, the spiral arms themselves are not so compact, and do not move through space so clearly and regularly, as in our diagram. Finally, all spiral nebulae are similar, but similar in the way people are who are of different heights, weights, ages, races, sexes, and so on.

Nevertheless, cosmological work on the Milky Way is getting closer and closer to the true state of things. Stars are born mainly inside the spiral arms; supernovas explode most frequently inside those arms; the Sun is

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