Technical Civilizations

Technical Civilizations

There is evidence that planets are a frequent accompaniment of star formation; in the satellite systems of Jupiter, Saturn and Uranus, which are like miniature solar systems; in theories of the origin of the planets; in studies of double stars; in observation of accretion disks around stars; and in some preliminary investigations of gravitational perturbations of nearby stars. Many, perhaps even most stars may have planets.

We take the fraction of stars that have planets, f(p), as roughly equal to 1/3. Then the total number of planetary systems in the Galaxy would be Nxf(p) =1.3 x 10(11) – the symbol = means here “approximately equal to”). If each system were to have about ten planets, as ours does, the total number of worlds in the Galaxy would be more than a trillion, a vas arena for the cosmic drama.

In our own solar system there are several bodies that may be suitable for life of some art: the Earth certainly, and perhaps Mars, Titan and Jupiter. Once life originates, it tends to be very adaptable and tenacious. There must be many different environments suitable for life in a given planetary system. But conservatively we choose n€=2. Then the number of planets in the Galaxy suitable for life becomes N f(p) n(e) – 3 x10(11).

Sun and planets. There may be a billion planets on which technical civilizations now exist only in our Galaxy. Image © Elena

Experiments show that under the most common cosmic conditions the molecular basis of life is readily made, the building blocks of molecules able to make copies of themselves. We are now on less certain ground; there may, for example, be impediments in the evolution of the genetic code, although we think this unlikely over billions of years of primeval chemistry. We choose f(1)- 1/3, implying a total number of planets in the Milky Way on which life has arisen at least once as N f(p) n(e), f(1) = 1x 10(11), a hundred billion inhabited worlds. That in itself is a remarkable conclusion. But we are not yet finished.

The choices of f(1), and f(e) are more difficult. On the one hand, many individually unlikely steps had to occur in biological evolution and human history for our present intelligence and technology to develop. On the other hand, there must be many quite different pathways to an advanced civilization of specified capabilities. Considering the apparent difficulty in the evolution of large organisms represented by the Cambrian explosion,, let us choose f(1) x f(e) = 1/100, meaning that only 1 percent of planets on which life arises eventually produce a technical civilization. This estimate represents some middle ground among the varying scientific opinions. Some think that the equivalent of the step from the emergence of trilobites to the domestication of fire gores like a shot in all planetary systems; other think that even given ten of fifteen billion years, the evolution of technical civilizations is unlikely. This is not a subject on which we can do much experimentation as long as our investigations are limited to a single planet. Multiplying these factors together, we find a billion planets on which technical civilizations have arisen at least once. But that is very different from saying that there are a billion planets on which technical civilizations now exist. For this, we must also estimate all the factors.