Life in the Cosmos
There must be many different environments suitable for life in a given planetary system. Once life originates, it tends to be very adaptable and tenacious.
In the Solar system there are several bodies that may be suitable for life of some art: the Earth certainly (if we all live in a real world and not in a computer simulation), and perhaps Mars, Titan and Jupiter.
There is evidence that planets are a frequent accompaniment of star formation. We can see this evidence in the satellite systems of Jupiter, Saturn and Uranus, which are like miniature solar systems. Theories of the origin of the planets are based on this premise and studies of double stars confirm it. We can also observe accretion disks around stars and find the evidence in some preliminary investigations of gravitational perturbations of nearby stars.
Thus many, perhaps even most stars in our universe must have planets.
But what about life? All 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 step now on less certain ground: there may be impediments in the evolution of the genetic code, although we think this unlikely over billions of years of primeval chemistry.
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. There must be many quite different pathways to an advanced civilization of specified capabilities.
On the Earth we must consider the apparent difficulty in the evolution of large organisms represented by the Cambrian explosion. Thus let us suggest that only one percent of planets on which life arises eventually produce a technical civilization.
The conclusion in interesting enough: the total number of planets in the Milky Way only on which life has arisen at least once may be a hundred billion inhabited worlds. And we repeat that we speak about the Milky Way only. That in itself is a remarkable conclusion. But we are not yet finished. One percent of plants where a technical civilization has developed, give us the total number of one billion “civilized” worlds.
Obviously, 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 goes like a shot in all planetary systems. Some other think that even given ten of fifteen billion years, the evolution of even ten technical civilizations in the Milky Way Galaxy is unlikely.
Anyway, this is not a subject on which we can do much experimentation as long as our investigations are limited to a single planet. Our estimation about a billion planets on which technical civilizations have arisen at least once is rather speculative. It is very different from saying that there are a billion planets on which technical civilizations now exist. For this, we must know much more about Cosmos.
How many technical civilizations exist in the Universe? We know for sure about only one by now. Image : Unknown author