How Many Extra-Solar Civilizations?

star field

1 Introduction

Do civilizations exist beyond the Solar System. If so, why have we not heard from them?  How many might exist? Might they be too far away for communication?

As members of national civilizations on Earth, our answers are based on our own experience. Our science suggests we emerge through biological evolution, with DNA as a means of reproducing ourselves. Our initial assumption is  that a civilization on an extra-solar planet would be based on the same chemistry and suffer from curiosity much like we do.

In recent decades we have found many planets around other stars. A fraction orbit in a zone where liquid water can exist, offering a chance for biology like ours to evolve. From this we can estimate the possible number of planets in the Milky Way galaxy living biological organism. The time required for planetary civilizations to evolve from such organisms, and the time they continue in existence, set the limit to the number present. If they are randomly dispersed throughout the Galaxy, we can estimate the average distance between them, and whether our communication systems could cross that gap. Unfortunately, planetary civilization lifetimes are so uncertain that we can only show how differing assumptions show different probabilities of communicating.

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