There are some hundred billion (10^11) galaxies, each with, on the average, a hundred billion stars. In all the galaxies, there are perhaps as many planets as stars, 10^11 x 10^11 = 10^22, ten billion trillion. In the face of such overpowering numbers, what is the likelihood that only one ordinary star, the Sun, is accompanied by an inhabited planet? Why should we, tucked away in some forgotten corner of the Cosmos, be so fortunate? To me, it seems far more likely that the universe is brimming over with life. But we humans do not yet know. We are just beginning our explorations. The only planet we are sure is inhabited is a tiny speck of rock and metal, shining feebly by reflected sunlight, and at this distance utterly lost.'
Written by: Carl Sagan, The Shores of the Cosmic Ocean, Cosmos