Are We Alone?
As far as we know, life may have only evolved once in the universe. We have just one case study, which doesn't tell us much about what it takes to create life from nothing, or how often that might happen on other worlds.
Suppose life did arise elsewhere. What are the chances it would evolve into technologically advanced, intelligent beings? In 1961, the astronomer Francis Drake wrote an equation to help estimate the number of intelligent civilizations in the Milky Way.
N = the number of civilizations in our galaxy that we can communicate with
R*=average rate for the formation of stars that are suitable for life
fp= the fraction of those stars that have planets
ne=the fraction of those planets with an environment suitable for life
fl=the fraction of those planets where life actually appears
fi=the fraction of planets with life that evolves to be intelligent
fc=the fraction of planets with intelligent life that develop technologies that release detectable signs into space
L=length of time those signals are released