The chances of an intelligent race with the capability of reaching Earth existing at the same time as ourselves are astronomically slim.
Actually they're not. Stephen Dole wrote a book called "Habitable Planets for Man" which was commissioned by the RAND Corp. It is a very detailed and highly scientific treatise on the factors needed, and the probabilities of finding planets that man could inhabit. Guess what, there's a shitload of them.
"Habitable Planets For Man is a work by
Stephen Dole originally published for the
US Government contractor
RAND Corporation in 1964 with input from
Isaac Asimov. It was republished in a posthumous second edition in 2007, as
Planets for Man.
[1] The 174-page book contains a detailed scientific study on the nature of worlds that may support life in the universe, the probability of their existence, and ways of finding them,
[2][3] including assessments of 14 stars within 22
light years with a relatively high probability of having
habitable planets (a collective probability of 43%).
[4][5] Writing in a
Scientific American blog in 2011, Caleb Scharf called it "extraordinarily detailed and prescient".
[2]"
Habitable Planets for Man - Wikipedia