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$R$ is the \textbf{rate of star formation}, telling how many stars are born every year in our Galaxy. This number is about 10 and might seem quite small, but the Galaxy is about 10 billion years old, so plenty of stars have born (and died) in the meanwhile.\\  R\approx $R\approx  10$ The other terms in the Drake equation were not known in 1961. Actually till the early '90s we didn't even know if planets were a unique feature of the Solar system.