Only in the last decade or two have we developed a reasonable understanding of these numbers. It’s now thought that, roughly speaking, there is around one habitable Earth-sized planet per star in our Galaxy. But after that, the numbers in the Drake equation start to get more fuzzy, and we only really have educated guesses to guide us. We next need to figure out the fraction of habitable planets that actually develop life, what fraction of planets with life go on to develop intelligence, what fraction of intelligent civilizations transmit detectable signals into space, and the lifetime of such technological civilizations.
“Every SETI person has their own version of the [numbers in the] Drake Equation”, says Korpela. “It’s a fun thing to play with. Mine includes uncertainties on what we know and don’t know. The last time I did the numbers, I calculated that there were somewhere between 200,000 technological civilizations in our Galaxy, at the upper end … and at the lower end, there’s one technological civilization for every two million galaxies. So it’s quite a range.”