What does the Drake equation estimate?

The number of communicating civilisations in our galaxy - and its value is not the answer it produces but the way it breaks an unanswerable question into pieces. Frank Drake wrote it in 1961 as an agenda for a meeting rather than as a result: rate of star formation, fraction with planets, fraction habitable, fraction where life arises, fraction where intelligence arises, fraction that communicate, and how long they last.

The first few terms have become real numbers. When Drake wrote it nobody knew whether planets were common; the exoplanet surveys since 1995 have established that they are, and that small rocky ones in temperate orbits are ordinary rather than rare. The later terms remain entirely unconstrained - we have exactly one example of life arising and one of a technological species, which is a sample size that supports no arithmetic at all.

The equation's real function is to make the Fermi paradox precise. If the universe is old and planets are common, where is everybody? The proposed answers range from life being vanishingly rare, to intelligence being rare, to technological civilisations reliably destroying themselves, to their being detectable only briefly - radio leakage from Earth is already declining as broadcasting gives way to cable and tight beams. Drake himself put the last term, longevity, as the one that dominates everything.