The Drake Equation

In 1961, radio astronomer Frank Drake organized at the Green Bank observatory, in the United States, the first scientific meeting devoted to the search for extraterrestrial intelligence. To guide the discussion, he wrote on the board a series of factors that, multiplied together, would estimate how many civilizations capable of communicating by radio exist in the Milky Way.

The equation considers the rate of star formation, the fraction of stars with planets, how many of those planets would be habitable, on how many life would arise, on how many it would become intelligent and technological, and for how long those civilizations would send out signals. Today we know that planets are common, thanks to exoplanet-hunting telescopes. The other factors remain guesses. That is why the formula works more as a research roadmap than as an answer.