The Drake equation focused on the number of civilizations that are currently in existence. Frank and Sullivan calculate how many civilizations there have ever been.
That immediately and significantly simplifies the Drake equation. When working out whether other civilizations currently exist, factors such as star formation rates and the length of time a technological civilization can exist, are hugely important. But they can be ignored entirely by considering only whether these civilizations have ever existed.
By plugging in the new exoplanet statistics, Frank and Sullivan come up with a specific number. “We find that as long as the probability that a habitable zone planet develops a technological species is larger than about 10^-24, then humanity is not the only time technological intelligence has evolved,” they conclude.
If the probability of a technological species arising on a given planet in a habitable zone is greater than one in 60 billion, then another technical species has probably arisen at some point elsewhere in the Milky Way.
In this paper we address the cosmic frequency of technological species. Recent advances in exoplanet studies provide strong constraints on all astrophysical terms in the Drake Equation. Using these and modifying the form and intent of the Drake equation we show that we can set a firm lower bound on the probability that one or more additional technological species have evolved anywhere and at any time in the history of the observable Universe. We find that as long as the probability that a habitable zone planet develops a technological species is larger than ~10^−24, then humanity is not the only time technological intelligence has evolved. This constraint has important scientific and philosophical consequences.