Someday, in the not too distant future, we’ll know whether the cosmic ocean is as richly populated as the oceans on Earth. Evidence for life could come from icy moons in our solar system, or in the form of an interstellar radio message, or from telltale signatures in the atmosphere of a faraway exoplanet.

But even if we find sharks beneath Europan ice or intercept an interstellar SnapChat, we still won’t know whether extraterrestrial organisms are common. We still won’t know how often beings evolve and develop technology.

There is a formula that estimates the answer to this second unknown. Called the Drake Equation, it predicts the number of detectable,