Box 3. The Drake equation

The Drake equation is an attempt to quantify estimates of the number of ETCs⁷. This relation is

where N is the number of ETCs communicating at any given time; R_* is the average rate of galactic star formation; f_p is the fraction of stars accompanied by planets; n_e is the number of planets per star system with conditions needed to support life; f_l is the fraction of habitable planets on which life actually arises; f_i is the fraction of the life-bearing planets which develop intelligent life; f_c is fraction of intelligent species which develop communication technologies; and L is the 'life span' of the communicating technological culture.

Astronomy is crucial in source selection for targeted surveys. We assume that ETCs need billions of years to develop, so only stars with such lifetimes would be suitable candidates for SETI. These stars must be similar to our Sun. There are approximately 1,000 such stars within 100 light years from the Sun¹⁸. More than 50 extrasolar planetary systems are currently known, but most of these are unlike our Solar System. At present we can detect planets that have Saturn-like masses with orbits close to the star, but because of limitations to measurements, we still have not found an Earth-like planet orbiting a Sun-like star. Finding these will require hundreds of times more accuracy than is available now; such searches will be conducted from satellites in the next decades. The value of N remains highly uncertain. Even if we had a perfect knowledge of the first two terms in the equation, there are still five remaining terms, each of which could be uncertain by factors of 1,000.