NASA and the ESA estimate this value to be between 1.5 and 3
Fraction of stars that have planets
Current science suggests this value is very near 1
Average number of planets that might support life per star that has planetsFraction of the above that actually go on to develop lifeFraction of the above that develops intelligent lifeFraction of the above revealing their existence via signal release into spaceLifetime of such a civilization wherein it communicates its signals into space
