How many alien civilizations are capable of contacting us? In 1961, radio astronomer Frank Drake presented an equation designed to estimate that number on the basis of seven assumptions. For now, the Drake Equation tells you more about your own views on life in the universe than about scientific fact. But the scientific community continues to use it as a yardstick for the SETI search. Plug in your estimates for the following seven factors and come up with your own answer.
1.   How many stars with suitable conditions for creating habitable planets are formed annually in our galaxy? (The typical estimates for this number range from five to 20.)
2.   In percentage terms, how many of those stars actually have planets? (The recent discovery of extrasolar planets has led astronomers to raise their estimates of this number. The typical range is 3 percent to 50 percent.)	%
3.   In those planetary systems, what is the average number of worlds in the "habitable zone," where water exists in liquid form? (Please enter zero or a positive number.)
4.   On what percentage of those habitable worlds does life actually arise? (In light of recent studies of Mars and Europa, a moon of Jupiter, some astronomers argue that this number is close to 100 percent.)	%
5.   In percentage terms, how many of those life-bearing worlds give rise to intelligent life?	%
6.   In percentage terms, how many of those worlds with intelligent life develop technologies that release detectable signs of their existence into space?	%
7.   On average, for how many years do such civilizations release detectable signals into space? (This ranks as one of the most controversial numbers in the calculation. In recent years, some astronomers have grown more optimistic that the only such civilization we know about — our own — will be able to avoid blowing itself into oblivion anytime soon. Please enter zero or a positive number.)