Suppose that stars were born at random times over the last 10^10 years. The rate of star formation is simply the number of stars divided by 10^10 years. The fraction of stars with detected extrasolar planets is at least 13 %, so the rate of star formation can be multiplied to find the approximate rate of formation of planetary systems. How often (in years) does a planetary system form in our galaxy? Assume the Milky Way contains 6 × 10^11 stars.