It’s possible to make a rough estimate of how often planetary systems form by making some basic assumptions. for example, if you assume that the stars we see have been born at random times over the last 10 billion years, then the rate of star formation is simply the number of stars we see divided by 10 billion years. the fraction of stars with detected extrasolar planets is at least 5 %, so this factor can be multiplied in to find the approximate rate of formation of planetary systems.
a) using these assumptions, how often does a planetary system form in our galaxy? (our galaxy contains at least 100 billion stars.)
b) how often does a planetary system form somewhere in the observable universe, which contains at least 100 billion galaxies? (express in seconds)