f_s = fraction of sunlike stars = 0.3

Not all stars appropriate: older ones have too few heavy atoms, massive ones are too short-lived, low mass ones may be too faint.

N_p = average number of planets per star = 10

There are 9 planets plus several big satellites in our Solar System. Studies of young stars indicate that disks are common (the birthplaces of planets), and recent discoveries of giant planets around other stars make us optimistic that planets are common.