He suggested the following figures should be multiplied : how many stars are formed in the galaxy in a year ; what fraction of these have planets and thus form solar systems ; the average number of planets per solar system that have the potential to support life ; on what percentage of those where it is possible do such biospheres actually form ; what percentage of such biospheres give rise to intelligent species ; what percentage of intelligent life is able to transmit signals into space ; and for how long could such intelligence keeps sending signals .