Há uma equação que calcula através da probabilidade quantos planetas habitáveis com vida inteligente como a nossa. A Equação de Drake.
The Drake equation is:
N = the number of civilizations in our galaxy with which communication might be possible (i.e. which are on our current past light cone);
R* = the average rate of star formation in our galaxy
fp = the fraction of those stars that have planets
ne = the average number of planets that can potentially support life per star that has planets
fl = the fraction of planets that could support life that actually develop life at some point
fi = the fraction of planets with life that actually go on to develop intelligent life (civilizations)
fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L = the length of time for which such civilizations release detectable signals into space
O SETI usa muito ela em suas pesquisas. É provável que exista outras formas de vida inteligente, mas ainda somos as únicas que conhecemos.