The Drake equation.
where:
N is the number of civilizations in our galaxy with which we might hope to be able to communicate;
and
R* is the average rate of star formation in our galaxy
fp is the fraction of those stars that have planets
ne is the average number of planets that can potentially support life per star that has planets
fℓ is the fraction of the above that actually go on to develop life at some point
fi is the fraction of the above that actually go on to develop intelligent life
fc is the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L is the length of time such civilizations release detectable signals into space.
The number of stars in the galaxy now, N*, is related to the star formation rate R*
The problem is no one knows the true values of these variables. but given that there are 10 billion galaxies in our observable universe with roughly 10 billion solar systems, at least bacteria should be able to spawn somewhere.