Number of stars visible in M31 1.1E+10 stars Fraction of stars which live long enough 0.500 Average number of planets per star 10 planets Fraction of planets suitable for life 0.025 Fraction of suitable planets where life rises 0.500 Fraction of suitable planets w/ intelligence 0.500 Avg life of visible evidence of intelligence 1.0E+07 years Estimated galaxy lifetime 1E+10 years