One of the biggest unknowns is the time variable. Recall that it took approximately 3 billion years to go from single-cellular life to multi-cellular life, and

One of the biggest unknowns is the time variable. Recall that it took approximately 3 billion years to go from single-cellular life to multi-cellular life, and another 600 million to get to our level of intelligence. We have no idea how long our civilization will last on a geologic time scale and no idea how long it could take another form of life to evolve in the same manner. Perhaps there was a planet where life existed, but intelligence did not evolve quickly enough before the fusion death of the star. Say it took 6-7 billion to go from single-cellular life to multi-cellular life. By then, the planet might have heated up just enough to become uninhabitable, or the star may have gone out entirely. A star only 30% more massive than the Sun will have ~50% of its longevity, so a 5 billion timeframe might be insufficient to develop life.