5 Civilizations Now: 10,000
The estimate for concurrent civilizations will increase significantly if we are more optimistic about the ability of civilizations to evolve from primitive life. We could put the probability at one success in a hundred attempts at evolution (0.01). And if we are more optimistic about the average length of their survival, we might put this at 500,000 years. We then estimate that a much larger number of civilizations are present in the Galaxy today. The number are shown in Table 3.
Table 3: Drake Calculation, Assumptions 3
| Stars per Year | 20 |
| Planets per Star | 1 |
| Fraction in Habitable Zone | 0.2 |
| Fraction with Life | 0.5 |
| Fraction with Civilization | 0.01 |
| Civilization Lifetime, years | 500,000 |
| Present Number of Civilizations | 10,000 |