4 Civilizations: 2
The initial calculation can be understood better by adding an extra term that shows what happens over the course of time, and extending the average civilization lifetime to 10,000 years to allow the Existence of an Earth 2.0. Table 2 shows that the combination of factors in the equation before civilization lifetime is inserted can be associated with the period within which one civilization can be expected to appear.
Table 2: Explicit Drake Equation, Assumptions 2
Stars per Year | 20 |
Planets per Star | 1 |
Planets per Year | 20 |
Fraction in Habitable Zone | 0.2 |
Habitable Zone Planets per Year | 4 |
Fraction with Life | 0.5 |
Planets with Life per year | 2 |
Fraction with civilization | 0.0001 |
Planets with civilization per year | 0.0002 |
Civilization Lifetime, years | 10,000 |
Number of Civilizations | 2 |
The assumptions have been modified by doubling the civilization lifetime to allow for the emergence of Earth 2.0.
In this second example, the civilization lifetime has been chosen deliberately to show the assumptions that lead to the conclusion that there are only two civilization present in the Galaxy at this time at this time. Given a population of 200 billion galaxies in the universe, this still implies many billions of active civilizations. However, as far as our own galaxy is concerned, the assumptions allow for many other civilizations in the past and in the future.