BTW: There are two interesting estimates relating to life elsewhere in the universe. How many planets in a galaxy can be expected to have living organisms? How many planets in a galaxy can be expected to have a technological culture? Id est: Intelligent life. I think the Drake equation is a poor approach to estimating either of the above. A better estimate can be obtained by carefully analyzing the history of the earth, taking into consideration both the habitable zone of a galaxy & the habitable zone of a solar system. After the analysis, make your best guess based on that analysis and/or use the analysis to make your best guess at parameters for the Drake equation. I prefer the SWAG approach over using the analysis to guess at Drake parameters. BTW: A SWAG (Sophisticated Wild Ass guess) is better than a WAG. My SWAG is that life of some sort is likely to have occurred several times or more in a galaxy like ours. Guessing that we are the only technological culture in our galaxy is not a bad SWAG. I would expect many galaxies to have never had a technological culture & very few to have had more than one. The history of the earth strongly suggests that the existence of a technological culture is a lucky fluke rather than an evolutionary inevitability. Most people seem to think that evolution is almost certain to result in a technological culture. This is counter-indicated by the history of the earth. BTW: Both the habitable zone of a galaxy & the habitable zone of a solar system are larger for the existence of simple life forms than for a technological culture. Both zones must stay habitable for 3-5 billion years in order for a technological culture to evolve. They need stay habitable for perhaps a billion (maybe less) years for simple life forms to occur.