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With this term, the average number of habitable planets within a system, it is time to start considering the number of worlds instead of the number of stars. A planetary system may have no habitable planets-planets appropriate for life-or it may have many habitable planets. By multiplying this term (the average number of habitable planets in a system) by the number of appropriate stars that have a planetary system, we are estimating the number of habitable worlds in the Milky Way Galaxy.
We have now reached a stage for which we have only our own solar system to help us estimate a value for the term. The Earth is clearly habitable, but what about Mars or Europa or the other planets and moons? Could Earth be the only habitable planet in our system? Could there be as many as 4-5 habitable planets in this system? (Your estimate will depends a lot upon what conditions you require to consider a planet (or moon) habitable.)
Analyzing our own system is not enough, however; you need to consider how typical you believe our system to be. (Do all systems have an "Earth"? Could some systems have no habitable planets? Could some systems have many?) And finally, you need to consider what all this means in terms of an average per system.
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