n_hab -- the number of habitable planets per star.

This distribution helps us a little:

http://exoplanets.org/

but the fact is that most of these planets are not what we would call habitable (if they are really gas giants).

How can we tell? What about habitable moons? What about Mars -- is it habitable? Was it, in the past? What about Venus? Is Europa habitable, even though it is way outside the habitable zone in our Solar System?

Basically the value for n_hab must be "we don't know." The answer is likely to be between 0 and 1, based on the radial velocity experiments, but ultimately that's not very helpful! Still, we can perhaps guess, based on this observational data. We can certainly guess a lower limit.

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