BoingBoing today is a link to NASA's 'Extreme Planet Makeover' website where you can design your own planet and then assess it's chances for life. You set the parameters for the planet and then click on the planet top find it assessed into one of a number of categories. As the BoingBoing post points out though:
If it seems like there's only a few, very limited ways to "win" this game ... well, that's kind of the point. The planet-builder is based on what we know about what it takes to produce life as we know it. And that list of requirements and contradictions really narrows your options. Ultimately, this site should make it clear why finding a "Goldilocks" planet is such a chore, and why everybody is so prone to get excited about the possibility that "life as we know it" isn't the same thing as "life".