In a really interesting follow up to the kerfluffles over Gliese 581g and arsenic-based life, NASA recently introduced an interactive website that explains what it takes to make a life-producing planet—by having you design your own.
The game starts with presets tuned to mimic either Earth, Gliese 581d (a planet that sits on the very edge of Gliese's habitable zone but seems, at this point, more likely to exist than 581g), or Mars. From there, you can alter factors like distance from the star, planet size, star type, and planetary age. That's probably-lifeless Planet Maggie pictured above.
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". Lesson: Don't get too bouncy when you see headlines about either possibility. We might find extraterrestrial life someday, but this isn't a simple or easy thing to pull off.
Thanks to monekyodeath for Submitterating!