All known 400+ exoplanets as of Dec 2009 plus the 5 new planets found with Kepler. The green band represents the parameters for habitable planets. Too close to the Sun and water vaporizes. Too far from the Sun and water freezes. Too low of a mass, and the planet does not have enough surface gravity to hold onto a life sustaining atmosphere. Too large of a mass and the planet has enough gravity to hold onto the most abundant element in the universe, hydrogen, and become a gas-giant planet.
