Figure 8-21: Exoplanets in the Habitable Zone The exoplanet Kepler-22b lies inside the habitable zone and is compared to our solar system in this illustration. This planet is 2.4 times the size of Earth, which raises hopes for a hard surface with at least some liquid water. Its mass is not yet known, but it could be around 10 times the mass of Earth. Each star determines the size of its surrounding habitable zone, and the two zones in this image are similar because the star, Kepler-22, is Sunlike.

(NASA/Ames/JPL-Caltech)