The habitable zone is one of the major things that astronomers look for when characterizing newly discovered exoplanets. This zone defines the theoretical upper and lower bounds for an orbit around a star where sufrace liquid water is possible on a planet that has sufficient atmospheric pressure. This zone is important because, after surveying all of the different life forms on Earth, the minimum requirement for life seems to be liquid water and an energy source (particularly solar energy).
The image to the right portrays Kepler-22b, which is a planet detected by the Kepler Space Telescope. This planet lies in its parent star's theoretical habitable zone. The image also shows Mercury, Mars, Venus, and Earth, of which only Earth and Mars lie in or around the habitable zone of the sun.