The habitable zone, or HZ, around a star is defined as the region in which an Earth-like planet can maintain liquid water on its surface. The continuously habitable zone, or CHZ, is the region that remains habitable for some finite period of time. The inner edge of the HZ is determined by loss of water via the moist greenhouse effect (a variant of the runaway greenhouse effect), while the outer edge is determined by the condensation of CO2 to form clouds and polar caps. Climate models that include the buffering effect of the carbonate-silicate cycle suggest that relatively broad CHZs exist around stars from at least F0 to K5. Thus, the chances of finding other habitable planets appear to be good, provided that planets themselves are commonplace.
Perhaps more interesting is the question of whether we can detect such planets and look for life on them. NASA's proposed Terrestrial Planet Finder (TPF) mission should eventually do just that. TPF could be either a single, 8-meter telescope that operates in the visible/near-IR or an 80-meter baseline interferometer that operates in the thermal infrared. Ongoing design studies should help to delineate the advantages and disadvantages of each approach. If either one of these designs proves feasible, it could be possible within the next 12-15 years to detect Earth-sized planets around stars out to a distance of ~15 parsecs and to characterize their atmospheres spectroscopically. Planets whose atmospheres contain substantial amounts of O2 or O3 are likely to be inhabited, although exceptions to this rule can be imagined. CH4 is a somewhat more ambiguous bioindicator on early-Earth-type planets.