  The habitable zone (HZ) is the circular region around a star(s) where
standing bodies of water could exist on the surface of a rocky planet. Space
missions employ the HZ to select promising targets for follow-up habitability
assessment. The classical HZ definition assumes that the most important
greenhouse gases for habitable planets orbiting main-sequence stars are CO2 and
H2O. Although the classical HZ is an effective navigational tool, recent HZ
formulations demonstrate that it cannot thoroughly capture the diversity of
habitable exoplanets. Here, I review the planetary and stellar processes
considered in both classical and newer HZ formulations. Supplementing the
classical HZ with additional considerations from these newer formulations
improves our capability to filter out worlds that are unlikely to host life.
Such improved HZ tools will be necessary for current and upcoming missions
aiming to detect and characterize potentially habitable exoplanets.
