Habitable Zones.

The habitable zone is a popular concept describing the range within a star's orbit in which a planet can support liquid water on its surface, and therefore have a chance of supporting Earth-like life. It is also known as the circumstellar habitable zone (CHZ) or the Goldilocks zone (after the fairy tale in which a girl chooses things that are "just right" rather than those on either extreme). Since the concept emerged in the mid-twentieth century, it has proved useful in shaping astronomical observation and astrobiological speculation, and many planets within habitable zones have been identified.