To our knowledge life can only evolve in an environment that is quite similar
to that on the Earth. Based on this unproven assumption, the habitable zone (HZ) has
therefore been introduced assuming that life generally depends on such conditions, namely
temperature, pressure, atmospheric chemistry similar to that on Earth.
A habitable zone HZ is defined as a region of space around a star or even around a planet,
where the conditions are favorable for life based on
• complex carbon compounds,
• availability of fluid water.
Several authors call a planet or satellite of a planet habitable when liquid water can be found
there. The liquid water belt concept was introduced by H. Shapley in the 1950s. By the
1970s M. Hart estimated extraterrestrial life as being extremely rare. This lead to the rare
Earth hypothesis.
In the following sections we will outline that habitable zones do not only depend on the
distance of a planet from its host star but also on the location of the planetary system in a
galaxy as well as there might be habitable moons around giant planets.
The main message of this chapter is that there exist several habitable zones, circumstellar,
galactic and even habitable zones around giant planets. Habitability however, strongly
depends on the definition of life.
Keywords: Habitable zone; circumstellar habitable zone; Galaxy; galactic habiatble zone;
Evolution of habitable zone; continuous habitable zone
