Habitable zones
The Goldilocks Zone
The usual method for searching life is to find those planets which are located in their star's habitable zone (also called as Goldilocks Zone). The habitable zone is usually described as a region around a star where a hypothetical Earth-like planet could support liquid water on its surface. If the planet is located inside the habitable zone, it may be suitable for supporting life. In the Solar System the inner boundary of the habitable zone is located between the orbits of Venus and Earth and the outer boundary lies just beyond the orbit of Mars. In numbers, the zone is located between 0.84 and 1.67 Astronomical Units (AU) from the Sun (Kasting et al. 1993). However, an improved model suggests 0.99 and 1.70 AU for the boundaries (Kopparapu et al. 2013).
In order to calculate the boundaries of the habitable zone it is necessary to know the stellar parameters. Other environmental elements, like the size of the planet, the presence of an atmosphere, its thickness, composition, atmospheric pressure, or cloud formation can influence the surface temperature, i.e. the phase state of the water as well. Consequently the temperature of the planetary surface is very difficult to calculate. In the case of other stars the procedure is even more complicated because of the uncertainties in the measured parameters of the star and the planet.
Other habitable zones
The abovementioned habitable zone is also called as Liquid Water Habitable Zone. The reason is that there are other habitable zones as well. Depending on the inspected conditions several habitable zones can be distinguished. These are the Galactic Habitable Zone, Ultraviolet Habitable Zone, Tidal Habitable Zone, Photosynthesis-Sustaining Habitable Zone and the Life Supporting Zone. My research work covers the Liquid Water Habitable Zone and the Ultraviolet Habitable Zone.
The so-called Continuous Habitable Zones describe whether a planet is located continuously inside the habitable zone for a long time. As the star ages, its radiation increases hence the habitable zone moves outward from the star. This means that a planet which was initially inside the habitable zone gets stronger stellar radiation with time, which can be harmful for living organisms. On the other hand, a planet that was orbiting the star too far and did not get sufficient energy may become habitable.
Stability
Suitable stellar and climate parameters are not sufficient for habitability. The emerge and evolution of life need long time, on Earth it took a few billion years. The orbit of the planet needs to be stable during this long time interval to stay in the central star's habitable zone.
Related links
Liquid Water Habitable Zone:
Empirical formulae for calculating the HZ
Ultraviolet Habitable Zone: