The habitable zone[9] is the distance range in which a planet must be located from its central star so that water can occur permanently in liquid form, as a prerequisite for life. The exact position of a habitable zone in a solar system depends on the star type, i.e. on the radiation and the temperature of the star. The habitable zone can, simplified, be calculated from the luminosity of a star. The average radius of this zone of any star can be calculated using the following equation:

d is the average radius of the habitable zone in AU
L_Star is the bolometric luminosity of a star
L_Sun is the bolometric luminosity of the sun

In astronomy, bolometric brightness is a measure of the total luminosity of a celestial body, i. the luminosity integrated over the entire electromagnetic spectrum. With a star twice as bright as the sun, the average radius is 1.4 AU.

AU = astronomical unit = 149,597,870 Km = distance sun-earth


From 603 planetary systems observed by Kepler, planets in habitable zones could be detected in 10 systems. This corresponds to 1.658 % of the investigated planetary systems. The probability factor is therefore:

F_h = 0.016.58 = 10:603

Related to all star systems A in our galaxy, the number N_h of solar-like star systems with habitable planets results to:

Insert the factors into equation 1.6.1: