One of the Drake Equation factors that has changed the most since 1961 is ne, the average number of planets per star that can potentially support life. This factor is still evolving.

The definition of conditions for developing life is related to the definition of a circumstellar habitable zone. It is generally defined as the zone in which physical conditions make presence of liquid water possible. As a first approximation, it implies a temperature between 0 and 100 degrees Celsius, the so-called Goldilocks condition. This preliminary estimate is based on the temperature of the star: in other words, its spectral type and the distance between the star and its planet. Changes in the ne term are principally due to unexpected characteristics on one hand of many exoplanets and on the other hand of some objects in the solar system.

We first discuss the basic condition for life: liquid water. Stellar spectral types where life is most likely to emerge and exist are reviewed, and less favorable conditions of hot and cold stars are discussed. We also look at planets orbiting one member of a binary star, or around both stars (“circumbinary planets”). We point out the physical conditions necessary for planets to shelter life, including mass and other physical parameters. Exomoons are interesting objects and are discussed as well. Relations between the star and its planetary system are reviewed. No longer is distance the only parameter to be considered. In the case of terrestrial exoplanets with large eccentricity that cross the habitable zone, life with some phases of hibernation may be possible. Some terrestrial exoplanets orbit so close to their star that they are co-rotating, keeping one face to the star at all times, which implies that a temperate annular zone may exist between the very hot face in front of the star and the very cold face on the opposite side. Characteristics of some satellites in the solar system discovered since the space age show that some tidal effects are liable to extend the habitable zone, as can be seen by the detection of oceans flowing below the icy surface of Europa or internal water springing from the geysers of Enceladus. It would be interesting to search for and study as a source of possible life moons of giant exoplanets located in the habitable zone of their star. As a conclusion, the continuously increasing number of small rocky planets provides great encouragement to search for extraterrestrial life. Indeed, they show a high rate per star and satisfy the conditions necessary for producing life.