Having computed all the essential probabilities for all primaries in the stellar mass range of interest, we have completed all preliminaries to calculating the probabilities that a given star will have habitable planets in orbit about it and the total number of habitable planets in the Galaxy (see page 82 for definitions):
Php — PjjPiPiyPMPh^BPRPAPL
From previous sections we have seen that PD and PA depend on the properties of the primary star, and we have adopted the following values: Pp = 1.0, Pt = 0.81, PM = 0.19, Pe = 0.94, PR = 0.9, PL = 1.0, and PB = 0.95 for a star taken at random, but PB— 1.0 if we know that there is no interference from a second star in a binary star system. Thus, for isolated stars, for stars in a widely separated binary system, and for very close (spectroscopic) binaries, PHP — Q.\2>0PDPA\ and for stars in general, PHP — 0A24PdPa. These products are given in Table 17 and PHP for isolated stars are shown in Figure 39. From Table 17 it may be seen that every star in the mass range 0.9 to 1.02 solar masses has about a 5.4 per cent chance of having a habitable planet in orbit around it. In this mass range about one out of eighteen stars should have a habitable planet. More massive stars have a lower probability of having a habitable planet because of a reduced PA due to their shorter residence times in the main sequence. Less massive stars have a lower probability because of the incompatibility between illumination level and tidal retardation of planetary
Table 17. Probability of a Habitable Planet
class mass PA PD Papd isolated" general6
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