Probability of Occurrence of Habitable Planets

Having summarized the properties of habitable planets and the astronomical requirements implied by these properties, we can now attempt to estimate the prevalence of such bodies in our Galaxy (the Milky Way); and to do this with any reasonable degree of accuracy (in the spirit of the present study), it is necessary to consider the following factors:

Ns, the prevalence of stars in the suitable mass range, 0.35 to 1.43 solar masses;

Pv, the probability that a given star has planets in orbit about it;

P{, the probability that the inclination of the planet's equator is correct for its orbital distance;

PD, the probability that at least one planet orbits within an ecosphere;

PM, the probability that the planet has a suitable mass, 0.4 to 2.35 Earth masses;

Pe, the probability that the planet's orbital eccentricity is sufficiently low;

PB, the probability that the presence of a second star has not rendered the planet uninhabitable;

PR, the probability that the planet's rate of rotation is neither too fast nor too slow;

PA, the probability that the planet is of the proper age;

PL, the probability that, all astronomical conditions being proper, life has developed on the planet.

Once values for all of these factors have been established, the estimated number of habitable planets NIIP in the Galaxy can be expressed as the product:

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