f_e = fraction of planets suitable for life = 1/40

We require planets or satellites in the habitable zone that are large enough to sustain an atmosphere and geological activity. This estimate may be too high because of the unknown frequency of systems in which terrestrial planets are swallowed by the star or ejected into space.

Our text prefers a smaller number, f_s x N_p x f_e = 0.005 (versus 0.075 here).

We find organic chemicals in many environments (even in interstellar clouds), so it appears that creating the raw materials for life is easy. Life began on Earth shortly after the intense meteoric bombardment ended; if the process of starting life (still unclear) were difficult, we would not expect life to have appeared at the first possible opportunity.