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N = 6.25 * f_p * n_e * f_l * f_i * f_c * L
Today, we’ll talk about the second term, f_p = the fraction of stars that have planets.
From http://arxiv.org/abs/astro-ph/0104347, http://en.wikipedia.org/wiki/Extrasolar_planet, and http://en.wikipedia.org/wiki/Planet_formation, we know that essentially all young stars have an accretion disk of gas. When the disk cools, the gas forms dust grains of rock and ices (small, volatile compounds: carbon dioxide, water, methane, nitrogen, etc.). The dust grains may agglomerate into planetesimals. Some of the planetesimals may then form planetary embryos, in a chaotic system that will tend to form terrestrial planets, similar in size and composition to Venus or Earth.
Gas giants complicate the above process. Although they can only form in the outer parts of a protoplanetary disk, they can migrate toward their parent star, which would disrupt the orbits of smaller bodies and could prevent formation of terrestrial planets. Gas giants can also eject smaller bodies from the stellar system by gravitational interaction.
Yet either way, a stellar system would probably form with terrestrial planets, gas giants, or both. Therefore, we’ll assume 90% of star systems will make it to that point, or f_p = 0.9
But how many planets could potentially support life? We’ll get to that next time.