# Drake's Equation Question Set: what is the average number of habitable planets per star?

This is the third question in a series estimating input parameters for Drake's equation, inspired by a recent paper, on the Fermi paradox.

The first question in the series, with more explanation, is here

The model in question uses probability distributions over the following parameters:

• $R_∗$ log-uniform from 1 to 100.
• $f_p$ log-uniform from 0.1 to 1.
• $n_e$ log-uniform from 0.1 to 1.
• $f_l$ log-normal rate, $1 − e^{−λVt}$ (giving $f_l$ mean 0.5 and median - 0.63).
• $f_i$ log-uniform from 0.001 to 1.
• $f_c$ log-uniform from 0.01 to 1.
• $L$ log-uniform from 100 to 10,000,000,000.

In this case we will be addressing the third parameter in the Drake's Equation, $n_e$. It is the number of planets, per star system, with an environment suitable for (though not necessarily possessing) life. We include suitable moons in this count.

Predictors should use the sliders to make their best estimate of this parameter and its uncertainty.

Most estimates consider how many planets fall within a star's habitable zone, probably with sufficient mass to retain an atmosphere or surface liquid of some type. (For example, our Moon is in our stars' habitable zone, though it is uninhabitable to life as far as we know, although there may have been a brief period of suitability.) However we might be more expansive:

• One must also consider the fact that habitable zones move as a star changes over time.

• Also there may be condition that allow a planet to be suitable to life outside of the traditional habitable zone such as sub-ice oceans of our gas giants moons.

• We may also consider the habitable zones not just for water-based life but other biochemistries.

The resolution to this question will be the scientific consensus 100 years from now, regardless of any remaining uncertainty.

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