can we estimate the number of habitable planets within a specified distance from Earth?

Question

For the purposes of this question, I will define "habitable" as:

• rocky composition like Earth
• about the same surface gravity as Earth (about 1 G)
• orbiting a relatively quiet star, like our Sun
• far enough from its host star to not be tidally locked
• receiving about the same amount of energy from its host star as we do (in the star's habitable zone).

Thus, a "habitable" exoplanet would be capable of holding on to an atmosphere conducive to life, and, if any water was present, it could exist in liquid form on the planet's surface.

Kepler data has revealed numerous planets orbiting numerous stars in only a small slice of the sky. Many of these planets have been characterized to some degree. There are compiled lists of potentially habitable exoplanets.

My question is whether the findings so far can inform a reasonable estimate of the total number of habitable (or potentially habitable) exoplanets within a given number of light-years distance from Earth, or if further sky surveys and studies would be needed. In other words, can we pin down the first few terms of the Drake Equation and apply it to our neighborhood in the galaxy?

Answer 1

We can do an estimate, but there are going to be pretty big error bars on that estimate. Kepler has given us a lot of exoplanets, but its results are biased: it's easier to detect large planets in small orbits than it is to detect small planets in large orbits. Most of Kepler's results is gas giants.

We only have one solar system which we've observed reasonably completely: our own. And our solar system is atypical, compared to the Kepler results: no gas giants in small orbits.