Just how many Earth-sized planets are there in the Milky Way?

Credit - ESO

Of course, at this point there really isn't an answer to this question. But, as more data is collected by exoplanetary surveys, the predictive precision increases. I seem to recall a statistic that one calculated very broad average is ~1.8 planets for every star in the Milky Way based on Kepler data. But, I can't find that value in published works.

This uncertainty has led me on a bit of a hunt recently to see where the current predictions are today. I'm not going to get to them all in this post and may just write up a more definitive article on it. But, we can play with a few numbers for Earth-sized planet.

For example, Petigura et al. 2013 estimate that 22% of Sun-like stars (spectral types G and K) harbor Earth-sized planets within their habitable zones. Quick calculations (using popular references) show this would be about 20% of all main sequence stars which is 90% of all stars in the Milky Way. Roughly that would be about 10¹¹ * 0.9 * 0.2 * 0.22 = 3.96 billion Earth-sized planets in habitable zones (which only works out to about 0.04 planets per star) in our galaxy. As a filter though, just how many of the 4B Earth-sized planets have water, proper atmospheres, just the right habitable distance, old enough, in the GHZ (see previous post), etc. are just some of the limits on extraterrestrial life

Since this simple estimate ignores other types of planets and associated systems it would be good to go back to the full estimate of average number of planets per stellar type in the galaxy. More digging is needed here.