Terrestrial Planets in the Milky Way

Kepler 186f - NASA

In my last post I noted a paper that I used to roughly estimate the number of Earth-sized planets in habitable zones of their parent stars. Going a bit deeper in to some of the more recent works I've been trying to find other estimates that might confirm or adjust my estimate.

If you are interested in some of the published works, two terms that might be confusing are the differences between 'terrestrial planets' and 'Earth-sized' planets. Though not a fixed view, terrestrial planets are considered to be composed of silicates and metals unlike the lighter giant gaseous planets. For example, Zackrisson et al. 2016 define terrestrial planets as having radii in the range of 0.5 to 2.0 times that of Earth with a mass range of 0.5 to 10 times. In our Solar System this would include three of the four inner planets of Venus, Earth and Mars with Mercury just under their range (though it is still considered a terrestrial planet in our local Solar System). From a different perspective, Petragura et al. 2013 note that Earth-sized planets have radii in the range of 1 to 2 times Earth.

Interestingly, Zackrisson's estimate posits the total number of terrestrial planets in the Milky Way to be about 65 billion for stars with spectral types F, G, K and M. This is at least in the ballpark of my previous estimate of 4 billion Earth-sized planets in habitable zones of G and K stars. However, given the 65B estimate includes all terrestrial planets it would seem that Earth-sized planets in habitable zone might be modestly less than 4B.  Of the 65B only about 2.5B are considered orbiting spectral types of F, G and K stars with remaining 62.5B orbiting M stars (the number of M dwarf stars in the galaxy is about 3x greater than FGK stars - Zackrisson also mention the higher probability of finding terrestrial planets around M dwarf stars is about 2.5). What fraction of those are Earth-sized and in habitable zones is still in question - but the estimates seem to fall approximately around 0.1 to 1 billion Earth-sized planets in habitable zones around G and K stars.

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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.

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