Estimating Galactic Population – Stars and Planets
[This is part 2 in a series. The series introduction is here.]
In figuring out the galactic population we can’t just take a census. We can’t get to or even see large portions of our own galaxy. Much of what we do know about it comes from looking at other galaxies. So we create a model that takes estimates from what we do know, and try to estimate it that way. Of course, this also means that any numbers we come up with should be taken with huge blocks of salt.
In the first part of this series, I gave the equation for number of planets as this:
Np = N* x Fp
N* is the number of stars in the galaxy. This is usually given as a range from 200-400 billion stars. With the recent announcement that the Milky Way is bigger than we thought, I use the top end number, 400 billion.
Fp is the frequency of stars having (and retaining) planets. In the astrophysics community there is not yet a consensus on a “Galactic Habitable Zone”, and I’m not sure the concept as stated in the Rare Earth Hypothesis survives criticism. But a related concept is useful in thinking about stars being born in a zone that allows having and keeping a planetary disk. In the core, bar structure, portions of the major arms, and in many stellar nurseries the population density of stars is such that any planetary material will be swept into a star or be ejected from any stellar system. Additionally, the largest stars are so short lived that any planetary disk they may have just does not matter to our calculations. I figure between one third and one half of the stars are in areas where planetary formation can occur mostly unmolested. This leaves us with stars that formed out a bit from the core, much like where we are now, which is really good because now we can look at stars in our neighborhood and get a good idea of what percentage of those have planets. Current estimates are that Sun-like main sequence stars in our neck of the galaxy have planets from 30% to 60% of the time. This gives us a range of 10% to 30%. We’ll say 20% of stars have planets.
Putting numbers in place:
Np = 400,000,000,000 x 20% = 80,000,000,000 planetary systems in the galaxy.