Estimating Galactic Population – Biology
[Note this is part of a continuing series, the introduction is here.]
In the previous part of this series, I described how I estimate the number of planetary systems in the Milky Way. In this part I will narrow this down to estimating the number of planets that have life. Any life, from things that scientists will argue about whether it is alive or not to non-biological life designed by large intelligent multi-cellular organisms. Here is the equation I use:
Nl = Np x Frv x Nrv x Fl
The first factor is the number of planetary systems, which I described in the last post. The next factor, Frv, is actually a series of factors that I aggregate into Frv – the frequency of rocky planets with significant, life-friendly volatiles. Then comes an estimate of the average number of such planets in such a planetary system – Nrv. Last is Fl, the frequency that life appears on those planets.
Before I go on, I need to make sure that there is no question about a word I am using repeatedly, that is not the usual sense, “planet”. What I mean by planet is not only the traditional planet, but also any large moons, planetoids, and anything else big enough to accumulate and retain volatile compounds. At any rate, from now on, when I say planet think not only Venus, Earth and Mars, but also the large moons of Jupiter and Saturn.
Now, with that out of the way, what is the frequency that rocky planets with volatiles and life friendly chemistry is found in planetary systems? The first component goes back to the time, place, and circumstances of a star’s birth. The star needs to have lots of the heavier elements, such as carbon, oxygen, silicon, and iron nearby to have any rocky planets. The heavier elements had to come from a supernova at some point as the only stars big enough to create the heavier elements will go supernova, and the supernova is how the heavier elements get cast out into the clouds that will become planetary systems. Most of our galaxy has plenty of the heavier elements. I would say probably around 80% of planetary systems have the necessary elements in sufficient quantity to produce rocky bodies with volatiles.
Next we need to know what the frequency is of getting enough rocky material and enough volatiles into a body big enough that the volatiles are not just boiled off into space and/or will have plenty of heat in the body’s core. This will depend on a number of factors, my best way to estimate it is looking at what we have found thus far in exoplanetary systems, and then trying to adjust for availability bias. We can more easily detect certain types of exoplanets, which means that we can more easily find exoplanets that will greatly reduce the chance of largish rocky bodies existing in that system. That said we have found some large rocky exoplanets. Let’s be generous and say that 40% of planetary systems will have substantial rocky bodies within it.
Next, what type of chemistry is going on on the planet? Looking around our own Solar System we some bodies with promising or at least tolerable chemistries, while many others are just plain toxic and corrosive to biochemical processes, or lacking in some critical material. I assume that all life at least begins as carbon based life. I believe this is a rather safe assumption as only hydrocarbons have the necessary complex chemistry for life, and the qualifier “starts as” allows for carbon based life to create new life forms that replicate and do work (as in energy transfer). Looking at the largish rocky bodies with lots of volatiles in the Solar System – Venus, Earth, Mars, Titan, and the Galilean moons, we see between 25 and 50% of these bodies have the chemistry to support life.
Combining the aforementioned factors, we see that Frv is between 8 and 16%, call it 12%.
Moving on to Nrv – now that we know how many planetary systems have planets that can support life, how many planets are life supporting in the system? Invoking principal that the Solar System is typical, average, or median, I look to the Solar System to determine what Nrv is likely to be. Given what we know of the various large rocky bodies in the Solar System, I’d say that between Earth, Mars, Europa, Ganymede, Titan, and Enceladus, three of them will prove to have all the prerequisites.
As to the frequency of life, I am of the opinion that if all the prerequisites for life are met, then given some time life will happen. I really do think that abiogenesis is fairly common. I am fairly sure that Fl is unity.
Plugging in the numbers:
Nl = Np x Frv x Nrv x Fl = 80,000,000,000 x 12% x 3 x 1 = 28,800,000,000 planets with life.