Estimating Galactic Population – Intelligent Life

[Note this is part of a continuing series, the introduction is here.]

In the last post of this series we got an estimate of the number of planets that have life, so we now start looking at how many of these have intelligent life.  Before I get into the equation and how I come up with numbers to put in,  I should point out my assumption of carbon based life. To the best of my knowledge, carbon is a very unique element in terms of its chemical properties. It is carbon that forms the stable long chain molecules that are the basis of life. Until the functions of DNA, RNA, proteins, and so forth can be demonstrated using other elements, I must assume that life will be universally carbon based.

The equation for intelligent life is:
Nc = Nl * Fh * Fi * Fc
Where Nc is number of planets with intelligent, technological civilizations, Nl is the number of planets with life, Fh is the frequency of those planets being surface habitable by large multi-cellular organisms, Fi is the frequency that intelligent life will arise on the habitable planet, and Fc is the percentage of those inteligences becoming advanced technological civilizzations.

A large brain will require a high metabolic rate, highly developed senses, locomotion, and the right balance between genetic stability and mutation. This means that the planet in question must meet certain criteria for supporting large multicellular lifeforms. First it must be in the habitable zone, that is the zone where the planet can have liquid water on the surface. The planet will then have to have liquid water on the surface, which also means a substantial atmosphere. It will need a magnetosphere to protect against the solar wind. The right frequency of extinction level events are also very desirable.

The first thing to do is elimate a very populous class of stellar-planetary systems. While red dwarf stars can have rocky planets that will support life, I do not believe they are capable of supporting large multi-cellular life forms on the surface. The “habitable zone” of a red dwarf is close enough to the star that tidal lock will occur fairly rapidly. Further, close proximity to the star makes the planet susceptible to solar weather, which is not conducive to surface living. The existence of a truly habitable “habitable zone” around red dwarves does not have a consensus amongst scientists. I find the “not conducive to surface life” hypothesis to be much more convincing. Additionally, even the suggested possible ways for planets to be habitable around a red dwarf are so unlikely as to be negligible for our purposes. Now is a good time to throw out all class M stars as the M giants are end of life stars that just swallowed any habitable planets. 76% of all stars observed are class M, so we now have 24% as the highest possible number for Fh.

Next we consider habitable zones, and we find that of the few observed exoplanet systems with multiple planets the odds are not all that good for a planet to be in a habitable zone for its entire orbit, if at all. Being generous, 20-40% chance of having a big enough planet in a stable habitable orbit.

A surface habitable planet needs a significant magnetosphere to protect against the solar wind and weather, which also protects from cosmic radiation. Looking through the Solar System for guidance, about 40% of rocky bodies large enough to hold on to an atmosphere have a sufficient magnetosphere.

The chemistry compatible with large mobile life forms is even more specific than the chemistry compatible with life. We have already accounted for some aspects of this in our requirement for liquid water. But there are other requirements, on Earth we believe that the chemistry has changed over the years that life has been evolving. Much of that change has been the actions of life itself, plants transforming the atmosphere to an oxygen rich atmosphere that supports large land mobile organisms. Another factor is that the habitable zone for Earth has changed, fortuitously along with the gradual change in habitable zone by our sun’s advance in age and brightness. That is the change in atmosphere from methane and carbon dioxide rich to oxygen rich changed the greenhouse effects in a way that cancels out the increasing warmth from the Sun’s increase in energy output. The likelihood of chemistry being just right for the formation of large multi-cellular organisms that can evolve intelligence is perhaps 20%.

Our total range for Fh  is 0.38-0.77%, we’ll use 0.57%.

For intelligence level I use intelligence level equivelant to or greater than the intelligence of any of the genus homo on Earth. While life may be very common, there are several things that must occur evolutionarily after abiogenesis for intelligent life to occur. For all the many species of life that have arisen on Earth, only the genus Homo has evolved to a level of intelligence that makes a technological civilization possible. By comparison, other useful traits arise multiple times in different branches of the tree of life. Brains appear to be a very expensive evolutionary development, and appear to be only marginally useful in narrow cicumstances until enough knowledge is figured out to then dominate nature. Further, it appears that certain extinction events are necessary for big brains to appear. For example, until the dinosaurs went extinct the likelihood of mammals being able to have enough breathing room to develop larger brains was just about zero. I have to give this very small odds, 0.01-0.1%.

On top of that we need to start considering time. It took billions of years for life to appear on Earth and then give rise to intelligent life. In Drake’s formulation time is just an added term. For my use I have included the time element into the frequency. Given the mediocrity principle, the time it took for intelligent life to arise on Earth and likely longevity of Earth being able to support intelligent life, it would appear that 15% of the time that a planet is capable of supporting life it will have intelligent life. This gives us 0.0015-0.015% for Fi, we’ll use 0.0083%.

Fc is the frequency that hominid level intelligences become mature civilizations with electromagnetic communications systems detectable outside their planetary system, and choose to remain detectable. I note that this is not necessarily a concious choice as regards to communicating with alien intelligences, but rather may be just that they, like us, prefer our communications channels to be secured for various reasons, and thus spread spectrum digital signals, encryption, and just plain cabled systems are used. I have no reason to prefer any particular number over any other. But I will note that there were some Homo species that were not our ancestors and are now extinct, who failed to establish a technical civilization though they likely had the intelligence to do so. Considering some of the close calls homo sapiens had with becoming an endangered species, I believe the odds are much less than 1 that an intelligent species becomes a technological civilization. Drake used 1% and I have no reason to go higher or lower.

Plugging in the numbers:
Nc = Nl * Fh * Fi * Fc = 29,000,000,000 x 0.57% x 0.0083% x 1% = 140 advanced civilizations with detectable communications.

[Continued here]

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.

Continued here.