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.