We Are Lucky That Humans Exist At All!
What is the probability that a habitable planet could develop complex life (something equivalent to a mammal) and go on to develop a technological civilization?
In the book Rare Earth: Why Complex Life Is Uncommon in the Universe Peter Ward and Donald E. Brownlee explain how unlikely the Earth is. Even beyond needing a hospitable, stable, and metal-rich star-system in the optimum part of the galaxy, consider how many things had to go just right for complex life to evolve on Earth.
The following list of necessary events is inspired by but not limited to the research of Ward and Brownlee. The probabilities assigned are guestimates from this author. Some of these probabilities may be too pessimistic but others too optimistic, so it averages out.
* an orbit in the habitable zone of an appropriate star (assumed)
* a cataclysmic but not terminal collision with another planet which produced a large moon for tidal forces (.01)
* a terrestrial planet with plate tectonics and a magnetic field for protection from radiation (.1)
* a large Jupiter-like planet for protection from asteroids (.1)
* which still allowed sufficient water-bearing comets to produce oceans but not too much water which would result in an evolutionary-limited water planet (.1)
* RNA-DNA-type life (.1)
* the miraculous prokaryote−eukaryote transition followed by cyanobacteria for an oxygen atmosphere (.01)
* finally multiple mass-extinction events which stirred the evolutionary pot but didn’t end complex life (.01)
We are assuming that from an evolutionary point of view, zero extinction events is just as bad as an event that wipes out all life.
A recent analysis suggested that there are 300 million habitable planets in our galaxy. (See statement by Jeff Coughlin and Michelle Kunimoto.)
So that gets us to the first event above which is followed by 7 events with the imagined probabilities. All events must happen so we multiply probabilities.
.01*.1*.1*.1*.1*.01*.01 = 1.E−8
Now consider the number of habitable planets.
300 million = 3.E8
Multiplying the number of habital planets with the probability of complex life arising we get:
3.E8 * 1.E−8 = 3
Three planets in the Milky Way with complex life. (We are conveniently ignoring the likelihood that many of these planets have already come and gone.)
Three planets with complex life.
But that is not the same as technological life. What is the probability that a planet with complex life forms will have a technological civilization?
Let's start with the Triassic-Jurassic extinction event which was 200 mya (million years ago). Mammals already existed by this point so this is a reasonable starting point for a countdown to technological life. And post Triassic-Jurassic corresponds with the last of our events listed above.
Imagine that we have surveyed the entire Milky Way and found X number of planets with a mix of life equivalent to after the Tr-J event. Looking at our own planet we know that humans appeared about 200 thousand years ago. 200,000 is one-thousanth of 200 million years. So 1 out of 1000 of our surveyed planets would have lifeforms similar to humans. But on Earth modern technology has only existed for about 200 years.
Michael Faraday invented the electric motor in 1821 (exactly 200 years ago) which is a reasonable date to consider as the beginning of modern technology. So modern technology has existed for one-thousandth as long as humans.
That means perhaps one-thousandth of the surveyed planets with humanoids would have modern technology. One millionth of the surveyed planets with life as complex as mammals.
But we only started with three planets. This gives us 3 millionths as the probability of technological humans existing! Anywhere in our galaxy. We shouldn't even be here.
Even if we start with 5 billion habitable planets (as has been suggested), we still get only 50 chances out of a million of existing.
This puts a completely different light on Fermi's question: Where is everyone? In reality we should be blessing our lucky stars that at least we are here!
See also by this author:
https://link.medium.com/bori2iey4hb
References
https://en.wikipedia.org/wiki/Rare_Earth_(book)
https://en.wikipedia.org/wiki/List_of_extinction_events
https://www.seti.org/press-release/how-many-habitable-planets-are-out-there
https://www.newscientist.com/article/2258911-there-could-be-around-5-billion-habitable-planets-in-the-milky-way/
