In the year 2000, as we were just entering the new millennium, two of my University of Washington faculty colleagues, geologist/paleontologist Peter Ward and astronomer Don Brownlee (W&B), published a remarkable book called Rare Earth: Why Complex Life is Uncommon in the Universe.  In it, they extended the Drake equation by compiling an amazing and compelling list of reasons why our planet Earth is a very special place for life, and why this may make it unique in our galaxy as a place where complex intelligent life has been able to evolve.  I note that while W&B provided the essential arguments, they did not attempt to quantify their arguments by giving a value for the probability of the evolution of complex life.

At the time, I wrote an AV Column about their work (AV-102), and I organized and moderated a special 2.5 hour-long session of Norwescon 23 at which Prof. Peter Ward was confronted by Gregory Benford, David Brin, Greg Bear, and other hard SF writers who really wanted there to be intelligent aliens out there for us to interact with.  Peter Ward held his own in the resulting exchanges.  However, he told me later that he didn't enjoy it very much.  In any case, for me it was one of the best Con events in which I have ever participated.

Since 2000 astronomers have learned much more about the behavior of stars as they evolve, the life-extinguishing supernovas they may produce, and the likelihood of finding Earth-like planets in habitable zones around them,.  This new knowledge has led to a number of studies that attempt to make quantitative estimates of the probability that complex intelligent life has evolved in our galaxy.  Because the methods of estimation used in these studies have varied widely and the relevant variables must be estimated with large uncertainties, these predictions have been all over the map, ranging from predicting many planets with complex life and even intelligence to predicting that we are probably unique and alone in the galaxy.

In this column, I want to discuss one of the most recent and complete of these studies, which, interestingly enough, was done by a high-school student, a CalTech geologist, and two JPL scientists.  In their calculations, they used a version of the Monte Carlo method, selecting random values from probability distributions to simulate the evolution and behavior of all the stars in our galaxy over a time period of up to 20 billion years after the Big Bang.  (For reference, our universe is presently only about 13.6 billion years old.)   Within this framework, they focused on the formation of G-type stars having a mass within ±20% of our Sun's mass and having Earth-like planets in their habitable zones.   Earth-like planets are assumed to have a mass of 1 to 2 Earth-masses, an orbital period of 200 to 400 days, and to receive stellar energy that is within a factor of 4 of that received by Earth. They included the possibility that supernova events in the galactic neighborhoods would extinguish the developing life, so that the evolution of complex life on the planet would have to restart.  Sun-like stars have a finite lifetime before they leave the main sequence and become red giants that would extinguish planetary life.  This lifetime was taken into account in the simulation. They also included the possibility that an advanced technological civilization might bring about its own destruction. 

In particular, their overall simulation: (1) initiated a 3D spatial random table of the Milky Way with interstellar gas appropriately distributed, (2) randomly generated Sun-like stars harboring Earth-like planets, (3) activated local life-sterilizing supernova explosions with the same distribution as observations, (4) for each Earth-like planet, allowed life to emerge according to a Poisson distribution, (5) for each life-bearing planet free from transient life-sterilizing events (e.g., supernova), followed life's evolution into intelligence and (6) after intelligence evolved, they threw the dice as to whether it would annihilate itself. 

In their galactic simulation, the development of intelligent species depended on three major parameters.  These parameters are: (1) λ_A = the likelihood rate of "abiogenesis" (i.e., the initial appearance of primitive life on a cooling planet, as guided by a Poisson distribution), (2) T_evo = the evolutionary timescale (i.e., the time required after primitive life first appears for it to evolve into intelligent life), and (3) P_ann = the probability of self-annihilation of intelligent life.  These parameters were varied to demonstrate their effect on the probability of the formation of complex life.  The parameters were the central values of appropriate probability distributions from which random value are selected. Since the simulation includes the geometry and time evolution of the entire galaxy over some 20 billion years (20 Gyr), the calculations are able to localize the time and place (at least as a distance from the galactic center) where complex life is likely to arise.  Typically in a simulation, the number of planets bearing intelligent life rises to a peak and then falls off, to settle at some much lower equilibrium value.

Because all three of the major parameters are unknown or very uncertain, the authors considered a few specific values for each major parameter: λ_A = either 1 or 10^-6 Myr ^-1 (i.e., per million years); T_evo = 1, 3, or 5 Gyr, defining the center of a Gaussian distribution having a width of 0.2 Gyr, and P_ann = 0.0, 0.5, or 0.99.  Combining these values produced 18 scenarios.  The most optimistic scenario (1, 1, 0) leads to a peak value of 7,811,780 intelligent civilizations reaching its maximum at about 6 Gyr, the "medium" scenario (1, 3, 0.5) leads to a peak of 2,880 intelligent civilizations reaching its maximum at about 7 Gyr, and the most pessimistic (10^-6, 5, 0.99) leads to a peak of only 25 intelligent civilizations reaching its maximum at about 9 Gyr.

Interestingly, the peak value does not depend very strongly on λ_A.  Apparently it does not matter much whether the first primitive life appears fast or a million times slower.  The dependence on T_evo is stronger: reducing the time required for complex life and intelligence to develop by a factor of 5 produces an increase of about 2.5 in the number of intelligent species.  However, the strongest factor of all is P_ann.  Changing it from 0 to 0.99 decreases the number of intelligent species at peak by about a factor of about 100,000, from millions to tens.

 The simulations indicate that the peak location for evolved intelligence in our galaxy should lie in a doughnut-shaped region approximately 4 kilo-parsecs or kpc (1 kpc = 2,252 light-years) from the Galactic center and should reach a maximum around 6-9 billion years (6-9 Gyrs) after the Big Bang, decreasing with increasing time and distance from that peak point. For reference, our own civilization has arisen about 13.5 billion years after the Big Bang and lies at a distance of about 8 kpc from the galactic center.  Thus, we are an anomaly.  We would be 5 or so billion years late on the scene, our civilization has only been around for a thousand years or so, and our home is far from the center of things.  So are we naïve late comers to a galaxy burgeoning with intelligent life?

The fact that SETI searches have so far not observed any evidence of intelligent species in our galaxy is not very consistent with that view, since the simulation suggests there might be thousands of elder races residing in the galaxy's 4 kpc doughnut.  Perhaps the explanation is that P_ann is large, and they have all killed themselves off by now.  Or perhaps there are important factors that were not considered in the simulation.

In that context, W&B identified several factors as playing important roles in the development of complex life on Earth that were not included in the simulation.  In particular, W&B specified three conditions: (1) that the star should be located in a sufficiently depopulated neighborhood that nearby stars would not perturb the orbits of Earthlike planets as life was evolving, (2) that the star system should have fairly large "metallicity" (astronomer-talk for the presence of elements heavier then helium), and (3) that the planet should have a moon large enough to stabilize its spin-axis orientation to insure regular seasons.  Condition (1) might make the galaxy's doughnut-shaped 4 kpc region, where the density of nearby stars is high, considerably less ideal for the development of complex life, since major orbit changes of planets would likely extinguish life permanently.  Condition (2) probably means that since the heavier elements producing metallicity are the ejecta of earlier supernova explosions, the availability of carbon, nitrogen, oxygen, and other elements essential for life in condensing planets is time dependent, and the early star systems in the simulation, however Earth-like in mass and energy input, would be unlikely to have developed complex life. Further, Earth's molten iron core, a product of high metallicity, is responsible for its protective magnetic field and for the plate tectonics that has some impact on evolution.  Condition (3) is difficult to quantify.  An Earth-like  planet with chaotic seasons would certainly be less ideal for the evolution of complex life, but it is not easy to assess the importance of this factor.  Our large, season-stabilizing moon certainly makes the Earth a special place, but it isn't clear whether its absence would have precluded the development of intelligent life altogether.  In any case, it is clear that a simulation that ignores these three conditions should err on the high side and should be over-optimistic in predicting the probability of intelligent life.

Further, in one of my early columns (AV-11) , I hypothesized that the evolution of complex life is not a smooth process, but rather is "pumped" by periodic partially-life-extinguishing catastrophes, e.g. the dinosaur-killer asteroid event 65 million years ago,.  These events vacate "stale" ecological niches and make them available for newly evolved species (like mammals).  Perhaps the Earth has the optimal rate of pumping catastrophes.  The rate is related to volcanism from plate tectonics and to asteroid bombardment related to the orbital configurations of Jupiter and the Asteroid Belt in the Solar System.  Other Earth-like planets would likely have less optimal rates of pumping catastrophes, and evolution might progress more slowly.   In the simulation, this would push T_evo to larger values.

In conclusion, Fermi's Question (Where are they?) remains unanswered.  The simulation suggests that as intelligent life forms we are not alone in the galaxy.  However, in my opinion, the Monte Carlo simulation of intelligent life in our galaxy is flawed, and it needs to be done over with the inclusion of some missing factors.

Is our galaxy really sprinkled with billion-year-old elder races?  Then why are they so quiet?

John G. Cramer's 2016 nonfiction book (Amazon gives it 5 stars) describing his transactional interpretation of quantum mechanics, The Quantum Handshake - Entanglement, Nonlocality, and Transactions, (Springer, January-2016) is available online as a hardcover or eBook at:  http://www.springer.com/gp/book/9783319246406 or https://www.amazon.com/dp/3319246402.

SF Novels by John Cramer:  Printed editions of John's hard SF novels Twistor and Einstein's Bridge are available from Amazon at https://www.amazon.com/Twistor-John-Cramer/dp/048680450X and https://www.amazon.com/EINSTEINS-BRIDGE-H-John-Cramer/dp/0380975106 .  His new novel, Fermi's Question is coming soon from Baen Books.

Alternate View Columns Online: Electronic reprints of 212 or more "The Alternate View" columns by John G. Cramer published in Analog between 1984 and the present are currently available online at: http://www.npl.washington.edu/av .

References:

The Rare Earth Hypothesis:
    Peter D. Ward and Donald Brownlee, Rare Earth: Why Complex Life is Uncommon in the Universe, ( Copernicus , New York , 2000, ISBN: 0-387-98701-0);

Simulation of Intelligent Life in the Galaxy:
Xiang Cai, Jonathan H. Jiang, Kristen A. Fahy, and Yuk L. Yung, "A Statistical Estimation of the Occurrence of Extraterrestrial Intelligence in the Milky Way Galaxy",  Preprint arXiv:2012.07902 [astro-ph.GA].

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