In January of 1981, I received this postcard from Isaac Asimov, the prolific science fiction writer:
The postcard was in response to a letter I’d written to Asimov after reading his 1979 nonfiction book, Extraterrestrial Civilizations. In this book, Asimov used the best scientific information available in the late 1970s to reason about whether civilizations might exist on planets other than our own, and then, how many there might be in our Milky Way galaxy. (Spoiler alert: He came up with an estimate of 530,000 technological civilizations existing in our galaxy today.)
Asimov starts with our best estimate of the number of stars in the galaxy (300 billion), then works his way through how many of those stars are similar to our sun, how many of those sun-like stars have planets around them, how many of those planets have conditions that could support life in any form, how many of those life-bearing planets could support multicellular land-dwelling organisms, and so on. All of this with explanations of what is actually known and what is speculation, along with plenty of fascinating sidetracks.
Asimov then gets into questions of interstellar travel and whether extraterrestrial beings might have visited Earth, including a discussion of reported UFO sightings. Here I will quote him:
Those UFO reports that seem to be the most honest and reliable report only mysterious lights. As the reports grow more dramatic, they also grow more unreliable, and all accounts of actual “encounters of the second or third kind” would seem utterly worthless.
Any extraterrestrials reported are always described as essentially human in form, which is so unlikely a possibility that we can dismiss it out of hand.
That last sentence stood out to me like a sore thumb. Dismiss it out of hand? After he’s so carefully gone step-by-step through all this reasoning about what makes a planet earth-like enough to support earth-like life forms, why does he drop the ball when it comes to the possibility of human-like extraterrestrials?
This irked me enough that I wrote to him. I don’t have a copy of the letter, but here’s the gist of it: Once you have assumed an earth-like planet that has evolved earth-like, land-dwelling life forms capable of forming a technological civilization, why do you consider it so obvious that these intelligent, technology-creating beings would have completely different forms than us?
It seems clear that in order for these beings to build a technological civilization, they would at least have to have large brains, sophisticated senses, and fully developed languages. They would need an efficient means of locomotion on land — I think that means something a lot like legs — and appendages that allow them to carry things, make and use tools, and manipulate objects on a very fine scale. I think that means something a lot like arms and fingers. Sure, it’s possible to imagine all of these capabilities in bodies that are quite different from ours. But how can you be so sure that these bodies would not have a design that’s similar to ours in its two-legged, two-armed, one-headed form?
Asimov’s reply to my question:
Considering the enormous variety of shape, form, and properties of the millions of species that have inhabited Earth, and the astronomic numbers of genes and gene combinations possible, why should humanoid beings appear on any two planets — intelligent or not? If you go to China, do you expect to see someone who looks exactly like you?
I was very disappointed in his response. Why should humanoid beings appear on two planets that carry technological civilizations? Perhaps because the humanoid form turns out to be quite advantageous when it comes to building a technological civilization? Perhaps because, of all those millions of species that have inhabited Earth, the only one that has built a technological civilization is one that has a humanoid form?
Ah well. Even though I’ve read less than 10% of the 500+ books Isaac Asimov wrote in his lifetime, I have fond memories of many of those books that I did read, most of them as a teenager. This one disappointment doesn’t count for much in the grand scheme of things. I still have great respect for the man and for his work.
In case you’re interested…
Here’s an outline of Asimov’s figures from his book Extraterrestrial Civilizations. Some of them, as he freely admits, necessarily amounted to pure guesswork since not enough data was available to do any better. I imagine that now, after another four decades of astronomical research, many of these guesses could be improved upon.
1. The number of stars in our Galaxy: 300 billion
2. The number of planetary systems in our Galaxy: 280 billion
3. The number of planetary systems in our Galaxy that circle Sunlike stars: 75 billion
4. The number of Sunlike stars in our Galaxy with a useful ecosphere: 52 billion
A “useful ecosphere” is a region around a star in which planets could exist in orbits that are neither too close nor too far from their star. This rules out most binary star systems.
5. The number of 2nd-generation, Population I Sunlike stars in our Galaxy with a useful ecosphere: 5.2 billion
Population I stars are found mainly in the spiral arms of galaxies and are rich in heavier elements, having been seeded by older stars. Our Sun is a Population I star. Population II stars are cooler, less luminous, and “metal poor.”
6. The number of 2nd-generation Population I stars in our Galaxy with a useful ecosphere and a planet circling it within that ecosphere: 2.6 billion
7. The number of such stars with an Earthlike planet circling within its ecosphere: 1.3 billion
8. The number of habitable planets in our Galaxy: 650 million
9. The number of life-bearing planets in our Galaxy: 600 million
10. The number of planets in our Galaxy bearing multicellular life: 433 million
11. The number of planets in our Galaxy bearing a rich land life: 416 million
12. The number of planets in our Galaxy on which a technological civilization has developed: 390 million
13. The number of planets in our Galaxy on which a technological civilization is now in being: 530,000
Note that this means that, by Asimov’s estimation, about 99.9% of all the civilizations that ever existed in our Galaxy have passed from the scene. That seems not to bode well for our own future.
But that’s based on Asimov’s guess that the average lifespan of a civilization is 10 million years. Ours has only existed for about 10,000 years, if you define its beginning as the invention of agriculture. Or at most, 2.6 million years, if you define civilization as beginning with the earliest use of stone tools in the Paleolithic era. Even then, we’ve potentially got 7,400,000 years left.
I say we go for it.