Islanded in a Stream of Stars
We're probably not alone in the universe, but the place isn't exactly crowded either.
Are we alone in the universe?
Many recent headlines tell of encounters between the US Navy and mysterious, spherical objects that seem to violate the laws of physics. What could they be? What if, instead of pilot/instrument error or experimental drones, they’re actually alien spacecraft?
In this article, we’ll explore this possibility with some math, and the math leads to a bleak conclusion: the odds of us ever encountering alien life similar to us is extremely low.
For the purposes of this article, we’ll assume that we’re interested in life based on carbon and water. The conditions for life to exist require a planet with just the right surface temperature for liquid water. The SETI Institute estimates there are 300 million such planets in our galaxy. This may sound like a lot, but consider that the Milky Way Galaxy has a hundred billion stars. The odds of any star having a habitable planet are about three in a thousand.
Given a habitable planet, what are the odds it has an alien civilization?
History by the numbers
Below is a brief summary of the past and future history of our planet:
Earth formed from particle accretion 4.5 billion years ago.
Single-cell life emerged somewhere between 4 and 3.5 billion years ago.
For the next 3 billion years, life was simple. Then came the Cambrian explosion roughly 540 million years ago. Life went from being simple to complex very rapidly, with the first vertebrates emerging not long after.
From then on, there were five major extinction events, happening anywhere between 50-140 million years apart.
The most recent and most famous extinction event was the asteroid that wiped out the dinosaurs, 66 million years ago. With the planet’s apex predators extinct, mammals would rise to the top of the food chain.
The first recognizably modern Homo sapiens lived in Africa 300,000 years ago.
Human beings only became smart enough to develop language, make art, and build tools somewhere around 40,000 years ago (this is called “behavioral modernity”).
Humans develop agriculture somewhere around 12,000 years ago.
The first cities were formed around ten thousand years ago.
Only in the past 75 years or so have humans had the three technologies you’d need for a spacefaring civilization: rocketry, radio communications, and nuclear physics.
One billion years from now, the sun will begin to expand, causing the temperature to rise to the point where the oceans will boil away, thus ending the story of hydrocarbon-based life on our planet.
540 million years of complex life on this planet, and just 75 years of spacefaring intelligence. If the history of our planet is any guide, the odds of a habitable planet having a civilization like ours is somewhere around one in 7.2 million.
On the original Star Trek, whenever the Enterprise would discover a planet, it would usually have an alien civilization mirroring some chapter of Earth’s history. If we were able to cruise around the galaxy like Captain Kirk, we’d find that for every primitive civilization we encountered, we should encounter around fifteen “pre-intelligent” hunter-gatherer primates1 and thousands of habitable worlds with no intelligent or “pre-intelligent” life whatsoever.
You might point out that this math is based on the assumption that our planet has hosted just one civilization: ours. How do we know that there wasn’t a species as smart as us a hundred million years ago, forever lost to time? I’ll concede that it’s possible for an unknown, extinct species to have achieved agriculture. But if such a species had spaceflight, we should find at least some trace of them on the moon.
Intelligence: mostly harmless
You may have heard of the Drake equation. This is the equation used by Dr. Frank Drake to argue that the Milky Way Galaxy is teeming with anywhere between a thousand and hundred million civilizations:
You can read what all the factors are on Wikipedia, but the ones we are interested in are f[l]
, the probability that a planet capable of developing life will do so, and f[I]
, the probability that life will become intelligent. In Drake’s original 1961 calculations, he used a probability of 100% for both of these.
It may be a good bet that any planet with liquid water and an atmosphere has a good chance of developing the building blocks of life. Scientists believe Mars could have been home to single-cell life. Jupiter’s moon Europa is also a promising candidate. Three worlds in our solar system alone seems to suggest the universe could be filled with proteins and amino acids.
The issue, however, is with Drake’s assumption that 100% of rudimentary life will develop intelligence given enough time. It actually makes sense if you believe intelligence is the ultimate measure of Darwinian success. Smart species must reproduce more, and as each next generation gets smarter, eventually you arrive at civilization.
But is intelligence really that much of a help from an evolutionary perspective?
Being the smartest species on the planet, we naturally take a human-centric view of the world. You may have heard the framing that human beings are the natural culmination of billions of years of evolution, that the first cell division inevitably would lead to Mother Nature’s Greatest Creation. But when you factor in mass extinctions, the rise of humanity looks more accidental, not inevitable. We are really the product of sixty million years of evolution, since the dinosaur-killing asteroid wiped out so much life you could consider it a “reset” button.
It took sixty million years for the types of mammals pictured above to eventually become intelligent humans. Dinosaurs roamed the earth for 165 million years, and so far as we can tell, never became that smart for a simple reason: intelligence doesn’t pay “Darwinian dividends” until you unlock advanced technology. For 40,000 years, human beings have been the brightest bulbs on our planet. How much has that advantage helped us on reproductive success?
Recall the concept of the “food web” or “food chain” from middle school biology. Species have resources they consume, like plants and smaller prey. As species consume resources, their population grows, which means there is more prey for predator species above them, reducing the population. These opposite forces keep the population in an equilibrium.
Although human beings have been intelligent for a very long time, we have only been undisputed masters of the universe since 1700. From prehistory until then, the population of the human race was either growing extremely slowly or not growing at all. Before the Industrial Revolution, we were just another species on this planet. Humanity had no predators like tigers or sharks, but was still kept in check by disease and famine.
You may have heard of the “Great Filter.” It’s the idea that there is some catastrophic barrier that intelligent civilizations have a 99.99% chance of encountering before colonizing space. Usual proposals for the Great Filter include nuclear war, environmental collapse, or a rebellion by artificial intelligence. I propose the Great Filter is actually something far more boring: “first mammal with a frontal lobe gets eaten by a giant lizard.”
So long, and thanks for all the fish
Humans and our primate cousins are the most intelligent species on this planet, but dolphins and whales are a respectable runner-up. Dolphins have very impressive brains. They can observe people solving problems, and mimic behavior to solve the problem themselves. Dolphins have the concept of individuality, assigning specific clicks and whistles as names. They can even be taught to understand full sentences using English words or pictures.
It’s not so ridiculous to propose that had things gone a little bit differently, dolphins could have evolved to be as smart as humans.
And I’d take the proposition further. Water is needed to complete chemical reactions in carbon-based life. Intelligent life that lives on land could be the exception and water-based intelligent life could be the norm.
But all the smarts in the universe are no match for the laws of physics.
Imagine a civilization of dolphins wants to go to the moon. They’ll need to fill their spacecraft up with water2. Water is heavy, with a density of one thousand kilograms per cubic meter.
The combined volume of the Apollo command and lunar modules are about ten cubic meters, meaning that an oceanic species would require an extra ten thousand kilograms of mass for their moon mission. The Saturn V was capable of getting 43,500 kg of mass to trans-lunar injection (or TLI, an orbital maneuver to leave Earth’s sphere of influence to intercept the moon). Using the parameters of the Saturn V as a basis in the single-stage rocket equation for a very rough estimate, I calculate that the extra 10,000 kg of TLI mass will cost an extra 700,000 kg of propellant, brining the total mass of the rocket up from 2.8 million kilograms to 3.5 million.
It’s also worth pointing out that not every land-based civilization would be able to launch rockets too. Launching rockets from Earth is hard. If we had a thicker atmosphere or higher surface gravity, launching rockets would become prohibitively expensive or even outright impossible. Aliens on these “super-Earths” might be trapped there.
Life, the universe, and everything
Recall the figures from earlier. Let’s plug them in to get a final number…
Assume there are 300 million habitable planets in the Milky Way.
We expect one in 7.2 million to have a society capable of spaceflight.
Divide 300 by 7.2, and round up. Therefore, the number of civilizations in our galaxy right now is…
The Milky Way Galaxy is 100,000 lightyears in diameter. One light-year is already incomprehensibly large, let alone a hundred thousand. And of the hundred billion stars in the galaxy, just a few dozen have species capable of spaceflight or sending radio signals.
Remember, too, that this number is a high bound. It does not account for added difficulty of living on a “super Earth” or living underwater. It also does not account for the risk of a civilization-ending nuclear war, airborne rabies pandemic, solar flare, or asteroid impact.
Life is rare in the universe, and is only getting rarer. The universe is producing just three percent as many new stars as it did at its peak. If life on Earth dies out, that could be it for life as we know it. Therefore, it is imperative that we must not only take care of our planet, but find new worlds to settle as well.
They encounter only one such species, in “The Galileo Seven” — a very good episode!
Yes, I know dolphins breathe air, but they need to be kept wet for other reasons. Besides, the point of this thought experiment is less about dolphins and more about seafaring life in general.