One of the great questions of our time is “Are We Alone?” There are really two parts to this question. The first part is “Does life exist which did not originate on earth?” The second part, perhaps even more important, “Do other intelligent and technological civilizations exist?” The Fermi Paradox is a way of asking, if the aliens are out there, why haven’t we seen some sign?
For the sake of this discussion, let’s assume that life does exist on other worlds. It seems a reasonable assumption, given our understanding of chemistry and the evolution of life on earth. We know that life appeared on earth relatively shortly after the formation of our planet. As soon as water, some basic precursor chemicals and energy were available, molecules clumped together to form stable systems that could reproduce themselves. Scientists still debate the exact point where these complex organic assemblies became “life,” but it seems that given the right ingredients and enough time in a stable environment, the result is simple life.
Going from single cells to technological civilization is far more complex. Most people have at least heard of the Fermi Paradox, which asks the question “Where is everybody?” or more succinctly, “Why haven’t we been visited by, or at least heard from, technological alien civilizations.” The assumption is that given the large number or stars, many of which will have planets, some of those planets will develop life, and a fraction of those will evolve intelligent life, and a few of those will become technological, and some of those will broadcast signals, or even develop space craft capable of interstellar travel. If those assumptions are true, there should be millions of technological civilizations in the visible universe, then it would also seem logical that we should have picked up some signals or been visited by now. But there is zero evidence of alien civilizations, hence the paradox. Something must be wrong with our assumptions.
The Drake Equation is used to determine the probability of alien civilizations with which we should be able to communicate. There a many arguments for plugging in numbers that suggest alien civilizations are either frequent or extremely rare.
I tend to think the proposal put forward by Peter Ward and Donald Brownlee, the Rare Earth Hypothesis, provides a reasonable solution. After all, we have a single data point, our own earth. And when we look at the history of our own solar system and planet, the odds of evolving humans that develop space ships, requires a long series of extremely improbable events.
To begin, rocky planets, close to earth’s size and density are rare. Many systems have hot gas giant planets orbiting near their parent star, making a smaller rocky planet in the habitable zone impossible. Earth is also a binary. The result of a collision of two planets early in the life of the solar system. Those two planets collided within a very small range of velocity and mass to create our earth-moon binary system which stabilized our rotation and axis for the following four billion years. Of the thousands of planets discovered so far, ours is the only binary. That makes sense, even a small difference in mass, velocity or angle, would have left a belt of asteroids, or caused the larger mass to exit the system entirely, or crash into the sun. A stable orbit with a stable moon large enough to stabilize our axis was the least likely of all the potential outcomes.
And consider that this very unlikely series of events happened in a system that wasn’t too close to the center of our galaxy, where nearby supernovas are frequent, along with gravitational disruptions from other stars passing nearby. Our solar system could not have formed much further away from the galactic center either. The stars in the outer parts of the galaxy’s spiral arms don’t have enough heavy elements of earth like planets. Remember that our hot iron core is responsible for the magnetic field that protects us from ultraviolet radiation. Without heavy elements is the right proportion, earth would be more like Mars, with a surface bathed in deadly radiation. Not only is Earth in the habitable zone around our sun, our sun is in the habitable zone of the Milky Way galaxy. Some estimates suggest that as little as 10% of the stars in our galaxy lie in that Goldilocks zone, favorable to the formation of long lived solar systems, in stable orbits around the galactic center, like ours.
So, while life may be quite common, systems that remain stable for long enough for technological civilization to arise might be extraordinarily rare.
There is another factor to consider. In spite of the earth’s fortunate position and stability it has had five major extinction events since the Cambrian Explosion 540 million years ago. The Cambrian Event is when biology went from simple life forms, to the complex forms we recognize today. All the phyla that define complex organisms were established during that burst of evolution. The Cambrian Event took place four billion years after the earth cooled.
The most severe event was the Permian extinction, 251 million years ago. That single event wiped out more than 90% of everything living on earth.
The most recent event, the Cretaceous-Paleogene event that killed off the dinosaurs, was much smaller. Even so, about 17% of all families, 50% of all genera, and 75% of all species went extinct. From that point, it has taken 66 million years for our technological civilization to develop. The unanswerable question is, “would technology have evolved more quickly had that extinction not taken place?” Maybe that event created yet another extremely rare set of circumstances which allowed humans and intelligence to evolve. Maybe dinosaurs with intelligence and social behavior would have evolved in our place. Even if they had, that doesn’t necessarily mean they would build skyscrapers or desire to go to the moon. There is no way to know for sure if these mass extinctions make the evolution of technological intelligence more or less likely.
The next question is how long do technological civilizations last? Humans are 150 years into an industrial age, and we are already causing major changes in the climate, and species are disappearing as fast as they did in other mass extinctions. One thing that we know for sure about the history of mass extinctions is that the top predators never survive. One possible solution to the Fermi Paradox is the technological civilizations don’t last very long. Something to think about.
Next week I’ll explore some additional possibilities that might explain why the aliens have not arrived.