The universe is vast. Hundreds of billions of galaxies, each with hundreds of billions of stars – each star with potentially multiple planets. This is a mind-bogglingly large number of potential planets in the universe.
With such a vast number of planets in the universe, surely some of them must have had the conditions appropriate to develop life. After all, life developed on Earth. Why would it not develop elsewhere?
So where are they? Why haven’t we found any aliens yet?
The only life that we know of in the entire universe is located on Earth. No forms of life, let alone intelligent life, have been discovered anywhere else.
We have found that life on Earth is incredibly resilient, able to survive the most inhospitable environments imaginable. Microbes live in Yellowstone’s hot springs, emperor penguins withstand temperatures of down to -40° C in the dark Antarctic winter without eating, and Arctic ground squirrels can withstand their own body temperatures dropping to -2° C. Microbes have even been detected in Earth’s stratosphere, 18km above the surface, and deep in the ocean around deep sea vents.
Bacteria is even able to survive in the void of space, posing such a problem that governments must sterilise planetary probes to avoid potentially contaminating other moons or planets in our solar system in order to abide by the United Nations Outer Space Treaty.
If life is so resilient on Earth, logic dictates that life must be resilient elsewhere in the universe.
If life is so resilient, then why haven’t we seen anything? If there are so many stars and so many planets, then where are they?
We can use very rough estimates to potentially calculate the number of possible alien civilizations out there in the universe.
If we assume that the Milky Way galaxy has about 250 billion stars, and each star has on average two planets orbiting it, we can estimate that there are about 500 billion planets in the Milky Way galaxy alone.
If we estimate that 1% of those planets support the conditions for life to develop, then perhaps there are 5 billion planets that can support life.
If only 1% of these planets develop life, then there are 50 million planets in the Milky Way with life.
If only 1% of these planets develop intelligent life such as our own, then there are around 500,000 planets in the Milky Way with intelligent life.
And if 1% of planets with intelligent life develop advanced civilizations capable of space travel to other stars, then there are 5000 advanced space-faring civilizations in the Milky Way.
That’s 5000 in our galaxy alone. If we multiplied that out to each galaxy in the known universe, that is potentially tens of trillions of advanced civilisations in the universe.
But even then, the chances that every star only has on average two planets orbiting it is a very conservative estimate, considering our own star has eight planets orbiting it. If there is a higher average of planets per star, then that suggests an even larger number of advanced civilizations.
It’s a mind-bogglingly large number. Even if I changed the percentages from 1% to 0.01%, that’s still billions of advanced space-faring civilisations.
So why have we not seen a single one? Even if there were only one advanced civilization in the galaxy, we would likely be able to detect them. The galaxy has been around for billions of years – this is billions of years for advanced civilizations to develop.
The stars are old – our sun and the Earth have existed for billions of years. Many stars are older than our sun, meaning those stars have planets that are older than Earth.
Odds are some advanced race of aliens received a billion-year head start on humanity. This would be enough to counter the suspected impossibility of faster-than-light travel – even restricted to sub-light speeds, a billion years would be more than enough to colonise an entire galaxy that is 100,000 light years across.
We can assume that an advanced space-faring race would have tremendous energy needs.
The most efficient power sources in the universe are the stars themselves. If humanity was sufficiently advanced, we could use our own sun to power all the energy needs we could ever realistically require.
Barring some as yet undiscovered technological miracle which could provide an unlimited amount of power, the stars themselves would be the power plants for alien civilisations as well.
The most efficient way to collect as much energy as possible from a star is to completely surround it with a megastructure that captures its entire energy output with countless solar arrays.
This type of megastructure is known as a Dyson sphere.
The side effect of a Dyson sphere is that a complete or even partially completed one would obscure the view of the star from viewpoints elsewhere in the universe.
A completed Dyson sphere would block the star’s light from ever reaching out into the rest of the universe.
If an advanced civilization had a billion-year head start on humanity, and was ever-expanding and always requiring more and more energy, then theoretically every single star in the Milky Way should be covered in a Dyson sphere by now.
If this was the case, no stars would be visible from Earth.
Obviously this isn’t the case, so this did not happen. Meaning that there was no energy-hungry alien civilization in the Milky Way in the last few billion years.
Our thought experiment we did earlier suggests that there should have been. So why hasn’t there been?
This problem is known as the Fermi paradox – that despite the high probability for intelligent life, there is absolutely no evidence of it. No signals from space, no visits from aliens, no signs of megastructures orbiting alien stars.
There was some excitement in 2017 when it was discovered that the star KIC 8462852 was routinely undergoing changes in brightness. One possible explanation for this is that orbiting this star is an incomplete Dyson sphere.
This cannot be confirmed, however. While a Dyson sphere is not out of the question, there are other theories, such as an enormous dust cloud obscuring its light, or that it is closely orbited by a number of large planetary bodies.
Until we can get further information, and while other possibilities seem more likely, it is not reasonable to assume that this is a Dyson sphere, as exciting as that would be.
Regardless, I believe there must be alien civilizations out there. The probability is too high for there to not be. As humanity becomes more adept at scanning the skies for signs of alien life, I believe one day soon we will discover it. I believe that day will have a profound impact on humanity, the day we discovered that we were not alone in the universe.