You may have heard that our Universe is old, a venerable 13.8 billion years old to be exact. It is so old that, in the rounding of that “exact” number, there is long enough for Rome to rise and fall over 100,000 times. Our galaxy has been around for 99% of the life of the Universe and yet in all that time, it would appear no species has yet colonized it. Which begs the question “where is everybody?”. Or, in other words, why is humanity so alone?
We call this the Fermi paradox and, in the years since it was first asked in 1950, we have developed a small number of plausible explanations some of which I will explore below.
What’s the big fuss?
Not only is our galaxy old, it is also big, containing over 100 billion stars. Greater still it contains a couple of times more planets than that with roughly 10% of those orbiting sunlike stars with conditions similar to Earth when life first got started here.
We have evidence that life first emerged on Earth 4.5 billion years ago, but we also have evidence that Earth took part in a collision that completely sterilized the planet and created the moon 4.51 billion years ago. This means life developed on Earth almost immediately after it became habitable again. If it can happen in just a few tens of millions of years here, why didn’t life develop somewhere else first on the tens of billions of similar planets in our galaxy and in the 10 billion years before it did on Earth?
Maybe we are too early
I just mentioned that life likely emerged about as quickly as possible on Earth which might be why we are so special. It may well be the case that life is “supposed” to take billions of years to emerge but that we simply got lucky on Earth and have woken up long before the rest of the species in our galaxy. Whilst it is very possible that this is the case if it were the only factor at play, our current situation would be highly unlikely. This is simply because of how little time we took to develop in comparison to how many planets have been around and for how long.
To put it into perspective it is like if a runner turned up two hours late to a marathon with every other person on Earth already competing. To be as fast as our species was on this scale they would have had to run the course in under ten seconds and win the race. If it was humanly possible for someone to run that fast surely at least one other person out of everyone on Earth would have completed the marathon in the two hours before they arrived. In this situation, you would assume that the runner cheated and in our situation, there must also be something more to the story.
Maybe Earth is even more special than we think
I also mentioned that we think 10% of planets in our galaxy have similar conditions to Earth when life first emerged here. This only considers planets orbiting sunlike stars and with the elements and stability required for life. Earth however has many other potentially unique properties that may well have protected us from extinction.
One such protector is our largest gas giant, Jupiter, which has kept Earth mostly safe from asteroid collisions for billions of years. Gas giants as large as Jupiter are not uncommon in our galaxy however most gas giants gradually lose material and fall toward their star over time. This process destroys any planets in their path and it is thought that Jupiter also wandered into the center of our solar system long before our planet was formed.
Fortunately today Jupiter is kept safely far away from us by the gravity of our second gas giant Saturn. We do not yet know if this is uncommon, however, it is possible that our powerful yet restrained gas giant has been the key to our unique success.
Another potentially rare feature of our planet is our abnormally large moon. Most moons are small objects captured by the gravity of larger planets. Ours however was formed from the debris after Earth’s collision with another large object. Of the 158 known moons in our solar system, only two were formed this way making them exceedingly rare. Perhaps the tides created by such a rare and large moon gave us a crucial leg up in our development.
Maybe there is only room for one
Finally, I have an argument that I think explains why we will not find any other life. I do however think it falls short of explaining why we are so special as to be the first. Suppose a species had developed at any point in the 10 billion years before we did. They would eventually outgrow their planet and it would be logical to assume that they would have attempted to expand to other planets. As far as we know, if they did this, there would be nothing stopping them from colonizing every planet in the galaxy in just a few million years. This would include Earth and if this had happened we would not be here. After all, life has not developed twice on Earth because ours is already too far ahead.
Back to our running analogy, this is like the marathon having a rule that the race ends one second after the first person finishes. In this situation, the runner turned up two hours late to the race and finished the race in 10 seconds. All they know is that the race didn’t end before they finished and so if someone else beat them they must have won less than one second before. It is very unlikely that the runner would finish in this exact right second in the at least two-hour race when it took only 10 seconds to complete it. It is much more likely that no one else finished before them and that they were first. Back to the galaxy that means if we exist (i.e we finished the marathon) then the most likely possibility is that no one else exists. It also means it is unlikely that another species will reach the same point as us before we colonize the galaxy in the next few million years.
Where do we go from here?
I hope I’ve got you thinking about our place in the Universe but there is still so much we, as such a young species, have yet to learn. With the James Webb Space Telescope in space, our knowledge of other planets will grow exponentially over the coming years. Hopefully, looking far out into the galaxy, it will show us what is so special about our own planet, and in a short time, this will cease to be a paradox at all. Even if we never find out why humanity is seemly so alone, I think it is awe-inspiring enough that we even exist to witness the Universe that made us.