Zero.
That’s right, the probability of humanoid or even recognizable aliens having visited or ever visiting Earth in the future is practically zero. Because there is a huge statistical difference between someone, somewhere winning the lottery and you winning the lottery. And yet some people will keep buying lottery tickets just as they will keep believing in aliens - and for the very same reason: they need to believe.
However, before explaining this any further, allow me to offer an aid in grasping the scale of some very large numbers that will be thrown around in a moment: for someone to simply count to a million (with no breaks of any kind, mind you) it would take about 12 days. To count to a billion, that would take almost 33 years. (I know you don’t believe me, so look it up. I ‘ll wait).
Now, since life emerged on Earth, it is highly probable that this also occurred in numerous other instances in the Universe as well. Yes, the requirements for life to emerge are pretty steep and specific, but the Universe is simply immense.
Our Milky Way galaxy has a diameter of 100,000 light years (or a quintillion, 1,000,000,000,000,000,000 km) and it contains about 100 billion stars. And the Milky Way is only an average galaxy among billions of other galaxies. Below is a rendering of just a part of the galactic supercluster our Milky Way belongs to, the Laniakea. Every single spot of light below is not a star, but an entire galaxy. And you can guess the density of galaxies in those thick white regions.
OK, so the Universe is immense. Not just in number of stars and planets (which means opportunity), but also in size. And size means distance. Keep this in mind for a while while we switch gears to evolution.
We have estimated the Universe to be 13.8 billion years old. The oldest planet we have discovered so far is 12.8 billion years old. However, that is not typical. Solid-crust planets are on average only 2.2 billion years older than Earth which has been estimated to be quite young, 4.53 billion years old. With the first cellular life appearing on this planet about 1 billion years later.
So, if there are older planets out there (and chances are there are) a fraction of them may contain water and a fraction of those will be within circumstellar habitable zones (CHZ or Goldilocks zones) with average temperatures remaining between 0°C and 100°C, thus keeping most of the water in liquid phase. I am not going to go into this in detail, but, trust me, water has unique molecular properties that make it essential for life. Yes, life may exist in entirely novel water-free forms (exobiologists have theorized about crystal lifeforms, for example), but this would be irrelevant to the question at hand. So, in an even smaller fraction of those planets water-based life may emerge.
At which point I am sure you realized that I have been giving you the long-hand version of the Drake equation. However, the Drake equation does not offer real probabilities. It is simply a shopping list of unknowns. And this is where it gets really interesting.
Did life emerge widely enough for evolution to become relevant? A population of bacteria feeding chemotrophically and thriving on huge methane lakes underground offers little need for natural selection to explore new directions.
Did the environmental conditions favor multicellular organisms? Unicellular prokaryotes are as successful today as they were 3.8 billion years ago, when they first appeared on Earth, surviving in most environments without ever the need to aggregate into multicellular formations. Will they ever develop the ability to space travel? Of course not. There will never be any need for them to do so. Multicellular organisms did not appear for 3.2 billion years, emerging a mere 600 million years ago.
Did life survive long enough for higher organisms to emerge? On Earth life came close to extinction five times that we know of, with the last one, at the Permian-Triassic boundary, killing off 90% of all species. From massive asteroids to star flares and from distant gamma-ray bursts to magnetosphere shifts, life may be hard pressed to find a way when facing a planet-wide sterilization event. Just look at what happened to Mars.
Did life escape near-extinction events with enough diversity to bounce back stronger? Near-extinction events may be perilous, but also burst open multiple new avenues of speciation, eliminating most vertical competition and putting all survivors on an equal footing. Yet, for an evolutionary gold-rush (that follows such events) to prove beneficial, the surviving populations need to retain enough diversity. Otherwise they are simply one more extinction waiting to happen.
Did stochastic events favour bilateral symmetry? intelligence? abstract communication? mineral-based technology? interstellar travel? Do you know why we have five fingers in our hands and almost every mammal, bird, reptile and amphibian follows the same anatomic pattern? Because 340 million years ago, at the end of the Lower Carboniferous period, the Sun’s output was reduced, when entering a new activity epoch, precipitating a planet-wide climate change which destroyed the carboniferous rainforests that covered the Pangaea. And the species that happened to survive this ecological near-extinction event just happened to be the ones sporting five digits in their limbs. Scientifically, the humanoid form is the end result of a very long chain of random events like this one. So, the most outrageous claim anyone has made about alien lifeforms is that they resemble humans. Sure, bigger or smaller, grey or purple, talkative or telepathic - yet in every tall abduction tale, the aliens are still basically humanoids. However, for yet another humanoid shaped lifeforms to emerge, an huge number of random events would have to repeat themselves - and in a similar enough order at that. And that is highly unlikely.
When did all this occur? Timing is very important because if some alien life managed to emerge yet then burned itself out (when their growth exhausted their resources or their star imploded or their entire galaxy was devoured by the galactic center of another), and this was completed a billion years before life appeared on Earth, it might as well never existed at all.
The questions above are very broad-stroked and yet, every single one of them drastically reduces the probability of recognizable lifeforms ever appearing elsewhere in the Universe. Because no matter how big the number you start with is, once you start taking the fraction, of a fraction, of a fraction, of a fraction…, the end result can quickly become a very small number indeed.
And even if, against all odds, all of the above still occurred in a way that allowed for concurrent alien lifeforms to exist, it would still amount to nothing. Because of the last two insurmountable hurdles:
UNFORGIVING DISTANCES: We have already established that the Universe is immense. The huge size of the Universe, the very thing that entertains the possibility of other lifeforms to occur, is also what makes it practically impossible for us to meet them. Even if multicellular intelligent speciation were to occur in other instances of the Universe, the distances are so daunting that it would not matter because we would never find each other. It is very unlikely that we would even get close enough to become aware of each other’s existence. So, now you can understand why Fermi’s paradox is, in fact, no paradox at all.
COMMUNICATION CHASM: How would you communicate with an ant farm? a bee hive? a coral reef? a fungus forest? an apple tree? Would it even be aware of your presence as an entity? How aware are you of the microcosm bursting with activity beneath your feet and in your bed mattress? Hollywood never gets this right for many reasons, the funniest of which is that they tend to forget that we are still unable to effectively communicate with our own dogs, let alone animals we encounter for the first time - and we are supposed to be able to communicate with alien lifeforms! At best, even in the most …sciency movies, they always resort to having the aliens “presenting us with a reality we are able to comprehend”. Care to examine how one would be able communicate with a deep sea fish by presenting it with “a reality it is able to comprehend”?
So, relax. There are no aliens visiting Earth. Go live your life.
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