Have aliens ever been to Earth?

A statistical journey.

Oct 07, 2021

According to the Fermi Paradox, it’s pretty weird that we haven’t found any trace of aliens yet. There is literally so much stuff in the universe that it’s statistically impossible for us to be alone. There’s something like 10^24 stars in the universe, not all of which are nice enough to allow life as we know it, but even 1% of those stars having habitable zones similar to ours represents 10,000,000,000,000,000,000,000 candidates for life. Us having a 1 in 1 septillion chance of existing is like my odds of not being sarcastic in a newsletter: for all intents and purposes, zero. This leads us to the unsettling conclusion of the Fermi Paradox—either, against those odds, we’re completely and utterly alone, or we’re not. Both of those are equally scary.

If there’s such a good chance we’re not alone, what are the chances we not only have alien counterparts, but they’ve visited us before? That question I can’t answer quite as easily.

The examples that first come to mind are legitimate sightings that humans have had. Not random people shouting “UFO!”, but things like the Omouamoua “comet” that was seen hurdling past Earth in 2017. To this day scientists have yet to entirely explain the motion of this interstellar visitor. Some people even concluded that it had to be unnatural—made by something. Or someone. The New Yorker has a great article about this comet, but the gist is that an extremely oblong object flew past Earth, traveling with a trajectory and spin that could not be explained by normal orbital mechanics. One theory is that it was being propelled by solar radiation (like a solar sail). This is physically possible, but the geometry needed to actually enable this cannot be found in nature. So if solar propulsion truly does explain its movement, it wasn’t a naturally occurring object. [3]

Things like this are the closest we’ve gotten to determining if alien life exists but until we see unequivocal proof, not many people are willing to risk their reputation to make the claim of alien life. But what proof is needed beyond “can’t be explained by naturally occurring phenomena”? Well, as they say in the biz, seeing is believing, and until we have something sitting in front of us that is confirmed out-of-this-world, I don’t think we’ll ever truly say for sure.

We’ve come to a disappointing conclusion relatively quickly in this article, but don’t worry, I’d never let you leave feeling dejected. We’re going to attempt to answer this question with statistics—the engineer’s way of making crazy statements. Earth has been around for 4.5 billion years. That means that aliens have only had 4.5 billion years to visit us. The first ~500 million years were too dang hot for anyone to want to stop by, and too long ago for us to ever find evidence of it. So we’ll say that aliens would have had a reason to visit Earth for 4 billion years.

Now, all we need to do is determine how much time it takes to evolve into an intergalactic species. We humans have yet to do this, much to my dismay, but typical estimates range from 50,000 to 100,000 years from where we are now. That’s a long time for us, but nothing on an intergalactic scale. We’ll be ambitious and assume it only takes 50,000 years. Therefore, if 0.1% of the stars in the Milky Way have habitable zones, and a rather conservative 0.1% of those stars are home to complex life, that leaves 1 million planets that have had 4 billion years to evolve and venture out into the cosmos. If we assume that these species have technology that can tell them which stars may be home to life, the same logic says that the odds of them picking Earth to visit are 1 in 1 million. Not great, but they’ve had some time to do it.

Earth’s average distance to everything else in the Milky Way is about 50,000 light-years. With light-speed travel, each round trip journey takes 100,000 years. Therefore, each of these species could have done 40,000 trips since Earth has been around. This many trips and each trip having a 1 in 1 million chance of being Earth means that the chances of us being visited at some point in Earth’s history are 40,000/1,000,000=4%. That’s the most reasonable number we’ve seen so far. And that’s just one species. To be fair, that species would’ve needed to be around all 4 billion years, and they would have actually wanted to do all that space travel. If we include out how many species would have potentially been out traveling during that time, and average how many trips they’d really want to do, that 4% actually increases to over 100%. This means that it is impossible for aliens to have not visited Earth.

Well, crap. They stopped by, and we didn’t even clean up for them. How rude. But we haven’t seen proof. Why haven’t we seen proof? That is the question. Either it’s much harder than we think for life to evolve, or it’s as easy as we thought, and the aliens don’t want to be seen. Are they hiding from something? Or are we really just that special. I’ll leave that question up to you.

Thanks for reading this week’s edition of It’s Not Rocket Science. See you next week!

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