Where Are All the Aliens?

When I was a teenager, I was convinced that aliens were here. It seemed completely plausible to my 13-year-old self that humanoid creatures from other planets could be visiting us in strange, saucer-like spacecraft. After all, look at all these people on the internet! They can’t be lying! There may have been some influence from my obsession with David Duchovny on The X-Files… but I digress.

As you can imagine, today I find the visitation of Earth by “little green men” to be extremely implausible. UFOs and alien visitation have occupied skeptical discourse for decades, as a quick glance at the archives of a publication such as Skeptical Inquirer will reveal. The more interesting question to me is: Why aren’t They here?

Famed physicist Enrico Fermi asked this very same question in 1950 around the lunch table at Los Alamos National Laboratory. Discussing a recent New Yorker cartoon in which saucer-flying aliens are carrying off New York city trashcans (I still don’t get it), Fermi set off a historic conversation by asking his colleagues, well, where ARE the aliens?

Here is the justification for his surprise: If you just look at our little corner of the cosmos, the Milky Way Galaxy, we have 100 billion stars, and the Universe has been around for over 13 billion years. After about 5 billion years of star-formation history, enough “heavy elements” like carbon, silicon, and oxygen, required for planet formation and “life as we know it” should have been available to start making planets. Our planet was formed another 3 or 4 billion years later, so there may have been planets that had a huge head-start on us in regards to time to form life.

Assuming the “Principle of Mediocrity,” or the idea that there is nothing necessarily special about the Earth, shouldn’t life have arisen elsewhere? Even if sentient life is a one in a hundred million chance, that gives us approximately 1000 civilizations, or one every few million years. Homo sapiens is only 150,000 years old. Given millions or billions of years, a sentient race should have colonized the Galaxy by now! So… why aren’t we inundated with visitors from another world? It’s a disquieting notion when you stop to think about it.

The possible answers to this Fermi Paradox* fall into three main categories:

Scenario 1: They are here, but they are being discreet about it.

So, maybe the UFO-believers are on to something. Maybe the extraterrestrials are buzzing by us all the time, but they have some sort of policy of non-intervention with young species like ours. Hmmm… this sounds familiar. Star Trek popularized this idea with the “Prime Directive.” This explanation does not sit well with me because of two words.

Captain. Kirk.

James T. Kirk. Professional Badass.

Sexy ladies? That’s a great reason to violate the Prime Directive! I feel like it? That’s a good reason, too! In 1000 Galactic civilizations, I find it hard to believe that there are no Captain Kirks.

Plus, it’s a tiny bit self-important to think that we’ve just been sheltered all this time.

Scenario 2: They haven’t/never made it here.

It could be the case that life is easy to create. It may even be plausible for sentient life to have formed several times, despite the challenges. That doesn’t necessarily mean that our extraterrestrial cousins wanted to spread out and colonize, though that notion may seem alien to us. There is also the distinct possibility that these civilizations never made it through their technological adolescence without exterminating themselves. Or, maybe interstellar travel REALLY is that hard to do. That’s right, no warp drive, fellow Trekkies.

Scenario 3: We are really the first sentient species in the Galaxy.

Well… damn. This is not very positive, especially for supporters of various SETI programs, but we have to acknowledge the possibility that we might be the very first sentient, intelligent, traveling, communicating beings, at least within the reasonable distances of our Galaxy and that is why we have not yet made “first contact.”

This is a topic that has fascinated people for decades, and we could easily amuse ourselves arguing the different sides for a long time. (In fact, I taught a whole college course on this!) We now know that planets are everywhere and that the building blocks of life litter the Galaxy. But then… where is everyone? Why do we still seem so alone? Until we find evidence of extraterrestrials, or until we colonize the Galaxy ourselves, I don’t think we can properly answer this question. But is fun to speculate.

Rare Earth? Photo from the crew of Apollo 17.

Thanks to everyone who commented on my last post, referencing this idea in some way! I have to give much credit also to Bob Rood, recently deceased astronomy professor at UVa and creator of the “Life Beyond Earth” course, and to Ed Murphy, current professor at UVa, for mentoring me through this fascinating topic as I prepared to teach it.

*It might be more accurately called the Tsiolkovsky-Fermi-Viewing-Hart Paradox, since it was discovered independently four different times. 


Nicole is a professor, astronomer, educator, geek, dog mom, occasional fitness nerd, and maker of tiny comets. She is also very loud under the right circumstances. Like what you read? Buy me a coffee:

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  1. Hi there!

    I’m sorry, my brain froze after you said that your 13 year old self said: “Look at all these people on the Internet”. #GodDamnImOLD

    For me, it was even MORE convincing: “Look at all these UFO stories written in BOOKS! They don’t let just ANYone publish a BOOK, you need to be a RESEARCHER for that s***!! 0_0

    — Craig

  2. I remember Carl Sagan explaining the Drake equation to me (yes he was just talking to me) during “Cosmos”. I was so disappointed at the numbers he ended up with … although I can’t remember what they were. *Goes off to google.*

    Can I also just say how thrilled I am to see these posts from you Nicole. Astronomy is my favourite science.

    “Space is big. You just won’t believe how vastly, hugely, mind- bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.” Douglas Adams

  3. I think it sort of sums it up best when you mentioned in your last article how long we’ve been telegraphing signals (i.e. not very long at all), vs how long it takes those signals to travel.

    And if they are amongst us? David Duchovny’s (former) hair just isn’t enough of an incentive to out them, I guess. :/

  4. Related to this: “Scenario 3: We are really the first sentient species in the Galaxy.” is that even if we’re not the first that there currently isn’t any other sentient species capable of communication anywhere near us at this current time. If the civilization we’re trying to talk to died out 1 million years ago (a very short time on the scale of the universe) then it’s just too bad for us.

    I also really do suspect that civilization like ours is a very rare evolutionary strategy, in part because in ~3.5 billion years of life on this planet it has evolved just one time. But there are many other survival strategies that have evolved independent of each other many different times. This is not to say that it could not have happened, but its rarity provides support for the idea that any life we’re trying to communicate with currently does not have the capability of receiving our transmissions.

    1. Possibly twice, if you count dolphins. Interestingly, there is debate around that. I’m not a biologist, so I can’t weigh in on that, but I did come across that idea when I was researching this.

      Also, not only is sentience/intelligence rare, it’s not necessarily required for survival. We love to think that it’s an inevitable by-product of evolution… but it doesn’t have to be.

      1. It’s my understanding that the whole dolphin intelligence thing seems to be the result of a fair amount of over-exuberance and optimism on our part. Not to say that they aren’t smart, or that their social lives aren’t complex, but the idea that they’re human beings with flippers is one that’s been pushed for a long time with little to no evidence to back it up.

        1. I agree it may be overdone a bit, and it also brings up another problem: It doesn’t matter how super intelligent you are if you don’t have hands, or access to smeltable metals.

      2. I love the idea of dolphin intelligence!

        I mean, if it turns out that dolphins are an intelligent species living on our own planet, then how important/AWESOME would it be to learn how to communicate with them. We wouldn’t have a hope or a prayer with an ‘alien’ from another world if we couldn’t communicate with a being that at least has the earth in common with us. :) I know it is super nerdy, but I just picture the Star Trek movie, where the whales were communicating with the advanced beings.

  5. My favorite theory is that there just is no good way to travel or communicate across these distances no matter how smart you are. Put it another way: maybe nobody in this universe has a flying car.

  6. I know this is not a popular opinion, but it certainly seems like (as in all evidence supports) FTL travel is just not possible. It doesn’t even really make sense.

    It is like saying, “one day, we will develop technology that will allow us to travel slower than not moving at all”. “c” is just that built in to our understanding of space/time.

    So, the enormous distances that separate the stars would be very costly to traverse. In both time, and fuel. Matter/antimatter annihilation is the best energy producing scenario in our universe – it gives you the least waste.
    A starship that used it would have to slow down (basically, point its rockets toward its target) in order to visit. Accordingly, it would be visible, as one of the brightest objects in its target’s sky, possibly for months.

    And it would have to be huge, and completely self-sufficient. And best case, would still take thousand of years to get anywhere (relative to one’s own home star).

    I don’t see the benefits, especially when your travelers can’t even report back to their home system what they’ve found, until many generations have gone by.

    1. Yeah, I studied aerospace engineering in college, and this is the conclusion I came to as well, that for our purposes – it’s nearly impossible. The amount of energy needed to bend even just a little bit of space faster than light around a spaceship (the most scientifically realistic FTL theory imo) is more energy that the human race can hope to produce in its lifetime.

      For instance, the latest calculations put the power required to create a bubble of space to propel a spaceship at 10^45 Watts. The total output of the sun is 10^24 watts. So even if we could build a sphere to catch all the output from the sun, it would take longer than the sun has to live to collect enough energy.

      If this is actually true, then it’s likely almost no races are flying at FTL speeds and therefore not in contact with each other for the most part.

  7. To be fair, for aliens to want to come here you’d have to have a world that was essentially a twin of their own.

    If they breathe ammonia and have biologies that require their bones be made of water ice, Earth isn’t a nice place.

    Similarly, if their biology uses sulfuric acid as a solvent and they are iron-based or use something like tin as the basic “four way bonding” chemical in their DNA, they’d go somewhere else.

    Also, given that our radio transmissions — which aren’t all that bright, BTW (I asked Gregory Benford about this, he said they’d be basically undetectable as artificial signals more than 100 or so light years out) — haven’t been going out there for long, it’s pretty unlikely anyone knows we are here.

    1,000 civilizations, separated on average by 6,000 light years — ok, less if you confine it to the outer 2/3 of the galaxy, but you still end up with an average distance ~ 4,000 light years or so.

    So any civ would have to be ahead/behind us by at lest that much. If they arose a million years ago and died out after 100,000 years and were on the other side of the galaxy, we might well just miss them. The transmissions went from 1,000,000 BCE to 900,000 BCE and stopped.

    Something tells me intelligence is rare, tho. And intelligence that spreads out rarer still. I know it only takes one. But even a lightspeed civilization would take thousands of years just to get here.

    Then there’s the “cosmic reboot” idea. That is, gamma ray bursters sterilize huge chunks of a galaxy every so often and anyone around is toast before they can figure out how to stop it or shield themselves.

  8. Hmmm, I have a degree in this shit and what I’d say the most likely causes are are:

    1. Interstellar travel is *extremely* difficult. The distances are vast, and the energy requirements to move masses the size of a spaceship even some fraction of lightspeed are enormous. The requirements to bend space around a ship are currently estimated to be the mass-energy content of Jupiter…

    2. There are a lot of stars. So even if a race does achieve interstellar travel, the chances of finding life on any given star is very low.

    3. Intelligence capable of interstellar travel is rare.
    There may be plenty of beings out there as smart as us, but they may not have the same ability to manipulate their environment. For instance, Dolphins may be as smart, but they lack the ability to manipulate objects as adeptly as we do, and being aquatic, it would hard for them to achieve things metallurgy, electrolysis, or most forms of chemical synthesis.

    So while they may be as smart, they lack 5 fingers to manipulate objects and a dry environment to manufacture things.

  9. I want to believe! But like Scully I’ll need some more evidence to substantiate the conspiracies.

    Maybe “they” don’t have the same motivations or psychological makeup as humans, be that colonization, Prime Directives, or the impulsive violation of directives. Who’s to say they have emotions, or the curiosity to explore their surroundings? What if extraterrestrials are dull?!

      1. I smoke the sticky icky, sometimes daily sometimes monthly (we’re at about weekly now because I be poor), and I think pretty much everyone can attest that I am far, far from dull. In fact, I’d probably be far, far more manic if it weren’t for the weed, man. LOL

          1. nah, the first warp drive will be dreamed by someone super gifted in the intelligence brainpan; being high, or not, does not factor. People always like to fantasize that being high somehow makes you more imaginative or give you some kind of super mental powers.

            No, being more imaginative, makes you more imaginative. the end. Being high does other more mundane things to you, mostly involving relieving stress.

          2. //People always like to fantasize that being high somehow makes you more imaginative or give you some kind of super mental powers. //

            //Being high does other more mundane things to you, mostly involving relieving stress.//

            Well no durrrrr. Seems you missed the big heaping pile of sarcasm in the comment.

            And as for ideas when high, it’s given me some ideas that I’ve later turned into products when sober :P.

  10. I call this the quarantine of space. Unless two planets with advanced life on them happen to be really close together (in space terms) they will never meet.

  11. I think a more interesting question is why do galaxies all look the same (to a certain extent)? If there is anyone out there, someone must have built something like a Von Neumann machine that would eventually leave evidence in the structure of a galaxy of its presence(self-replicating robots using up a large proportion of a galaxy’s raw materials).

  12. IMO it’s helpful to ask the opposite question. If we were smart, why would we go to another Solar system?

    In this one we have more resources then we will ever use. So there’s no pressing economic need to do so.

    Scientifically astronauts are cool, but most of of what they do is done by un-manned probes cheaper and safer. The only compelling reason we’ve ever found to leave Low Earth Orbit is making the Soviets look bad, and I sincerely hope that Earth has out-grown that kind of thing.

    Which means that even if our Great-Grand-Children master light speed they probably aren’t gonna be sending millions of colonists to Alpha Centauri. Un-manned probes to satisfy their curiosity? Probably. Massive space habitats in the Solar System itself to house billions of extra humans? Almost certainly.

    But the alien Fermi isn’t likely to notice that kind of thing. He’ll dismiss the probes as natural phenomena, and he’ll never see the habitats.

    Keep in mind that even if there was some economic resource in space that we needed to transport to Earth most alien Fermis would never see our ships. Since most of space navigation is avoiding large objects with massive gravity wells the only systems that would actually see any of our mines and freighters would the ones were actually transporting stuff from and our Solar system.

    1. “In this one we have more resources then we will ever use.”

      Ever is a long time. Will this system support a trillion people? Probably. 100 trillion? Donno. But at some point we will run out of room. Though I suppose I agree that travel to other systems is likely to be too hard to serve as a means of reducing population pressure or acquiring needed resources. However, I suspect that for advanced enough societies, planting a seed of their civilization on another star would be possible. And It seems likely that organisms which have made it far enough to be space-faring civilizations on the local level will have probably retained that instinct to spread themselves. Eventually, after a couple million years or so, that new system is feeling their oats and they are ready to spread themselves. It is the billions of years during which this could have happened that makes the Fermi paradox compelling. That and the fact that it if even one of these civilizations developed we would expect them everywhere.

      My personal unfavorable resolution of the fermi paradox is the idea of a single, ancient, widely spread civilization, which is developed enough to be satisfied with inhabiting only a fraction of the systems, and which prevents the rise of more expansionist cultures by neatly snuffing out all technological civilizations it detects.

      1. And what possible reason would we have to let our population grow to a trillion?

        All available evidence is that once you get rich enough population growth no longer becomes an issue. A human kid is a lot of work, and very few people choose to spend the energy it takes to raise two if they’re given a choice.

        That’s kinda the whole problem with the Fermi Paradox. It’s based on the idea that the trends of a very weird period — the age of Imperialism — are normal, and that all intelligent life would always act just like the Western Empires of that time. Massive population growth, coupled with spending vast amounts of money to send your citizens places just so you can be the country who did that shit just isn’t normal.

        Which means Fermi’s hypothesis that some other intelligent life would be doing that shit in our very Solar system, at this very moment, if there was any other intelligent life in the universe, makes little sense.

        Because pretty much the only way we’d notice them is if they were doing that Imperial shit. Otherwise at best they’d be sending a patrol through the Solar system on a regular schedule, if NASA noticed a couple spacecraft among the millions of bodies that orbit the system it would probably rationalize them into comets of some sort.

        1. THe only problem with the economic analysis you have is that what drives lower birth rates is the need for labor (or lack thereof). Humans do have a fundamental need to reproduce — we’re hard-wired for it, just like every other animal.

          And it isn’t just the Age of Inperialism that drives people to go out and re-settle somewhere. Humans have done it from the get-go. Chimps, baboons and other primates do too when groups get too large. Pre-industrial, hunter-gatherer/small scale agriculture groups seem to have a limit of about 400 people or so before they split up. It wasn’t until someone invented a non-kin-based loyalty system that you had towns of more than a few hundred.

          That said, see above — sublight travel is awfully slow. If a civilization had a million years they would just be getting to the other side of the galaxy around now. And there are lots of stars between us and them. They’d not only have to be space-faring for a long time, they’d have to be close by too, and the odds of that aren’t good.

          In fact, think of this: the Earth was born 4.5 billion years ago. The universe is 13.5 billion years old. It took the first few hundred million years at least to get any heavy elements to work with. It would take another 4-5 billion to get to intelligent civilizations.

          Then you have to hope that one of that first generation makes it without some disaster.

          And that they are in our galaxy.

          And that they are close enough to see.

          And that they are even able or interested in traveling

          And that they are anything like us — I mean that their body chemistry remotely resembles ours and they like stars sort of like the sun.

          That is a LOT of conditions to meet.

          Maybe most life int he universe is algal mats.

  13. I loved Geoffery Miller’s essay in 2006 for edge’s What Is Your Dangerous Idea?: Runaway consumerism explains the Fermi Paradox

    I suggest a different, even darker solution to Fermi’s Paradox. Basically, I think the aliens don’t blow themselves up; they just get addicted to computer games. They forget to send radio signals or colonize space because they’re too busy with runaway consumerism and virtual-reality narcissism. They don’t need Sentinels to enslave them in a Matrix; they do it to themselves, just as we are doing today.

    1. Or, they progress far enough to invent Holodecks. After that point, evrybody spends all their time in them, and nobody ever invents anything else.

  14. The nanotechnology-based aliens are here. They’ve been here for millions of years. This is a tiny “nature preserve” for them; they don’t want to pollute or destroy it. They’re waiting and watching to see what happens. Maybe someday, after we’ve achieved the singularity and much more, we might approach the level of intelligence necessary for them to consider us worthy of communication.

    An appropriate comparison would be, “When’s the last time you tried to establish “communication” with a bacterium?” Same idea here: From their perspective, we’re not “intelligent” yet.


  15. Wait, dolphin intelligence? I actually thought this one had been put to rest and that dolphins aren’t really that smart after all. Shouldn’t we be discussing if crows might be another sentient species at this point?

    1. Well, dolphins can manage to understand some human language, and so far we are unable to understand any of theirs. So I think the dolphins might be winning on intelligence. (I did see some research that we’ve determined dolphins do call each other by individual names. So we’re starting to sort it out.)

      If they had thumbs, we’d be toast.

    2. From what I can tell, it’s hard to separate our own anthropocentric biases in studying dolphin intelligence or self-awareness. However, they do seem to have advanced problem solving abilities AND they split off from the mammalian branch that contains primates well before these developments.

      But yeah, as a couple of people have pointed out, they don’t have thumbs…

  16. Isn’t it possible that aliens are here and we can’t even perceive it? I don’t mean that in the ‘higher spiritual plane” sense. Pretend a bacterium had a consciousness. Couldn’t it just as reasonably say, “Whoa, if i evolved for this biosystem, couldn’t other’s? Where’s the aliens?” and have no idea that it lives in something that has a (subjectively) alien consciousness of it’s own?

  17. I love, love, love your articles, I love space and shit. Anyway, I think given the vastness of the universe I think the most likely scenario is that there’s a shitton of life out there, probably a lot of senitient beings too, but they don’t even know we exist, we’re bacteria in a petri dish relative to the universe.

  18. I suspect the reason we don’t see many highly advanced technological civilizations in the galaxy is that, before any species attains technology sufficient to either travel or effectively communicate across interstellar distances, they first attain sufficient electronics and information processing technology to create something like the Internet.

    …At which point their civilization stagnates, degenerates, and rapidly collapses.

  19. meh, I think it’s pretty simple. Interstellar travel is very very hard and FTL drives may not be possible. If most interstellar travelers are traveling at sublight speeds, then given the small # of civilizations who are capable of doing, the distance between them, and the abundance of stars in between them that are completely barren it’s no secret why we haven’t heard from anyone yet.

    If there is no easily habitable planet for 1000 cubic light years of most civilizations and these civilizations can only travel subliminally, then it’s going to be hard to do a mass expansion across the galaxy.

    If humans ever do expand across the galaxy, my feeling is that it will not look like a unified galactic federation, but rather a group of disjointed and fairly isolated colonies because of the distances and times involved.

    It’ll be a bit like the expansion of Polynesian civilization I think. Very slow, and very isolated.

  20. Although i am sceptical to all the UFO fuss, i still think they are already out there watching us. And your “Kirk-Argument” is valid too. There might have been several Kirks already, resulting in some of the more believable UFO-stories. It’s too bad that we can not be sure either way at this time.

  21. I think that answer to your question lies with Ray Kurzweil's predictions for the furture of human technology.  Based upon objective trends in computing, biological, medical, and nano technology, in less than 100 years humans will begin to transfer our conscious selves to machines.  As machine life, we will become more intelligent, faster, and capable by large factors.  This is the logical direction and conclusion of all technical civilizations that have appeared in the universe.
    However, the amount of time from the first radio broadcast, to emergence into the galaxy as fully formed gods (for lack of better term) is brief.  Beings who have already transcended would have no more interest in dialogue with us, in our present state, than men would have in dialogue with a caterpillar.
    It is possible that we may be monitored by other civilizations, but more out of scientific curiosity than parental concern.  For them, we either make it, or we don't.  Or, they may be competely unaware of our pending emergence.  The Universe is large and the amount of time from initial broadcast to transcendency may be too brief for them to have become aware of us.
    Either way, we won't find out until we "transcend" ourselves and go looking for the transcended descendents of our biological cousins out there.

  22. It seems irrational that life on earth is a blip in a universe so big.  However, it also seems irrational to assume that life on other planets would evolve an intelligent species just like us. We are surrounded with evidence of an amazing diversity of life, yet we expect the aliens to be us, in funny suits.

  23. Of course there are aliens on earth. They are making comedy programmes for German television.

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