Darren asks:

This pops up in video games relatively frequently, and I’ve often wondered:  would it actually be possible for an organism to evolve in (or evolve the capability to survive in) the vacuum of space?  If it’s possible at all, what qualities would they need to have (or what qualities would be especially beneficial) and how complex could such organisms realistically become?

As you’ve touched on, the answer to this one depends on two things.

1)     What you mean by “vacuum”.

2)      What you mean by “organism”.

As it stands each one could potentially describe a rather large number of variable states, so we’re going to have to narrow down a bit.

First I should make it absolutely clear that the chances of something that we’d recognise as life evolving in situ in the vacuum of space are bugger-all. There’s simply too many missing elements that we regard as being necessary for life to evolve in the first place; no gravity, very little energy, no nutrients, none of life’s basic building blocks, nothing.  It’s kind of like saying “Can life evolve out of nothing inside a blast furnace?” Well no, probably not. Organisms which start out in more temperate environments can evolve to tolerate very high temperatures (as covered in my extremophiles post), and this is the great power of evolution: it’s a gradual, iterative process in which organisms can adapt to live in just about any kind of environment imaginable, but they need to begin that process in a place which is conducive to life forming in the first place.

So here we’re covering the second case, in which we start out with some bacteria or something swarming and multiplying inside a primordial ooze. If we were vengeful gods (or the typical consumer of Maxis games) and we could somehow tinker with the evolution of this bacteria over hundreds of millions of years, could we end up with something capable of living in a vacuum? And if so, how complex could we make it? In order to save time I’ll just assume I can give our little super-organism any attribute exhibited by currently existing lifeforms; what would it take to make its survival possible?

For the vacuum, we’ll consider a single factor that determines how hostile the environment is: is there a large quantity of harmful solar radiation present that could do awful things to a cell directly exposed to it?  For the organism, we’ll simply deal with the hardiest forms of life currently known to man, the various genii of extremophiles. As for what that organism is going to need to make it go, I’ll be using Wikipedia’s definition of life here. Most of it can be taken for granted (in that anything we recognise as life will have these attributes by default) but the three things we have to cover from that list are:

Metabolism. Our vacuum-dwelling organism needs to eat something to sustain itself.

Growth. It needs to take the resources it metabolises and use them in anabolic processes to grow itself. The rate of growth needs to at least match the rate at which bits of the organism are dying off, otherwise the organism as a whole will gradually shrink and eventually die.

Homeostasis. Its vital systems need to be stable and self-correcting inside the parameters of the environment it’s going to be living in. Because this is vacuum that environment is potentially very very demanding, with the temperature in particular undergoing rapid and extreme changes on a regular basis.

I should probably stress at this point that I’m a physicist, not a biologist. I understand everything I’ve told you so far because they’re fairly simple physical systems; energy intake during metabolic processes needs to equal energy spent to grow the organism, and so on. I don’t need to know the actual mechanisms behind “metabolism” or “growth” to tell you this, since even biological entities have to follow the conservation laws. However, your mileage may vary considerably when applying the following hypothetical scenario to actual biological processes since there’s almost certainly a huge amount of fine detail that I’m going to be ignorant of. What I’m saying here is, don’t actually try to grow a vacuum-dwelling super-organism based on what I’m about to tell you because you’ll probably come a cropper. It’s an interesting thought experiment and nothing more.

Metabolism and growth are intimately linked, so we’ll start with both of those at the same time. You’re not going to grow as an organism if you’re not metabolising enough matter to do so. Sadly for our prospective vacuum organism you have to metabolise something physical – for example, plants do not feed on sunlight directly but instead use the energy absorbed from sunlight to power a photosynthetic reaction that extracts carbon from CO₂ and oxygen from H₂O and fixes them together to form a healthy nutritious sugar. Without the carbon dioxide and the water the sunlight isn’t all that helpful; it powers the metabolic process but does not feed the organism itself. Other organisms have evolved that have substituted in other power sources to power metabolism instead of sunlight – this one uses actual honest-to-god decay radiation from natural deposits of uranium – but no matter what you use you still need something to process from the outside environment.

This is probably the primary reason why vacuum dwelling lifeforms in deep space are impossible¹. There’s simply not enough raw materials out there for them to live on. Life is pretty much confined to planetary surfaces. Moreover, most forms of life require oxygen if they want to evolve anywhere useful. Wikipedia’s pages on aerobic vs. anaerobic respiration are complete garbage written by morons. Fortunately I can use my doctor skills to check sources that aren’t Wikipedia² and I can tell you that while anaerobic respiration (metabolic processes that do not use oxygen) is fairly common at a low level within microbes and even as a temporary measure within more complex organisms (human muscles when sprinting), the amount of energy it provides is tiny compared to aerobic respiration which does use oxygen. Aerobic respiration is absolutely necessary if we want to build a complex organism, as the metabolism requirements will be so large that only supercharged oxygen molecules (which are very good electron acceptors in terms of generating energy) can fulfil them.

So this basically rules out any organism more complex than a microbe (unless they can get their oxygen from a non-atmospheric source such as an abundant supply of water, but that’s outside the scope of the question). It’s possible that alien life might evolve a decent method of respiration that does not involve oxygen, but even if it did it’d be drastically, drastically different from what we’d recognise as “life”. Since we’re now dealing with microbes there is some good news, and that’s that microbes have continually surprised us with their capacity to live and thrive in hostile environments. We’ve already seen how extremophiles have adapted to hot, cold, acid, alkali and even radioactive environments, but the thing that immediately sprang to my mind when I read this question was the curious case of the Streptococcus mitis bacteria that was (allegedly) inadvertently sent to the Moon on one of the Surveyor probes.

This bears some explanation because it’s one of the more astonishing events in astrobiology, yet it gets almost no discussion outside of academic circles — prior to me looking up that article the only reason I knew about it was an offhand piece of trivia in a Bill Bryson popular science book. Surveyor 3 was sent up to the Moon to gather scientific data and scout things out for a future lunar landing back in 1967, and it spent two and a half years sitting on the lunar surface doing precisely that. Surveyor 3 was a machine probe, not a life-bearing spacecraft. It had precisely no provision for keeping any hitchhiking microbial life alive during that thirty-month stint on the Moon. So when the Apollo 12 astronauts retrieved one of the cameras from Surveyor 3 in 1969 and brought it back to Earth, NASA scientists were rather surprised to discover about 200 spores of Streptococcus mitis hidden away inside the camera lens mounting.

Now, to be fair, there’s actually two ways this could have happened. Either common Strep. bacteria managed to survive two and a half years in an environment that is completely inimical to Earth life, or else they somehow got onto the camera at some point between the Apollo 12 astronauts retrieving it from the lunar surface and the camera being checked back on Earth. This is a debate that has sadly devolved into the scientific equivalent of “NO UR A POOPYHEAD!” Modern sceptics assert that it was sloppy procedure in the NASA clean room that lead to contamination of the camera, while the actual crew that found the bacteria cling to certain abnormalities in the way the Strep. was subsequently cultured that point to the camera bacteria being in a state of dormancy when they were recovered, which they wouldn’t have been if they’d just come out of a filthy, smelly human body. Since the original camera parts have long since been contaminated by being put on display in a museum there was no possible way to repeat the tests independently and find out for sure. If it’s true, though, then it does point to bacterial life forms being able to survive for short periods in the vacuum of space.

Unfortunately that’s not quite good enough for the purposes of our question. If the Strep. really did survive up there then it did so in a state of hibernation, shutting down nearly all active biological processes and effectively becoming inert. We want something that can live in a vacuum environment – bacteria may be able to cope in a deoxygenated environment through anaerobic respiration assuming they have a sufficient quantity of nearby raw materials, but can they deal with all that vacuum implies? This is the homeostasis part of the question: we have to set the parameters of the environment and then determine what qualities our vacuum organism would need to survive in them. I’m going to use the surface of the Moon as my touchstone here; having no atmosphere it is an almost pure vacuum and so it is exposed to all that space can throw at it including sunlight, cosmic radiation and meteorites. The things we have to consider are:

Temperature. It can get pretty cold in a vacuum, with lows in deep space approaching just a few degrees above absolute zero (or -273^oC) but without an atmosphere around to filter out sunlight getting hit with the full whack of it can easily heat up the surface of the Moon to over 100^oC at the equator. If you’re on part of a rotating body that alternately flits in and out of sunlight then you end up being chilled and cooked in equal measure. Extremophiles exist which live and thrive in high temperatures, and extremophiles exist which can live and thrive in low temperatures, but I’m not aware of any extremophile which can do both at once, and certainly not over a temperature range of about 250 degrees. If we could pick and choose both attributes and give them to our vacuum organism, the next thing it would have to deal with would be…

Radiation. There’s no atmosphere to filter sunlight’s UV content out, either, so our vacuum organism has to be resistant to ionising ration to boot. This is a somewhat easier condition to deal with since radioresistant extremophiles soak the stuff up like a sunbather on a hot day at the beach with nary an ill-effect in sight.

Low pressure. Not so much of a problem for certain extremophiles; this zombie microbe in particular has no problem thriving in low pressure environments (and is radioresistant to boot).

If there is a condition that would give pause to the concept life in a vacuum, then, it would appear to be the temperature range such an organism would have to endure. Still, while complex life wouldn’t have a chance in a vacuum (these adorable little water bears notwithstanding), an extremophile-type organism that could cope with it is far more plausible. It’s also worth remembering that the high- and low- temperature extremophiles are niche organisms specifically evolved to survive in a specific temperature range – in other words, the reason they can’t deal with lower/higher temperatures is because they’ve never had to. Just because an organism that can deal with both doesn’t exist doesn’t mean it’s not possible.

To sum up, then:

• Complex lifeforms cannot survive in a vacuum because they cannot derive the required amount of energy from their local environment to support their large cell structures.

• Certain forms of microbial life would be able to survive in a vacuum in the short-term, but I have significant doubts about their ability to do it over a long-term period in which the environmental parameters of the vacuum are fluctuating widely.

• It might be possible for a hypothetical microbe with all the right qualities to survive indefinitely within a vacuum. It’d certainly be foolish to rule it out just because we haven’t seen it yet; fifty years ago scientists thought the extremophile life was impossible, but today we know there are hundreds of the bastards.

Honestly though, those tardigrades scare the willies out of me and if anything is going to be able to survive long-term – if not thrive — in a vacuum environment it’s going to be them.

 ——————————————————

1. I’m not ruling out the possibility of life somehow hitching a ride inside a comet or asteroid as these would provide some raw materials that could be metabolised and which would keep the organisms alive for a time.
2. This is a joke, in case you’re wondering. I’m not actually going to have my sole source be a GSCE Bitesize revision page for all that it’s probably broadly correct. No, I also used the TalkTalk web… encyclopedia… hang on a minute. Seriously though, the web doesn’t appear to have a single good explanation of anaerobic respiration for idiots (i.e. me) and if any biologists are reading you should probably get right on that. Meanwhile I did ask a biologist friend if I understood it correctly and he said I was in the right general area, so that’s going to have to do.

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Topical science time! This recent news article caught my eye the other day. Russian scientists have succeeded in drilling through the 3.7 km thick layer of ice covering Lake Vostok in Antartica and hope to be able to extract samples from the lake later this year. Lake Vostok has been completely sealed off from the surface for about twenty million years. This makes it a very interesting place to look for life forms, and what they’re doing in Lake Vostok right now is not a million miles away from the way we’ll likely find the first alien life forms.

The environment inside Lake Vostok is, to put it mildly, just a little bit exotic, and not the first place you’d expect to find bacteria. The ice sheet covering it is so thick that we didn’t even know for sure that there was water down there until aircraft- and space-based radar picked it up in the early 90s. Because it’s got a 3.7 km thick ice layer sitting on top of it, that water is under a tremendous amount of pressure – enough so that it remains liquid¹ despite having an average temperature of -3^oC. This high pressure also results in that water being supersaturated with high concentrations of oxygen and nitrogen than you’d find in an equivalent surface lake, much of which has been forced into “cages” of water molecules called clathrates. There is no light at the bottom of Lake Vostok, but that’s okay because there’s no light at the bottom of the ocean either and we find plenty of stuff living – in fact, thriving – down there, most of which tends to cluster around geothermal vents on the ocean floor – and we suspect the same might be true of Lake Vostok.

Not that you particularly need a geothermal vent to support life these days. Hopefully you’ll have heard of these guys – the extremophiles. Over the last few decades we’ve kept finding them in locations which were thought to be completely inhospitable to life – too hot, too cold, too acid, too alkali, too saline, even too radioactive. In all of these places extremophiles thrive, and some of them even need those harsh conditions in order to survive; for example the halophile loves salt and can’t grow without at least a reasonably high concentration of it present in its surrounding environment.

This has completely revolutionised our view of the conditions required for life to exist, and it’s entirely possible that at the bottom of Lake Vostok there will be a colony of extremophiles that has been evolving entirely on its own for the last 15-20 million years, and which has adapted to exist in the freezing, oxygen-rich environment down there. If they do exist, then those extremophiles will be completely unlike anything we’ve so far observed; bear in mind that twenty million years ago the ancestors of the human race looked something like this and then consider what those twenty million years of divergent evolution might do to bacteria. If they’re down there, and the Russians do find them, then it’ll be as close as we can get to finding alien life without actually going to another planet to do it.

What does this mean for the chances of alien life existing? Well, it improves them considerably, at least in terms of micro-organisms — you might not be able to have a conversation with one but you can at least be comforted that somewhere out there in the Milky Way is the alien equivalent of a tardigrade. However, it also improves the odds of us, personally (or at least a probe controlled by us) going out into space and actually digging some of these things up. You see, Lake Vostok isn’t the only place in the Solar System where there’s a large quantity of water trapped under several kilometres of ice. Several of the moons of the gas giants also qualify; notably Europa, Ganymede² and Callisto.

Europa is the really interesting one. The surface of Europa is entirely composed of ice, making it by far the smoothest body in the Solar System, but we think that ice surface is only 10-30 km thick. Beneath that is a layer of conducting fluid – overwhelmingly likely to be saltwater – that was detected by the Galileo probe via its magnetic moment. This water is kept relatively warm by tidal heating³ provided by Jupiter, and it’s possible, although nobody knows exactly how possible, that there might be microbial life down there living in environments similar to the ones we find extremophiles inhabiting on Earth – in cold, dark environments like the bottom of Lake Vostok, or next to deep sea thermal vents that spit out heat from the moon’s interior (it does have radioactive decay heating, although this is deemed insufficient to create the subsurface oceans). Missions to visit Europa to try and find some of this life have been mooted for years, although I don’t believe any of the proposals have satisfactorily tackled the question of how exactly you drill through ten kilometres of ice on another planet with a robot probe when it’s taken the team at Lake Vostok decades to bore through four kilometres with the benefit of having humans on hand to operate the equipment. Regardless, it’s likely that it’ll be tried sooner or later depending on what future technology does or does not render possible, and so if you’re unlucky enough to be alive seventy or eight years from now you might end up having the news of the first discovery of alien life piped into your brain through your neuro-cortical machine interface.

The other possibilities are somewhat less interesting. Ganymede has an ice-silicate lithosphere over 200 km thick covering its ocean, so it’s doubtful we’ll ever be able to tunnel down there. Callisto too has a surface layer up to 150 km thick. Enceladus on the other hand is kind of promising; if you’ve watched that EXPLETIVE DELETED Brian Cox on his EXPLETIVE DELETED EXPLETIVE DELETED series Wonders of the Solar System, you’ll be aware that Enceladus has a surface which, like Europa, is composed entirely of ice, but that Enceladus is unique in that the Cassini probe observed water vapour outgassing from the surface in 2008. This points to the presence of liquid water in some form or other, the most likely source of which is yet another subsurface ocean – and since it’s outgassing directly from the surface, it points to this ocean being somewhat more accessible than that of Europa (after all, it needs some way to get there from the interior). At the very least it should be possible to collect material that has come directly from the ocean and determine its composition, and thus its likelihood of being able to support life. While Enceladus isn’t necessarily a better bet for finding life than Europa, it’s potentially far more useful for telling us what the odds of that bet actually are.

The first step, though, is to look at these subsurface water environments on Earth. The Lake Vostok team has been painstakingly drilling through the ice for years while constantly at the mercy of the Antarctic elements (Lake Vostok is the coldest place on Earth). They used a thermal sensor to detect when they were approaching a source of free water and stopped the drill, the general idea being that the decrease in pressure would allow the ice separating the borehole and the lake to melt and then refreeze; they can then retrieve this portion of ice to examine water that has come directly from the lake. Meanwhile they’ve examined the last piece of ice extracted from the borehole – which contains ice thought to have frozen onto the bottom of the ice sheet hundreds of thousands of years ago – and they’ve managed to turn up evidence of extremophile microbes, so the odds are good that there’s a colony of something down there. The team has plans to go even further by sending a robot down in late 2013 to retrieve samples from the sediment making up the lake bed. I suggest you pay attention to the results, if only to make sure that this hasn’t happened.

1. If you think back to the geophysics post you’ll recall that the normal rules of physics don’t really apply when you crush something underneath hundreds of millions of tons of pressure. Rock cannot melt despite being at a high temperature because melting would require it to have some room into which it can expand – and because it’s compressed so much by the high pressure, this is the one thing it doesn’t have. Because water is unusual in that it actually expands in volume upon freezing into ice, the same rule applies to Lake Vostok.

2. CORRECTION: I stated in the geophysics post that Earth is the only body in the Solar System known to have a liquid iron core. While this is broadly correct, it is also suspected that Ganymede, Venus and Mercury might have them as well; Ganymede is very uncertain, Venus is likely based on its similar size to Earth (with the absence of plate tectonics being explained by there being no water on the surface of Venus to soften up the crust enough to allow plate formation/subduction) and Mercury has a strong magnetic field relative to its size which also points to the existence of a partially liquid core. Tectonic behaviour is observed on many of the icy moons of the gas giants, but their tectonics all involve chunks of ice shifting around and not rock. Earth is still a unique case in terms of the strong magnetosphere and plate tectonics.

3. Ever seen a picture of Io? I don’t particularly recommend it as a holiday destination because it’s absolutely covered in volcanoes. In terms of raw geological activity it puts the Earth in the shade, but unlike the Earth this is entirely down to outside forces: tidal heating caused by a constant tug of war on Io between Jupiter and the other Galilean moons which constantly squeezes and stresses it like… well, like somebody constantly squeezing and stressing a stress ball. Io has to relive this internal pressure somehow, and it does it by forming volcanoes which constantly spew out poisonous sulphur gas, which has stained the entire planetary surface a sickly yellow colour.

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 I apologise in advance if this post doesn’t contain much science. As we’ve seen astronomy is mostly based around an astounding amount of ingenuity and bloody-mindedness backed up by not a small quantity of guesswork, but when it comes to dealing with questions like the Fermi Paradox we don’t even have enough information to make an educated guess. As a result things can start to get a little metaphysical, almost bordering on sci-fi at times.

As mentioned in the exoplanets post, one of the reasons we’re so interested in finding Earth-like planets around other stars is so that we’ll have more accurate terms to plug into the Drake Equation. Right now the number of intelligent civilizations we know of stands at one: us. Basing our assumptions of how likely intelligent civilizations are likely to evolve on other planets on this one example is a very bad idea, for the same reason that concluding all sheep in Scotland are black on the basis of seeing one black sheep in a field in Scotland would be really, really stupid. Unfortunately at the moment we just don’t have any other data; until we find more Earth-like planets we can’t even begin to constrain the probabilities down to something that’s not a complete shot in the dark.

But I’m getting ahead of myself. The Drake Equation is a semi-famous equation thought up by Frank Drake (duh) to describe how often intelligent civilizations is likely to evolve. I particularly like it because it doesn’t actually involve maths of any kind, instead being a shorthand way of describing the various factors we think would contribute to the evolution (or not) of sentient lifeforms. The equation runs as follows:

where

N = the number of civilizations currently existing in our galaxy that we might be able to talk to.

R* = the average rate of star formation in our galaxy.

f_p = the fraction of those stars which go on to form planets.

n_e = the fraction of those planets which are Earth-like planets capable of supporting life.

f_l = the fraction of those Earth-like planets which go on to evolve life of some kind.

f_i = the fraction of lifeforms which evolve into intelligent life of some kind.

f_c = the fraction of intelligent lifeforms which develop the technology to start beaming communications signals out into space.

L = the average lifetime of a communicating civilization; i.e. how long they send out those signals for.

The reason we use the rate of star formation is because that’s easily multiplied by the lifetime to find the number of intelligent communicating civilizations currently existing. Say you have ten new stars being born each year. A certain fraction of those stars will have planets, of which a certain fraction be will Earth-like, of which a certain fraction will evolve life, of which a certain fraction will be intelligent life, of which a certain fraction will end up communicating. Multiply all terms except L together and you get the number of communicating civilizations coming into existence each year – this will be much lower than ten, say 0.007 civilizations per year. If these civilizations stick around for an average lifetime L of 1000 years each, then we can say there are 7 intelligent civilizations  capable of communication existing in the Milky Way at any one time.

Now, of these terms we know roughly what R* is, and thanks to the exoplanet search we should eventually arrive at educated guesses for f_p and n_e. We have absolutely no idea what the other four terms should be. Even if we could use the Earth as an example it wouldn’t be a very good one, since on the one side you have the astronomers who argue that the assumption that Earth-like conditions are necessary for life is tainted by anthropic bias, while another side argues that there’s nothing particularly special about the Earth, while on yet another you have those who argue that the chances of another Earth existing are so incredibly remote that it’s not even worth considering as a possibility¹. All the Earth tells us is that the final answer to the Drake Equation is greater than or equal to one.

As I said, though, the Drake Equation isn’t a real equation. Instead it’s more of a description, a way of formulating the argument and a talking point for astronomers and astrobiologists to argue over endlessly. It is, however, the most scientific treatment of the intelligent life problem that exists, and provides a framework for me to talk about some of the theories as to why we haven’t been contacted by any little green men just yet.

Some numbers. The universe is 13.7 billion years old. Stars began to form a few hundred thousand years after the Big Bang; second generation stars with the heavy elements necessary for rocky terrestrial planets would have started to form a from a few hundred million up to a billion years after that. A decent guesstimate would be that the galaxy has been capable of supporting the evolution of life for about 10 billion years.

This presents us with a bit of a dichotomy when we look at the Earth. The Sun formed about 4.7 billion years ago, with the Earth coalescing together out of the leftovers 4 billion years ago and the first evolution of microbial life taking place (we think) about a billion years after that. It would be foolish in the extreme to assume that we are the first intelligent civilization to evolve in our galaxy; the Milky Way consists of about 200 billion stars, any one of which could have produced an intelligent alien civilization with a head-start of up to five billion years on us². After five billion years of evolution the galaxy should be teeming with intelligent civilizations. Back in 1950 Enrico Fermi was having lunch with some colleagues at Los Alamos when he asked the following question: if the galaxy is so jam-packed with intelligent life, then where are they?

Fermi was a very intelligent man, and so this isn’t anywhere near as naïve a question as it sounds. Everyone knows that faster-than-light travel is an impossibility given those pesky laws of physics, so a visit by an alien race in person is pretty much out of the question. Even communication via radio signals/lasers/whatever is unlikely if you assume a fairly low population density, since any signal crossing the thousands of light years separating civilizations would be preposterously faint once it reached us. None of this matters, though, because of a little concept called the von Neumann probe.

Assume there is an alien civilization that really wants to explore the galaxy and contact its neighbours. Assume that they have a level of technology that is just a few hundred years in advance of our own, and assume that they’re not particularly bothered about waiting around for a few million years to find out the result. If all these things are true then it would be perfectly possible to explore the galaxy using a von Neumann probe. This is an entirely autonomous, robotic, self-replicating space probe that is sent towards the nearest star at sub-light speeds. It takes several hundred years to get there. When it arrives, the first thing it does is look around for a handy source of raw material – say an asteroid, or even a planet – with which it can build two or more new von Neumann probes. These new von Neumann probes then set out for other close-by star systems, and when they get there they repeat the exact same process; one probe becomes two, two probes become four, four probes become eight and so on. This results in a wave of von-Neumann probes spreading throughout the galaxy exponentially so even though they’re doing it very slowly they can explore the entire thing in the time it takes for one probe to cross the Milky Way from end to end, which is estimated as being not more than a few hundred million years.

This makes interstellar exploration very very easy for a civilization in possession of the required level of technology; the only effort required is that to send out just one probe (maybe two or three for redundancy since the first probe is the most vulnerable point in the von Neumann chain) and then sit back and await the results. The von Neumann concept therefore presents us with an unpleasant paradox because it essentially makes the result of the Drake Equation independent of L, the lifetime of the communicating civilization. Assuming the civilization lasts long enough to launch the probe in the first place then it wouldn’t matter if their planet got hit by an asteroid or their sun went nova or they blew themselves up with nuclear weapons; the von Neumann probes are completely autonomous and will carry on exploring the galaxy long after the civilization that launched them has died out. So given the assumptions that other intelligent civilizations would have evolved hundreds of millions to billions of years before the human race, that some of these civilizations would have had a psychology that both led them to want to explore the galaxy and communicate with other civilizations and which let them come up with the idea of the von Neumann probe, and that at least one of them survived long enough to launch it, then there should, somewhere in our Solar System, be an ancient von Neumann probe broadcasting the galactic equivalent of “Hi! How are you?” That there is not means either that intelligent life is much rarer than we suppose, or that there is some common factor that prevents a civilization from launching a von Neumann probe in the first place.

Some of the latter explanations are more than a little fanciful. The most plausible is something called the Great Filter; this holds that there is some unknown factor in the Drake Equation with a very low value that stops the vast majority of intelligent life from progressing further down the chain. In the case of human civilization we are either one of the very few civilizations to get over this hump (unlikely given the Copernican principle) or else we have yet to encounter it, which is a somewhat depressing thought. Perhaps civilizations exhaust their resources before they get to the point of being able to launch a probe. Perhaps they blow themselves up. Perhaps they get hit by a giant space rock. It could be anything with the capacity to take out an entire planet’s worth of sentient life in one fell swoop.

Then you have theories advanced by people who have been watching too much sci-fi, most popular of which is the Zoo Hypothesis. This holds that there is a community of advanced extraterrestrial civilizations out here, but that they’re governed by something similar to Star Trek’s Prime Directive and that they not allowed to interfere or communicate with less-advanced lifeforms. I find this somewhat unlikely to say the least – where are the Klingons? Or the Borg? Even if this were true we’d still have seen some sign of alien life, albeit probably in the form of a vast Independence Day-style invasion fleet that wants to strip our planet of everything valuable. Other explanations involve the alien civilizations experiencing a technological singularity and undergoing a process similar to Sublimation in Iain M. Banks’ Culture novels whereby they simply no longer give a shit about mundane physical things, or else that we have been contacted by extraterrestrial life and that it’s being hushed up by national governments.

You can find all these crackpot theories and more on Wikipedia’s Fermi Paradox page, but at the end of the day the question of the existence of extraterrestrial civilizations is, while interesting, ultimately an excuse for scientists (and everyone else) to avoid doing any science by blowing a lot of hot air. All that really matters is that we haven’t yet found anyone else out there, and that this state of affairs is likely to continue for the foreseeable future.

1. I read the Rare Earth book as part of the research for my third year undergraduate dissertation. I didn’t find it particularly convincing; when the assumption that the Copernican theory holds true forms one of the cornerstones of modern cosmology you can’t just throw it out the window the way these guys did.
2. I am pretty much pulling some of these numbers out of my ass, but they’re so big that what the real answer is doesn’t really matter: the point is that the Earth has gotten around to forming intelligent life reasonably late in the grand scheme of things and that there must have been intelligent civilizations that came before us.

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