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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Here’s an interesting fact: if you smooshed together everything in the Solar System that was not a star — all the planets, moons, asteroids and other assorted junk out there — you would end up with a ball of stuff with just one seven-hundredth the mass of the Sun. The Sun has 99.86% of the overall mass of the Solar System.

Here’s another interesting fact: if you took a pea and put it next to a basketball, you would achieve roughly the same visual effect as if you took the largest planet, Jupiter, and put it next to the Sun.

In the grand scheme of things — certainly from the perspective of any astronomer who doesn’t make a living studying Solar System science — planets are insignificant specks. They’re tiny. Yet we keep finding more and more of the bloody things every year. The first extrasolar planet orbiting another star (or exoplanet, as they will be referred to from here on) was discovered in 1992. When I was learning about this stuff during my undergraduate degree in 2003, we’d bumped the discovered number up to just over a hundred. Today, if Wikipedia is to be believed, the number of known exoplanets stands at 725. Seven-two-five, despite the vast difficulties inherent in finding these tiny dust motes in a galaxy of comparative colossi. When you think about it you might wonder how the hell we’d found even one exoplanet, let alone seven hundred. Good thing you have me here to tell you all about it.

 Any astronomer who wants to discover a new exoplanet and name it after his cat has several hurdles to overcome; not least of which is that the IAU are killjoys who won’t tolerate a planet called Mrs Fluffingham even if it is several dozen light years away. Other stars appear to us as tiny points of light. If you used a really powerful telescope to beef up the magnification several thousand times this tiny point of light would resolve itself into… a tiny point of light. Interstellar distances are so vast that stars will always appear as points of light to us; we’re never going to be able to resolve them as objects and observe fine detail as long as we’re stuck here on Earth. That we see them at all is merely a happy byproduct of every star being a raging nuclear inferno that throws out billions of photons every second. A very few of these photons, after spending many, many years whizzing through space, will happen to enter the objective lens of a telescope – or even your naked eye – which is enough for you to perceive a tiny point of light, but nowhere near enough to make out anything more.

So if this is the case for stars, which are a) really sodding big and b) natural light sources, how on Earth (har) do we find planets? It’s certainly not through the conventional methods used to locate the other seven in our Solar System, which helpfully reflect the light emitted by the Sun and appear to us as distinct discs. Extrasolar planets have about one-millionth the brightness of their parent star, and finding this preposterously faint wisp of light is further complicated by the fact that it’s sitting right next to a light source one million times brighter – it tends to get lost in the glare of the star. That’s not to say it can’t be done; in some cases we can “block” the light from the star (basically phase-shifting the image so that the star is subtracted from it while leaving everything else intact) allowing us to directly view the planet, but this is really goddamn hard and a major technical challenge, not to mention only being possible if you’re looking for a planet that’s really big, emitting a lot of infra-red radiation and orbiting far enough out from its parent star that it can be viewed as a separate object.

 Directly viewing an extrasolar planet is pretty much out of the question, then. Fortunately astronomers are very ingenious people who aren’t going to let a little thing like not being able to actually look at the planet get them down, and so several ingenious methods have been developed for indirectly detecting an extrasolar planet over the last twenty years. I’ll be talking about the three most successful ones.

 The first is radial velocity measurements. Stars exert gravitational force on planets which causes the planet to orbit the star, but what people often forget is that all gravitational effects are mutual and that the planet is also exerting a gravitational force on the star. This means the centre of mass of the star-planet system won’t be at the centre of the star but will instead be offset from it by a small distance, causing the star to orbit around this point and move back and forth relative to the Earth. This movement Doppler-shifts the light emitted from the star in exactly the same way as stellar redshifts; when the star is moving towards us the wavelength of the light will be shorter, and when it is moving away from us the wavelength will be longer. By looking at the wavelength of a star’s light as it changes over time we can measure the star’s radial velocity, and so determine whether or not the star possesses an orbiting planet. It’s a bit imprecise for determining what the exoplanet is like – since a smaller planet orbiting close in will produce the same radial velocity as a bigger planet orbiting further out – but it’s an excellent method of finding out whether or not it’s there in the first place, with the vast majority of exoplanet detections being attributable to the radial velocity method.

Then there’s the transit method. If a planet is orbiting so that it passes directly between the Earth and the star, the planet will block out some of the star’s light. Astronomers study what is called the star’s light curve; essentially a big graph of the brightness of the star as it varies over time. If they see a small temporary dip in the light curve, and that dip repeats itself at periodic intervals, it’s a fairly good bet that’s it’s being caused by an exoplanet. The drawback to this method is that it’s only applicable in very specific situations; you need a big planet orbiting close in to the star so that it’ll block out a lot of light, and you also need the exoplanet to have this very specific orbit which places it on a direct line between the Earth and the parent star, which is thought to be the case for only 10% of exoplanets. However, there is also an advantage in that we can use spectroscopy to examine how the starlight changes as it passes through the planet’s atmosphere – whenever you see a headline about a newly discovered exoplanet with a vaguely Earth-like atmosphere, chances are it was discovered using the transit method.

 Lastly there’s gravitational microlensing. This isn’t that great in comparison to transits and radial velocity measurements, but it’s notched up thirteen detections so it does deserve a mention. If light emitted by a star passes close to another star on its way to the Earth, the gravitational field of the second star will “bend” the light from the first star causing a lensing effect which effectively magnifies the light. Any exoplanet orbiting the lensing star will contribute its own gravity to the lensing effect causing it to change over time as the planet moves around the star. If astronomers are vigilant and lucky – and they have to be considering that stars and planets are constantly moving relative to each other making these events very hard to catch – they can observe this change in the lensing star’s magnification and infer the existence of the planet.

 Most the exoplanets so far discovered have been of the “hot Jupiter” type; very large gas giants orbiting their parent stars at about the same distance as Mercury orbits the Sun. This isn’t because the majority of exoplanets are genuinely of this type, but instead reflects a bias in the detection methods: all three methods described above are heavily dependent on the exoplanet in question having a large mass, and two of them are also more effective if the exoplanet is orbiting close to its parent star. While the simple existence of the hot Jupiters caused scientists to reassess the way they thought planets formed and evolved over time (as ever, I may write more on that later), a major goal of the exoplanet search is to refine the detection methods so that they’re sensitive enough to reliably detect Earth-like planets orbiting at Earth-like distances, for the obvious reason that these are the places most likely to support life of some kind. This made the detection of Kepler 22-b a big step forward – it was small enough that it was unlikely to be a gas giant, and it was orbiting in what is called the habitable zone¹ of Kepler 22 thus qualifying it as the first confirmed exoplanet discovery in this category.

 But why are we so interested in finding Earth-like planets? It’s not like we can ever actually visit these places in any meaningful way² (at least not without some physics-breaking discovery that allows FTL travel) so why the obsession with finding habitable worlds? Well, it’s mainly to put some kind of constraint on how many of those worlds are out there. There’s something called the Drake equation which describes all the factors that go into an intelligent civilization arising on a certain planet — does the star have planets, are those planets in the habitable zone, do those planets go on to develop life etc. etc. – but it hits the tiny snag that we have absolutely no idea what values we should assign to these factors because we’re currently working with exactly one example of a habitable planet that supports life: the Earth. With more examples of habitable worlds we can start to assign some vaguely meaningful values to the first few terms of the equation and start narrowing down exactly what the chances are of the Milky Way hosting extraterrestrial life. As this is probably the only way we’ll ever figure that out, it’s very much worth doing.

1. Exactly what constitutes a habitable zone is a matter of some debate. The straight definition is the zone where temperature and pressure conditions are such that liquid water can exist on a planet’s surface, and so this varies with the temperature of the star; hot stars have habitable zones which are further out, while smaller, dimmer stars would have habitable zones much closer in. However, astrobiologists call into question the prequisite that liquid water has to exist on the surface of a planet as necessary for life to exist, citing the example of Europa as a planet-sized object with a subsurface ocean of liquid water that may yet turn out to support microbial life of some kind. And of course this isn’t even getting into the “life, but not as we know it” argument.
2. By “meaningful” I mean “Seeing a probe we launched arrive at another star within our lifetimes”. If we were willing to wait a while – say, a couple of hundred years – then interstellar travel would be easily doable the old-fashioned way. This gives rise to all sorts of wacky concepts like O’Neill cylinders, generation ships and von Neumann probes, the last of which actually come into play as part of the argument for why intelligent life likely doesn’t exist anywhere else in the galaxy.

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