The Fermi paradox
refers to a train of reasoning along the lines
of:
There are various approaches to answering this, of course. There's a range of depressing possibilities such as that species capable of space exploration are far too likely to wipe themselves out before they do so, whether by inventing nukes and using them, by polluting their home-world so badly they die of it or by some other doomesday mechanism, probably involving making very bad decisions about how to govern ourselves. Er, sorry, I mean themselves (of course). Or the practicality of interstellar travel, in the eminently plausible case that there are no short-cuts or faster-than-light ways of travelling, may just discourage anyone from trying, for example after the first few attempts to do so have failed miserably. In fact, most explanations are depressing in one way or another. So let's start with some of the less depressing options and I'll probably stop before I get to the real downers.
One possibility is that the aliens are out there, they've noticed our little
blue dot and recognised that it might produce an addition to the galactic
community, but they have A Rule similar to the Prime Directive of The Federation
in the fictional Star Trek
universe. They know we're here but they're
not meddling in our affairs, even though doing so might be the only way to save
us from doing some of the stupid things I just mentioned that would select us
out of the game. After all, anyone stupid enough to make those mistakes is
probably not going to make a good neighbour. So the depressing part of this one
is that we're not going to get any help from the aliens – we'll have to
make it to the stars for ourselves – and they tacitly suspect we're
idiots. It's hard to argue with the justice of those positions.
That still leaves our failure to detect aliens unexplained, but if they don't try to make contact that's not entirely surprising. We used to think aliens would be obvious, because of things like building Dyson spheres around stars, but it's entirely possible that, once you've invented a good fusion reactor, burning hydrogen in it is more efficient than gathering energy from a star. After all, stars do fusion the dumbest and crudest way possible and, relative to the amount of fuel they have available, spectacularly slowly. We also used to expect migrants to settle planets, but that's another spectacularly inefficient way to do things, compared to building cylinder ships. Why fight the (biochemical) inertia of an entire planet when you don't need to ? Back when we communicated with one another via radio broadcasts, we expected aliens would be doing the same, so tried listening in on theirs, but we've now mostly given that up in favour of point-to-point communication; and it's surely both more secure and more efficient to point a laser (or maser, or the variant on that theme for whatever frequency band you're using) at the intended recipient of a message than to just blast it out in all directions. Swarms of cylinder ships, built out of asteroid belts and the rubble between stars, powered by efficient fusion engines, communicating with one another via accurately directed lasers, would be quite easy to not spot.
A close relative of this explanation is one where the aliens are doing all of the above and don't need a Prime Directive because everyone has already learned (some of them The Hard Way) that contact with life from a different origin planet to your own is generally a terribly bad idea. Some stray microbe from either biosphere is far too likely to wreak havoc on the larger life-forms from the other. So once you've noticed there's life in another solar system, you keep away from it as a straightforward form of quarantine. Even its asteroid belt's rock may be dangerous, since meteors that hit the biosphere's home planet do occasionally kick a lump of infected rock out of its gravitational influence and you never know where that might end up within its solar system. Cosmic rays and unfiltered sunshine are quite good at sterilising anything that makes it out into space, but some extremophiles don't seem to care so much about that. There's plenty of rock that isn't anywhere near a biosphere, so why take the risk ?
This scenario has one significant upside, whose lack in the next may be considered a potential downside, which is that the aliens have already worked out how to share the galaxy peacefully. If we can manage to join the club, just playing along with their solution to that should be a good way to avoid the potentially dreadful consequences of conflict between civilisations from different solar systems. Admitedly, such conflict is somewhat unlikely, at least as long as there are no short-cuts to interstellar travel, just because that would make interstellar war absurdly impractical.
Another possibility is that we really are either the first species to get this far in our galaxy or, at least, close enough to first that the others aren't far enough ahead of us to have showed up yet. That, however, would have to mean that something about our world is special, that gave us a head-start on most other worlds, or that made it possible for us to arise at all. Scientists are naturally a bit wary of that sort of hypothesis, given that people are rather prone to exceptionalism that later turns out to be vanity: we're not at the centre of the solar system, after all; our sun is a relatively mediocre star; the solar system is just one of billions of similar systems in our galaxy; our galaxy is a fairly mediocre example of how galaxies can be, in a somewhat backwater galactic group; and so on. None the less, we can look at what might actually be exceptional about our solar system and/or our home planet.
That is, of course, hampered by how little we know about planets in other solar systems: until quite recently, after all, our belief that other stars do have planets was based on the assumption that our solar system surely isn't all that special and our best model of how stars form does imply that they'll typically have some planets. Within the space of three decades we've gone from that level of ignorance to knowing about a respectably large number of planets orbiting other stars and having actually measured spectra of the atmospheres of some of them; but, even so, there's a great deal we don't know. That makes it a bit hard to be confident that anything we think might be special about our home actually is, but we can at least look at what might plausibly be special.
Oh, and the downside of this story is that, of course, it may mean we're all alone. Which could equally be read as saying we aren't going to have to compete with other space-faring folk for the galaxy's resources; but then, there's enough galaxy out there that having to share wouldn't really be a big problem, and having someone to discus things with, who thinks very differently from us, would probably be good for us. But in any case this scenario does include the case of us merely being one of the first, rather than the only ones in the game. So, all things considered, not such a big downside really.
One thing we do know is that our solar system is uncommonly rich in the element phosphorous, 15P. We can look at the spectra of stars to discover what elements are present, and how abundant they are, in the outer atmospheres of those stars. Comparing the results to those for the Sun we see that our home star has an exceptionally high abundance of phosphorous, implying that the cloud of gas and dust from which our solar system formed was likewise and thus that the abundance of phosphorous on the planets of other stars is significantly lower than on Earth. Given that phosphorous shows up in many of the molecules that are important to life on Earth, it may well be a prerequisite of some step along the way to creatures like us, or at least help to get there faster.
That step needn't be the first: even without phosporous, it's entirely plausible that one form of life or another could arise on planets of other stars; it'd just have very different biochemistry. So while that first step is one possibility, it's just the first. We still don't really understand how eukaryotic cells invented sexual reproduction, for example; maybe that depended, in some way, on the sugar-phosphate bonds that form the outer backbone of each of the paired spirals of DNA.
Of course, the planets of other stars surely do have some phosphorous, just not as much as life on Earth had available to it, so – quite apart from the possibility of some form of life that doesn't need phosphorous at all – it'd be possible for the molecules responsible for life on Earth to form and play a part; it's just that life on planets of other stars would likely have tended to make more use of the chemical properties of molecules that don't involve phosphorous. So life could surely arise on planets of other stars, and it's a bit hard to see why it shouldn't have been just as capable of leading to creatures as capable of space-faring as we are. But maybe it would have taken longer, with phosphorous-based chemistry providing us with the head start that puts us among the early entrants in the galactic space race.
In case you're wondering why phosphorous is more common in our solar system than most others, that's down to nucleosynthesis, i.e. how stars build the various elements, starting from hydrogen and helium. Some of the more abundant elements – such as carbon, oxygen, silicon and iron – are made by all stars big enough to eventually blow up and supply the the rest of the universe with their precious star-dust. Others are only made (or, at least, released into the universe at large) by stars in particular size ranges. In the case of phosphorous, it only gets made by supernovae (really big stars going boom, at the end of their short lives) of stars that formed from gas-and-dust clouds that contained some of the other elements – I forget the details, but this is why it's generally rare; and, in particular, why the universe takes a while to get to the point where it can happen. So while abundances of many elements, particularly the lighter ones, tend to be relatively similar from one solar system to another, the abundance of phosphorous is relatively variable. Apparently the gas-and-dust cloud from which our solar system formed had been seeded by such a supernova, making it unusually rich in this rare element.
Another possibility is that the Earth's relatively thin crust enabled more tectonic activity than you'd normally get on a planet with enough of its other properties to favour life. The rocks laid down by life thus get recycled, via the upper mantle and vulcanism – instead of just sitting at the bottom of the ocean and never going anywhere, thereby soaking up valuable chemistry that life needs to function. That greater tectonic activity has also subjected the life that evolved on Earth to a whole lot of dramatic changes of environment, that may have provided important evolutionary pressures towards adaptability, that may be important to the emergence of anyone capable of even considering the possibility of space travel. So while no-one likes an earthquake, and volcanic eruptions are generally disastrous for those who live nearby, the Earth's relatively vigorous tectonic activity may well be a significant contributory factor in how it came to be the home of creatures as crazy as us.
As for why Earth's crust is thinner than usual for bodies roughly similar to it in other respects, I'll have to explain how the Earth formed, which in turn needs an explanation of how solar systems form.
The way a solar system forms is that a cloud of dust and gas swirls around under mutual gravitation to form a disc within which haphazard local density variations clump together; and mergers of clumps form larger clumps. Of course, there are also collisions that break clumps up, but even those contribute to warming up the swirling disc, as do the clumping collisions, so that some of the substances that make it up – such as water; or, if things get really hot, silica – melt and make the clumps of which they're a part sticky, which makes mergers more common. Eventually some clumps get big enough to gravitationally dominate their neighbourhood, so that nearby dust and gas orbits them and, because collisions within such a disc are lossy, tend to fall onto the big clump. A clump can eventually get so big that it'll even hold onto Hydrogen, at which point it's on its way to becoming either a gas giant or a star. The process tends to make the big get bigger faster, enabling (at least if the initial gas cloud was big enough) at least one clump to get so big its self-gravitation compresses and heats its interior enough to start fusion, making it a star. Once that happens, the light and wind from the star pushes most of the lighter stuff between clumps away, and can even strip the lighter stuff from the surface of some of the clumps nearer to the star. That puts a stop to the other clumps' ability to grow, which tends to leave one relatively big clump containing most of the mass in each of various ranges of distance from the star (or stars, if more than one clump gets that big – but I'm mostly ignoring that possibility because it's not what happened in our solar system, even though it seems to be fairly common). Such locally-dominant clumps, that aren't stars, get to be called planets.
So far, so generic. As the star ignites, the planets are probably quite hot messes, thanks to all the energy from the collisions that formed them; and being hot enough to make much of what they are at least a bit runny means that the heavier stuff can sink to the middle while the lighter stuff floats to the outside, where it gets to cool while serving as thermal insulation for the stuff further in. In our solar system, Jupiter started out migrating inwards, crowding the inner planets towards the sun as it did so, until Saturn's similar inward migration got close enough to it to pull it back out and stabilise both their orbits. Between what are now Mars and Venus, there were two bodies whose orbits weren't far enough apart, at least during the Jovian squeeze period, so they ended up crashing into one another. (Aside: accounts I've seen of this tend to describe one of those bodies as Earth and say that it got hit by another body, roughly the size of Mars. I prefer to just say there were two bodies and reserve the name Earth for what resulted from their collision.)
So the thing that's unusual about Earth, here, is that it results from a collision relatively late in the process of the solar system forming. That meant the two precursor bodies had time to separate out, after the churning caused by the formation process's perpetual bombardment, into a dense iron-rich core, surrounded by a mantle and coated in a lighter crust. The late collision thus mostly caused the outer crust to splatter off, while leaving the denser bits nearer the middle mostly in place to merge and form the resulting (Hadean) Earth. Of course, some of the inner parts also escaped, but the lost matter was disproportionately of lighter stuff than denser stuff. Which is why Earth is the densest planet in the solar system.
Some of the lost crust (and denser stuff from further in) surely escaped entirely, but most of it ended up in a cloud around the roiling mass of hot lava that would eventually cool to become Earth. Some of that cloud fell back down, but most of it did its own clumping and ended up forming the Moon. Which is why the Moon is one of the solar system's less dense rocky bodies. Its presence has helped keep the Earth's spin relatively stable, and the tides it produces may well have helped life get over some of its early thresholds, so it deserves at least some credit for making the Earth special, quite apart from the fact that the process that created it has left the Earth with less crust, and thus more tectonic activity, than is probably typical for otherwise similar planets.
One other effect of that late merger is that Earth's core (the dense bit in the middle, with a lot of iron in it) is probably relatively big for a planet of its size: and that core is responsible for Earth's magnetic field, which is thus probably relatively strong by the standards of such bodies. That magnetic field has the important consequence of intercepting the solar wind, deflecting much of it and routing the rest to near the poles, where it forms the aurorae. This significantly reduces the extent to which the outer atmosphere gets stripped away by radiation, of various kinds, from the Sun.
Admittedly, that's not necessarily decisive: although Mars's thin atmosphere almost certainly has arisen from a thicker one being stripped by solar radiation, despite being further from the Sun than Earth is, Venus is the opposite example. It actually has a thicker atmosphere than Earth (indeed, a hellishly hot and high pressure atmosphere, containing some terrifyingly corrosive ingredients), despite having a much weaker magnetic field and being closer to the Sun. It is more massive than Mars, though less massive than Earth, and there are surely other factors in play, but Earth's stronger magnetic field clearly isn't the whole story of how it kept its atmosphere, for all that we're fairly sure it's helped.
That magnetic field is also something life has exploited, with various creatures known to respond to it and use it to help them navigate, but I doubt it's a major player in how it comes about that life got so peculiarly strange as to produce humans.
Although there surely are similar-sized planets of other stars, I think it's fair to say that Earth's size is just right. It's big enough to hold onto an atmosphere but small enough that it's feasible to escape its gravity. If a significantly larger planet ever brings forth creatures capable of technology comparable to ours, they're going to have a much harder time getting into space.
They're also going to have a harder time, if and when they get there, building habitats in space that spin fast enough to sustain an ecosystem whose creatures evolved under their higher gravity, so probably need it to thrive. The faster spinning and greater gravity mean greater stresses on the structure, requiring stronger materials than we'll need for our equivalents.
So civilisations from planets with stronger gravity than ours are apt to have a harder time becoming space-faring, while planets with weaker gravity are less likely to manage to hold onto an atmosphere (that isn't hostile to life) for long enough to produce a technologically advanced civilisation.
But that probably isn't decisive, given that planets of similar size surely aren't all that uncommon, although perhaps Earth's anomalously high density – which lets it be smaller, compared to other right-sized worlds – might play some part in making this more significant.
Written by Eddy.