Let me start by glossing over some of the classic arguments against extraterrestrial visitors:
By International Astronomical Union definition on light-year is
Given the trouble it takes to accomplish inter-stellar travel for anything but the most advanced civilisations, the only viable purpose would be colonisation in order not to have all of one's collective eggs in one stellar basket, as it were. To be at a point where that would be a concern would mean the civilisation in question would be incredibly ancient or incredibly advanced. If aliens came specifically to Earth for the purpose of colonisation it would only be because Earth was similar to their home planet. Either they would have been around long enough that their star burning out would be a concern or they would be advanced enough that they expect to still be around when their star burns out. Getting all of one's eggs out the same planetary basket is a much easier prospect that doesn't require the expense of interstellar travel. In our case, for example, putting humans on Mars is trivial compared to inter-stellar travel. Once the species is on more than one planet, planetary-level extinction events are no longer a threat to the species. It would take a stellar-system level event, like the star running out of fuel, to wipe out the species. And any species concerned enough with its own survival across time-frames of billions of years to mount the effort to cross billions of kilometres of space is pretty damned unlikely to give a rat's ass about the indigenous life on the planet they have their sites set on and would have no problem subduing us or just plain getting us out of the way.
To date, most conjecture about life on other planets use Earth as the model for life-bearing planets. Finding terrestrial planets is the holy grail of extrasolar planetary research. There very likely are lots and lots of small, rocky, "terrestrial" planets out there, and a lot of those could be in their parent star's Goldilocks Zone. But just how likely is it that there are an abundance of truly Earth-like planets out there? There is a lot that goes into making Earth habitable for us, and a lot of that is really, really unlikely to be commonplace. We need our big moon to generate the tidal friction that keeps the core of the Earth molten and moving so that we get the benefits of a strong magnetic field to protect us from cosmic radiation and plate tectonics that keep the carbon cycle moving, amongst other things. Now consider that our nice big moon that we owe our very existence to is the result of another planet the size of Mars hitting the developing Earth in the right way at the right time - about as likely as making Julienned fries with a Sledge-O-Matic.
Along with the requirement of this unlikely collision, consider the planets we have been able to find in recent years - "hot Jupiters" orbiting close to their parent stars - illustrates one thing: planet formation is common, but during accretion things get bashed around quite a bit. Jovian-sized planets can migrate into close orbits, which is seriously going to mess with any smaller planets that might already be in close orbit. Some extrasolar planetary systems have Jovian and larger bodies in highly elliptical orbits. Not all planetary systems are crunchy on the inside and fluffy on the outside like ours. And even if we find one, chances are the terrestrial planets will be moonless like Mercury and Venus, or at best have uselessly tiny captured moons like Mars.
The universe if big, of course. So there are inevitably other planets like Earth, but the unlikeness of the conditions required means they are going to be wickedly far apart - making travel between them all that much more difficult.
So then what is likely?
We know planet formation is commonplace. We've already found a few hundred Jovian-type extrasolar planets. If Jupiter, Saturn, Uranus and Neptune are any indication, great big planets have great big satellite systems as a matter of course. That opens up two more options. Let's look at the less likely first:
If a Jovian planet happens to be in the Goldilocks Zone, it makes it possible that one or more of its moons would therefore be habitable. The tidal forces between the Galilean moons of Jupiter keep Io volcanically active and the interior of Europa liquid (which I'll come back to.) So, worlds like the forest moon of Endor from Return of the Jedi are not only possible, but very probably more common than planets like the one you happen to call home. That's good news for us. Maybe we'll even find some places like this soon. But what does it mean for potential visiting aliens?
Imagine you are an alien who evolved on an "Endor." Let's say your technology has advanced to the same level as ours and you've just figured out how to detect extrasolar planets. Like us the first planets you would find would be hot-Jupiters for the same reason we're able to find them. But where do you go from there? Would you expand your search to look for small, rocky, terrestrial planets? Not a chance. You would extend your capabilities to find Jovian planets orbiting further from their star because you'd look for systems like home, like what you would consider normal. And, you'd eventually find some and consider them likely candidates for life. Maybe at some point in your search you'd turn your equipment toward our star and be disappointed to find Jupiter far too far away to be of any interest. Since you found other Jovian planets the right distance from their stars you'd then concentrate on detecting moons around those planets, looking for a place like home. Why would you even waste you time an resources looking for something so unlikely and alien as a rogue "moon" orbiting a star all by itself, and especially one with a satellite of its own.
But why worry about the Goldilocks Zone at all? Look at Europa: Jupiter is way out beyond the habitable zone, but Europa may harbour life because under its frozen surface tidal forces are keeping the interior liquid. Moons like Europa are infinitely more likely to be completely commonplace. In fact they are so commonplace that there are four watery moons in this solar system: Jupiter has Europa, Ganymede and Callisto, and Saturn has Enceladus. The Goldilocks Zone is irrelevant. All you need is a Jovian planet with a few moons.
But would anything that evolved on a Europan-type moon be inclined to come visit us? First of all how do they even get out of the stone age? Living in a global ocean isn't exactly conducive to discovering fire and subsequently developing metallurgy, to say the least. And if you evolved in a sphere of water bounded on one side by rock and ice on the other, would you even conceive that there might be anything beyond the ice? What would you expect to find there? And would you ever look? Anything indigenous to a Europan environment would be living in utter darkness. Even here on Earth when creatures who had evolved eyes to see light find themselves living in environments where there is no light, like cave fish, they soon lose their eyes as the species adapts to the dark. Something evolving in a global ocean under a mile of ice would not even evolve eyes. They would have other sensory apparatus. Sonar would make sense. Or a platypus-like electrical sense. Maybe the ability to see heat.
So lets give these creatures the benefit of the doubt and presume they are curious enough to dig up through the ice with their stone tools. Lets give them an even grander benefit of the doubt and imagine they come up with some way to protect themselves from the lack of pressure and the extreme cold using stone-age technology. What does the Europan who sees by sonar find when they reach the frigid vacuum of the surface? Nothing. Their senses are useless outside of the water. The only conclusion they can make is that the whole universe consists of a rock surrounded by water, surrounded by ice, floating in an endless empty void. They not only can't develop the technology to manage space flight, but they aren't likely to even conceive that there is anything beyond their own world.
Despite all this, I can think of one scenario where aliens might come to Earth - to examine the freak show. A civilization that developed on an Endor-type world would be inclined to develop space travel since there would be so many things in their sky that would be relatively easy to get to. Later, they would find other Jovian planets around other stars just as we have, and given sufficient time and inclination might even set out to create interstellar colonies. With a colony established there would be incentive to try and find better ways to travel between the colonies and the home world. Freed from the threat of planetary-wide (or moon-wide as the case may be) and stellar-system-wide extinction such a civilisation could get very, very old and potentially find ways to make interstellar travel fairly trivial. They'd eventually come across intelligent life on Europan-type moons and maybe lift them out of their shroud. Maybe in the course of looking for other Europan-type intelligences they'd come and look at Europa, Ganymede, Callisto and Enceladus and come across our little freak show by accident.
Would they reveal themselves? Maybe not. Let's assume they have run into Europan-type civilisations. I'd expect they'd know to be very careful in how they approached Europan aliens who heretofore hadn't even conceived of anything beyond their own world. Having never run into anything quite like our situation they'd exercise extreme caution. It would probably be more interesting to study us from a distance for a good long time than to make contact. Given that they are advanced enough to harness the incredible energies required for interstellar travel the chances of them accidentally revealing themselves to us would be pretty damn slim.
But say their incredibly advanced civilsation and experience in contact with xenophobic Europan-type life has led them to some inscrutable protocol where first contact via anal-probing bucolic rubes is, in fact, the way to go. Would they actually be little grey men or humaniod lizard-people?
Look at the diversity of live on this planet, where we all share a common ancestor. Here I sit, a little under two metres tall when standing. I have red blood and a two-lobed brain. I have thick hair in on my head, groin and armpits and sparse hair almost everywhere else. I have five major protuberances from my torso: two legs, two arms and my head. The arms and legs further split into five more protuberances each at the ends (fingers and toes.) Not far from me are my cats. From the point of view of an alien xenobiologist they are practically the same species. Their hair is more evenly distributed. Their internal organs are all pretty much identical. Their four limbs subdivide into five branches at the end. They have red blood and two-lobed brains. The only real significant difference is their spines extend beyond their pelvis to form a tail.
My cat and I are pretty similar because we share a common ancestor not more than a few tens of millions of years back. But look at branches that split much longer ago. I really don't have much in common with the spiders that my cat likes to eat, and even less in common with cephalopods.
Cephalopods are a perfect case-in-point. Small squid and octopi have been shown to have intelligences on par with similarly sized monkeys. They are able to use their tentacles with dexterity on par with our much-ballyhooed opposable thumbs. Yet they have no bones, a five-lobed brain, green blood based on copper, no heart as we commonly know it (instead they have two pumping organs based on gills). They are, in fact, almost utterly alien, yet they evolved right here on the same planet.
If two branches of the same evolutionary tree can develop intelligent creatures as wildly different as a squid and a monkey why is that there is the commonplace belief that aliens will be humanoid?
This wasn't always the case. Looking back at golden-age science-fiction and earlier one can find many examples of aliens that truly are alien in form. H.G. Wells' Martians are one good example:
They were, I now saw, the most unearthly creatures it is possible to conceive. They were huge round bodies--or, rather, heads--about four feet in diameter, each body having in front of it a face. This face had no nostrils--indeed, the Martians do not seem to have had any sense of smell, but it had a pair of very large dark-coloured eyes, and just beneath this a kind of fleshy beak. In the back of this head or body--I scarcely know how to speak of it--was the single tight tympanic surface, since known to be anatomically an ear, though it must have been almost useless in our dense air. In a group round the mouth were sixteen slender, almost whiplike tentacles, arranged in two bunches of eight each. These bunches have since been named rather aptly, by that distinguished anatomist, Professor Howes, the hands. Even as I saw these Martians for the first time they seemed to be endeavouring to raise themselves on these hands, but of course, with the increased weight of terrestrial conditions, this was impossible. There is reason to suppose that on Mars they may have progressed upon them with some facility.
It wasn't until people started making science-fiction films that humanoid aliens began to get the upper-hand in the depiction of aliens. It's really painfully simple: it's cheaper to put a man in a costume than to try an build or animate a genuinely alien looking alien. Not only that, but it is easier for an audience understand the gestures of a humanoid alien. Star Trek creator Gene Roddenberry went so far as to mandate that the faces of his aliens not be covered, particularly the eyes, so as not to impede the actor's ability to connect with the audience on a human level.
Over the last 50 years or so of mass media has inadvertently codified what we think an alien "should" look like down into a few rather limited forms, all humanoid. The UFO conspiracy sorts believe in three main types, the big-headed, big-eyed, small-statured "Greys" (or "Grays"), the recycled sleestack "Reptoids" or "Reptilians" of David Icke fame, and the indistinguishable-from-human "Nordics" - all codified from the sorts of aliens that have most predominantly appeared in films and television for over half a century. It's a feedback loop: TV and the movies started showing the aliens they could afford to fabricate, this became the model for what aliens "should" look like, and now people producing alien fictions stick to these forms, further emphasizing and codifying this limited palette.
The likelihood that even a single alien species would visit us at all is already literally astronomical, that they would look even more like humans than my cat does is patently absurd. The likelihood that not one but three different species, each evolved on a different planet, are all here, all happening to be strikingly humanoid, and are wasting their time covertly mucking with our politics for their own nefarious purposes is so close infinitely remote as to be absolutely absurd.
Oringinal post: http://mbarrick.livejournal.com/881932.html