NB: this article contains a mild spoiler for Oblivion.
Before the science fiction genre even had a name, writers were wondering about life on alien planets. Philosophers, astronomers and dramatists were suggesting that civilisations might exist on other worlds in the 17th century, and some of the earliest pieces of movie theater - such as Georges Méliès' A Trip To The Moon (1902) - tried to imagine what those aliens might look like. But in the decades since that key silent movie, only a handful of filmmakers have successfully managed to convincingly describe what an alien life form might look or behave like, and all the terror and awe we might feel if we were to encounter one for ourselves.
These aren't necessarily films full of stunning special effects - in some cases, the aliens aren't shown on screen at all. Rather, these are alien life forms that are convincing from an ideas standpoint.
Rather than try to draw up an exhaustive list, we've instead looked at the different kinds of aliens we've seen in movies - the godlike, the bacterial, the societal, the mechanical, the animal - and how each variety has managed to convince us that we're in the presence of something truly extraterrestrial.
First published in 1961, Polish author Stanislaw Lem's Solaris contained perhaps the ultimate example of the unknowable alien intelligence. Adapted for the screen three times - in 1968, as a Russian TV movie, then by Andrei Tarkovsky in 1972, followed by Steven Soderbergh in 2002 - it's about the scientific investigation into the nature of the titular ocean planet. Protagonist Dr Kelvin arrives on a station orbiting Solaris to find its occupants distant and fractious; the planet appears to have some sort of supernatural effect on them, but what?
Kelvin discovers that Solaris' ocean is essentially a vast consciousness, and that it somehow has the power to read its visitors' thoughts, and render faces from their haunted past as physical reality. In the book, this discovery plays out as a procedural detective story; in Tarkovsky's adaptation, it's an operatic philosophical drama; in Soderbergh's movie, it's a tragic romance. Solaris can bend to all these interpretations because the idea at its center is so hypnotic - the ocean intelligence is so beyond our understanding that it can only communicate to us through ghosts.
That same sense of awe at the cosmos, and the unknowably intelligent beings that might be lurking somewhere in it, is all over Stanley Kubrick's 2001: A Space Odyssey. Its aliens are not only older than us as a civilisation, but they've also used their technology to shape our fate and, ultimately, lead us to their location somewhere beyond Jupiter.
Solaris and 2001: A Space Odyssey's alien intelligences - along with Robert Zemeckis' Contact or William Eubank's 2011 indie feature, Love - are convincing because they hint at so much but show so little. The imposing black monoliths, which we see towering over apes and space explorers or tumbling through space, are unforgettable pieces of alien technology because they're so minimal - we can only guess at what they're made of or how they might work.
Shorn of silly designs or special effects, these depictions of alien life are timeless and haunting - just as primitive humans quaked in fear at the unknown power that caused thunder and lightning, so we can only shudder at the vastness of these unknowable intellects.
It's when movies try to introduce logic that this almost primal spell is broken. Steven Spielberg's Close Encounters Of The Third Kind introduced that same feeling of cosmic awe, but his 1980 Special Edition added a concluding scene in which Richard Dreyfuss' protagonist is shown the inside of an alien space craft.
Spielberg later took this scene back out again, rightly saying that, "The inside of that mothership is the exclusive domain of the audience’s imagination". Arthur C Clarke famously said that advanced technology would be "indistinguishable from magic" - and in movies, that magic is at its most potent when alien technology is implied rather than explicitly shown.
In movies, the most convincing alien invaders are often the most primitive. The Andromeda Strain, based on the 1969 novel by Michael Crichton and precisely directed by Robert Wise, takes a scientific and highly compelling look at what might happen if an extraterrestrial organism spread among Earth's populace. With the alien germ wiping out all but two inhabitants of a small town in New Mexico, a group of scientists in an subway lab are in a race against time to discover the nature of the threat, which causes blood to coagulate in the veins.
Like so many of Crichton's stories, this disturbing scenario has a basis in scientific possibility. Scientists have long said that alien life may exist in simple forms somewhere in our own solar system, whether it be bacteria on comets or microorganisms on Europa. Scientists have been writing about the possibility of diseases arriving from outer space for years, and two professors even suggested that the late 90s mad cow disease epidemic could have had an extraterrestrial origin.
Stories like these, and most of all our own fear of disease and bodily invasion, are what make the alien threats in films such as The Andromeda Strain, and Invasion Of The Body Snatchers (1956) so timelessly disturbing. The 1953 British TV series The Quatermass Experiment - and its later movie adaptation, The Quatermass Xperiment - was among the first screen stories to suggest that astronauts might encounter something nasty and corrupting on their travels, with its surviving space explorer crashing back to Earth and mutating into a creature whose spores could wipe out life on our planet.
Even the creature at the center of The Thing (1982) could be counted in this category - is it an intelligent organism, or a disease-like entity that copies the minds as well as the bodies of its hosts? Whether it's strictly speaking a disease or not, The Thing's monster is so terrifying because its spread is so insidious.
These movies introduce other-worldly life forms that are theoretically plausible, can't be reasoned with, and physically affect us in horrible, unpredictable ways.
If there's one thing that keeps would-be invaders from attacking our planet every week, it's the vastness of space itself. With scientists suggesting that even the nearest habitable planets are several light years away, only the most highly advanced civilisations would have a chance of reaching us - for its own part, NASA's currently trying to prove that it's possible to build a faster-than-light warp engine.
This brings us to a less common yet intriguing type of alien visitation in movie theater - the mechanical. Kronos, directed by B-movie maestro Kurt Neumann and released in 1957, sees a machine invade Earth to rob it of electrical energy, which it uses to grow into ever greater size. Even dropping an atom bomb on it merely makes its hulking proportions increase, until it's stomping on cities and terrorising the populace.
Kronos himself is a simple special effect, consisting of little more than two black boxes stacked on top of each other - which makes sense, given that the whole movie was shot in two weeks. In spite of this, the idea behind Kronos is a thought-provoking one - if aliens couldn't visit us in person, wouldn't they send artificially-intelligent machines to do the job instead?
The idea of an alien planet-killing machine was compelling enough to become the basis of a Star Trek: The Original Series episode 10 years later (The Doomsday Machine), and elements of it even come into play in this year's Oblivion. Stories such as these posit the idea that, if aliens do come visiting, they may take the form of AI devices rather than almond-eyed humanoids.
The convincing depiction of an alien society is a difficult thing to get right, whether in books or on screen. After all, it's likely that advanced life forms on other planets would not only look nothing like us, but their culture and social structure would also be so different as to be unrecognisable.
HG Wells' The First Men In The Moon was among the first books to imagine what an extraterrestrial race might look like in its own habitat - and in Wells' mind, these lunar creatures (or Selenites) are five-foot-high, ant-like beings that lived below the Moon's surface. Arranged into a strict hierarchy, the Selenites are bred for their own specific purpose, with some physically suited to labor, for example, while others have big brains and raised as intellectuals.
The idea of aliens as social insects is one that quickly took hold in science fiction, and its usage has resulted in some of the most memorable and convincing alien societies in the movies. Although Nathan H Juran's 1964 adaptation of First Men In The Moon was a rip-roaring adventure at heart, it managed to sketch in a quite intriguing glimpse of a race of the Selenite creatures living beneath the lunar surface.
Nigel Kneale, who wrote the screenplay for the 1964 First Men In The Moon, also created the classic Quatermass And The Pit, which began as a BBC TV series in the late 1950s and was later adapted as a movie in 1967. Kneale's story, taking inspiration from Wells, suggested that Mars was once populated by a locust-like beings that had a direct hand in the evolution of humans. Although the aliens themselves are only briefly glimpsed, their devil-like silhouettes and disturbing scientific interests - such as eugenics - make them truly memorable.
Ultimately, the alien societies in movies tend to mirror our own, or provide a disturbing opposite. Where human cultures are typically individualistic, alien cultures are often a collective horde, driven by cold logic and science.
This fifth type of science fiction movie introduces an alien life form that is neither as toweringly evolved as those in Solaris or 2001, nor as basic those in The Andromeda Strain. Ridley Scott's Alien brought us what is surely the ultimate extraterrestrial creature - an entity with a thoroughly thought-through lifecycle, from parasitic origin to imposing final form. Plenty of other science fiction movies had introduced shadowy monsters before, but none had felt so real as the one designed by HR Giger for Alien.
Its growth in the stomach of the luckless Kane (John Hurt) not only created one of the great shock moments in 70s movie theater, but also accounted for the alien's humanoid stature. It looked a bit like a man because it was born from a man - something gleefully played with in Alien 3. Best of all, the alien's spooky, almost silent presence introduced so many intriguing questions that lingered after it was blown out of an escape pod airlock: just how intelligent are these starbeasts? Does their bio-mechanical anatomy hint that they're some form of artificially-created weapon?
With both Scott's direction and Giger's unforgettable design suggesting so much, it's not surprising that no other sci-fi movie has managed to create such a startling creature since. Pitch Black, this year's Riddick and even Ridley Scott's own Prometheus have introduced their own squelchy, slithering alien life forms, but none have felt quite so chillingly other-worldly as the beast in Alien.
As Ash (Ian Holm) once marvelled, "I admire its purity. Unclouded by conscience, remorse, or delusions of morality..."