Fictional aliens come in all shapes and sizes - but what would a creature from another planet really look like? Would we even recognise one if we met it? In a new book published this month, science fiction writers Jack Cohen and Ian Stewart set out to find the answers.
It's life but not as we know it....
The first thing to get straight when thinking about life on other planets, according to Jack Cohen, is that it will almost certainly not be humanoid.
In the same way that Jesus Christ is portrayed as a Caucasian in Western culture, Mr Cohen argues, we have made fictional aliens in our own image.
"ET looks like a cute three-year-old child. But that is just for dramatic purposes, within the film."
The development of spines and skeletons is, he says, an evolutionary accident that could well be unique to Earth.
"If you ran Planet Earth again, the chances are you wouldn't get vertebrates. You wouldn't get creatures with a jointed spine."
The Roswell alien autopsy footage, which purported to show a creature recovered from a crashed space ship, is too humanoid to be taken seriously, he argues. The same goes for the classic almond-eyed alien creatures from the X Files and countless Hollywood movies.
When authors and film-makers are not making their aliens cosily familiar, they are creating modern-day bogeymen.
The alien has become a "quasi-scientific stand-in" for ghosts, ghouls and fairies, Cohen and Stuart argue.
The Roswell autopsy was widely believed to be a fake
The best example of this is the film Alien, which plays on our fear of the unknown to spine-chilling effect. But the science behind the film is "beyond nonsense", according to Mr Cohen.
The idea that a creature would wait 12,000 years before hatching its egg, "without something eating it" is absurd, he says. That it would adapt instantly to the human immune system is similarly far-fetched.
Life on Mars?
But the famous scene where the alien creature emerges from John Hurt's stomach is the most unrealistic of all.
"If you have something much bigger than your heart moving around inside your chest and you don't know about it, you are in big trouble."
The idea that aliens live on Mars is also wide of the mark - the chances of life being found on the red planet are slim, the pair argue.
It may have been around two billion years ago, when Mars had water and an atmosphere, but it would not have had a chance to develop beyond the most primitive of organisms, before dying out.
But just because aliens don't look like us or live on Mars does not mean they don't exist.
"We've got to get away from all those comfortable ideas that aliens will be just like us, except for a few minor differences that don't challenge our imagination."
"Real aliens will be very alien indeed," Cohen and Stewart write.
Many different habitats can theoretically support life, not just a water and oxygen based planet.
Anywhere that physical matter exists and there is an energy source could lead to the development of something of sufficient complexity that we would categorise it as "life".
Perhaps a gas cloud on a moon of Jupiter or a "sentient sea" of the kind found in the classic science fiction novel Solaris.
But only aliens that have evolved in a similar environment to earth would be interested in visiting us and making contact, Cohen and Stewart argue.
Other, more exotic types of alien life might be here already, they argue, but we are just not able to recognise them.
Alien: scary but unrealistic
"Certainly aliens would not look like the canonical little green men... unless they wanted to.
"They might look exactly like people. Or cats. Or houseflies. Or they could be invisible, or lurking just outside our space time continuum."
They could also be concealed inside atoms or they could "exist only in the gaps where the human perceptual systems are in their refractory phase and cannot register their existence."
But the chances are they are not here at all.
Like humans, whose initial enthusiasm for space exploration, which culminated in the moon landings in the late 1960s, petered out, they just might not be interested in making the trip.
What Does a Martian Look Like? The Science of Extra-terrestrial Life, by Jack Cohen and Ian Stewart, is published by Ebury Press