Just one possibility (Image: c.20thC.Fox/Everett/Rex Features)

Quiz: Do you know your aliens?

TENTACLED monsters, pale skinny humanoids, shimmery beings of pure energy… When it comes to the question of what alien life forms might look like, we are free to let our imagination roam. The science-in-waiting of extraterrestrial anatomy has yet to acquire its first piece of data, so nobody knows what features we will behold if and when humans and aliens come face-to-face. Or face to squirmy something.

Despite this lack of hard evidence, a blend of astronomy and earthly biology offers some clues to what is out there. A few bold scientists are even willing to make an educated guess at the nature of aliens that might exist on faraway worlds.

What these extraterrestrials will be like depends on where and how we expect to meet them. Barring the appearance of flying saucers, there are two broad possibilities: either we have a close encounter with our neighbours by visiting the planets and moons next door; or we make an interstellar phone call to creatures inhabiting much more distant planets that circle alien suns. These two options have different implications for the shape of what we find living there.

What extraterrestrials will look like depends on where and how we meet them

If first contact turns out to be within our solar system, then at least we have some prior knowledge about the available habitats. Several spots might be suitable for Earth-like life based on carbon biochemistry and using water as a solvent. The subsoil of Mars may be warm enough to host microbes akin to Earth’s bacteria, for …