In pop culture, aliens are often depicted as delicate, elf-like creatures with grey skin and almond eyes.

But they are more likely to be 47-stone monsters, a University of Barcelona researcher suggests.

Even relatively stupid life-forms are likely to be pretty big-boned, the researchers suggest - using a mathematical model which predicts that humans are most likely at the smaller end of the scale size-wise.

The researchers say, ‘Most inhabited planets are likely to be closer in size to Mars than the Earth.

'Furthermore, since population density is widely observed to decline with increasing body mass, we conclude that most intelligent species are expected to exceed 300kg (47 stone).’

Other experts believe that aliens might be a bit closer to humankind - both in size and shape.

Seth Shostak, a researcher at the SETI project (Search for Extraterrestrial Intelligence), says that humans have evolved into an intelligent, space-faring species not because of our size, but because of our upright stance.

Shostak says, ‘Polar bears are large but do not write great literature and build radio towers, and a lot of that is probably because they are walking around on all fours.’

