Aliens are often portrayed as diminutive beings, but in ‘reality’ they could be much larger and heavier than science fiction films suggest.

Scientist Fergus Simpson has calculated the minimum size needed for intelligent life to survive, based on the laws of conservation of energy seen on Earth.

And from this, he calculates that if intelligent extraterrestrials exist they will typically weigh 650lbs (300kg) - the median weight of a polar bear.

Aliens (illustrated with a stock image) are often portrayed as diminutive beings, but in ‘reality’ they could be much larger, one scientist claims. Fergus Simpson from University of Barcelona applied a mathematical formula to assume aliens obey the same laws of conservation of energy as animals on Earth

Mr Simpson came to this conclusion by applying a mathematical formula that assumes aliens obey the same laws of conservation of energy as animals do on Earth.

This means that larger animals need more resources to survive, so are less common than plentiful ants, for example.

The cosmologist’s calculations are also based on the idea that there’s a minimum size needed for intelligent life, but don’t factor in an alien planet’s gravity.

‘Throughout the animal kingdom, species which are physically larger invariably possess a lower population density, possibly due to their enhanced energy demands,’ he wrote in a paper, published at arXiv.org.

He said there must be a minimum body size necessary for supporting intelligent life, based on the range of body sizes of intelligent animals such as apes and humans, found on Earth.

‘As a result, we should expect humans to be physically smaller than most other advanced species.’

'...we conclude that most species are expected to exceed 300kg in body mass. The median body mass is similar to that of a polar bear.'

He claims that just as it is on Earth, there are probably more small animals than larger ones in the universe, but that the larger a planet or biosphere the more likely it is to have evolved an intelligent species.

Mr Simpson said there must be a minimum size necessary for supporting intelligent life, based on the range of body sizes of intelligent animals such as apes and humans, found on Earth. And, if they exist, he believes intelligent aliens typically weigh more than 650 lbs (300kg) - around the size of a polar bear (stock image)

His argument follows that if clever aliens are large, there may be fewer of them, as they will likely live in a society with low population densities to ensure they have the resources they need.

Study claims aliens are larger than those portrayed in sci-fi (stock image). But the calculations don't take into account an alien planet’s surface gravity

Commenting on the study, Duncan Forgan, a Research Fellow at the University of St Andrews, told NewsWeek: ‘I think the average size calculation is reasonable’.

Seth Shostak, a researcher at the SETI Institute in Mountain View, California, added that it is not the first time scientists have suggested that aliens may be large, because big animals tend to live longer.

A longer lifespan makes creatures potentially more likely to develop knowledge and invent technology needed to make contact with humans one day.

Mr Simpson continued: ‘Higher intelligence enables the development of technologies which can sustain larger population sizes.’

While the research has been described as interesting by Dr Shostak, he said ‘there’s no concrete data to work with’ and that Earth is being used as a model to predict what life looks like elsewhere in the universe – but aliens may be completely different.

He pointed out that it is humans’ thumbs, bipedal stance and brains, rather than our bulk, that has enabled us to thrive on Earth, so body mass is only one indicator of intelligence.

‘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,’ he said.

Mr Forgan also noted that the research doesn’t take an alien planet’s surface gravity into account.