What the scientist says: Aliens are likely huge,

If you're traveling to distant planets anytime soon, you might think twice about raising a ruckus: The inhabitants likely weigh an average of 650 pounds, a cosmologist says.

Apparently it all comes down to planet size and the conservation of 

energy, CNET reports. "Throughout the animal kingdom, species which are physically larger invariably possess a lower population density, possibly due to their enhanced energy demands," says Fergus Simpson of the University of Barcelona.

That's quite true on Earth, where we have seven billion (relatively big) people, and, the BBC noted last year, up to 100 trillion (tiny) ants.

Which brings us to outer space, where, Simpson says, "most inhabited planets are likely to be closer in size to Mars than the Earth." And "since population density is widely observed to decline with increasing body mass, we conclude that most intelligent species are expected to exceed 300kg (660lbs)," he adds.

A scientist in Scotland says Simpson's "average size calculation is reasonable," but doesn't account for gravitational pull—and planets with stronger gravity would probably have smaller animals, .

SETI Institute researcher Seth Shostak says Simpson's paper, published at arXiv.org, also leaves out evolutionary theory: With humans, for example, it's our ability to walk upright and use opposable thumbs that gave us the upper hand on Earth.

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