Lots of aliens will be hitting the streets this Halloween. There'll be big-headed, beady-eyed grays ... pointy-eared Starfleet officers ... reptiles with mouths that bristle with fangs ... fur-faced wookiees and more.



But you hardly ever see depictions of extraterrestrials that live underwater — and there's a good reason for that, says Don Lincoln, author of a new book titled "Alien Universe: Extraterrestrial Life in Our Minds and in the Cosmos."

The reason? It's hard to build a fire underwater.

Some experts speculate that many of the habitable planets in our galaxy are water worlds, with no land in sight. But those wouldn't the best places for technologically advanced civilizations to take root.

"There could be alien cavemen underwater," Lincoln, a physicist at Fermilab in Illinois, told NBC News. "But truly, you can't smelt metal." And that means it's unlikely that intelligent dolphins will ever develop the technology for spaceflight.

In "Alien Universe," Lincoln blends together a compendium of alien tales going back to H.G. Wells and even earlier, plus a look at the scientific parameters that define the search space for intelligent aliens.

The book isn't aimed at veteran UFO fans looking for the latest revelations about the alien conspiracy. It doesn't address the search for microbial life on Mars, or Europa, or Enceladus — and it doesn't delve deeply into the search for planets beyond our solar system. Instead, "Alien Universe" is meant for those who wonder how all the stories about intelligent aliens got their start, as well as those who wonder how much science is behind those stories.

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"It's not just fiction. It's not just pretend," Lincoln said. "There's some real scientific thinking going on."

The chemistry of alien life

For example, there's a reason why all life on Earth is carbon-based, and why alien life is likely to be based on carbon as well. Carbon atoms can handle four chemical bonds (unlike a puny single-bonding hydrogen atom), and yet it's relatively easy to swap those bonds around (unlike, say, silicon-based chemistry ... sorry, Horta!).



There are also chemical reasons why water works so well as a solvent for life's processes, but it's possible to imagine other liquids serving a similar role. Methane, for example, could have some advantages over water — and liquid methane exists in abundance on Titan, a smog-shrouded moon of Saturn.

"This leads us to speculate that if life is an inevitable outcome of chemistry, then Titan should have at least primitive life," Lincoln writes. "If it turns out not to have life, then we must begin to suspect that there is something unique about the environment of Earth, perhaps including the use of water as a solvent."

The sociology of alien tales

Lincoln makes a distinction between primitive forms of life, which may well turn out to be common in the universe, and advanced forms of life that could head out from their home planets and contact us. In the book, he refers to those life forms as Aliens with an capital "A." Those types of Aliens are the main focus of "Alien Universe," as well as thousands if not millions of books and movies about extraterrestrials.

Surveys suggest that most Americans think such aliens have already visited Earth, and are behind at least some of the UFO sightings that have been reported over the past few decades. Today, the 1947 Roswell UFO Incident looms large on the list of UFO tales — but Lincoln said that story didn't make much of an impression when it happened.

"The Roswell saucer disappeared from history," Lincoln said. "It only reappeared in the 1970s when the National Enquirer reran the report from the Roswell Daily Record."

He said the interest in UFOs actually got more of a boost from other tales in the 1950s and '60s, such as George Adamski's stories of flying saucers and Betty and Barney Hill's account of an alien abduction. Such accounts triggered a long string of Hollywood productions, ranging from "The Day the Earth Stood Still" to "Men in Black." And such movies, in turn, make the public more receptive to UFO stories.

"There's a loop between the stories, the media and Hollywood — they feed each other," Lincoln said.

That kind of alien appeal is what drove Lincoln to write the book in the first place. He's a particle physicist, not an astrophysicist — but his interest in the prospects for intelligent life beyond Earth began long before his interest in the Large Hadron Collider. "Aliens are something that absolutely fascinated me when I was a kid," he said.

More about the alien quest:

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the NBC News Science Facebook page, following @b0yle on Twitter and adding +Alan Boyle to your Google+ circles. To keep up with NBCNews.com's stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.