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With so many nice planets out there, why would aliens attack Earth?

They're baaaack: Movies about aliens are invading the theaters again — in forms ranging from the evil villains of "Pacific Rim" to the benign federation of "Star Trek Into Darkness."

The saga of extraterrestrials menacing Earth goes at least as far back as "War of the Worlds" — as in the 1898 H.G. Wells novel, not the 2005 Tom Cruise flick. But there's one question about that perennial plotline that bugs the SETI Institute's senior astronomer, Seth Shostak: Now that we're learning that planetary real estate is so abundant in the Milky Way, why would the aliens bother attacking?

The real-life search for worlds beyond our solar system is eroding the motivation for a space alien menace, Shostak says.

NASA's Kepler planet-hunting mission suggests that there could be billions upon billions of alien Earths and super-Earths out there. "What is it we have to offer, aside from some pretty good fish up in Seattle?" Shostak asked NBC News. "The answer is, not much. Just our culture, really. That's the only thing that they won't have at home, where the shipping charges are less."

Shostak complained that the standard motivations have become terribly outdated: "They come here for the water, or they come because something is wrong with their reproduction. People love that sex angle. But I'm not going to go to the insects in the backyard because things have gone wrong with our reproduction — and those guys, at least, have DNA. Most of this doesn't make any sense whatsoever."

The aliens could be in it for the cultural exchange. Or they might just dig the music, like the clueless extraterrestrials in Rob Reid's comic novel about contact, "Year Zero." But if they come in with ray guns blazing, they'll spoil the atmosphere, in more ways than one.

Besides, if it ever came to humans vs. aliens, particularly extraterrestrials capable of interstellar flight, there wouldn't be much of a plot to the real-life story. "Any aliens that really came to Earth ... they're going to be way ahead of us technologically, even if their poetry is no better," Shostak said. "As a consequence, the idea that we could take them on is a bit like Julius Caesar figuring he could take on the U.S. Air Force. He might think he can do that, but he's going to be disappointed."

Ironically, Shostak served as one of the consultants for last year's big alien-attack movie, "Battleship," and although he made several suggestions for the villainous aliens' motivation, the filmmakers didn't pick up on any of them. Instead, the motivation was left unsaid — which is kind of a Shakespearean thing to do. "These guys are just like the bad kids down a dark alley. They're just malevolent," Shostak said. "They're baaad."

The motivation is clearer in movies where meddling humans disturb the aliens' lair, whether it's on LV-426 in the "Alien" movies or the Jovian moon Europa in "The Europa Report," which comes out this summer. Maybe that's why "Aliens" and "Forbidden Planet" rank among Shostak's favorite sci-fi flicks: In those movies, you could argue that the humans are the intruders.

So what about the real-life search for aliens? If Earth-type planets are common rather than rare, that will affect the strategy for the search for extraterrestrial intelligence, known as SETI. "We're doing a bunch of projects," Shostak said. "One is to look at the Kepler candidates, star systems that are thought to have planets. ... That's a good idea, although, to be honest, I think that idea has been overtaken by events, because now we know that most stars do have planets. Given that, you might as well look at nearer stars, because Kepler's candidates are typically 1,000 light-years away."

Besides, if any alien neighbors are crazy enough to want to invade us, it's best to know about the closest ones first.

What's your favorite space alien movie? Which ones are real stinkers? Find out more about fictional and real-life quests for extraterrestrial contact with Seth Shostak and Alan Boyle at 8 p.m. ET tonight on "Virtually Speaking Science," an hourlong talk show that airs on BlogTalkRadio and in the Second Life virtual world. You can join the Second Life audience in the Exploratorium's virtual auditorium. And if you miss the show, you can always catch up with the podcast on the Web or via iTunes. Here's a listing of previous "Virtually Speaking Science" episodes:

For more from Seth Shostak, check out the "Big Picture Science" radio show.

Alan Boyle is's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as's other 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.