Expedia's galaxy-wide website must be offering Earth at a major discount. In one movie after another, aliens decide to pass up competing Milky Way attractions -- including neutron stars, antimatter clouds, hot Jupiters, and a 4 billion-trillion-trillion-ton central black hole -- in favor of our planet. The small speck of rock we inhabit is more popular with tourists than Disneyland.

Even an abbreviated laundry list of invasion films will give you the idea: Independence Day, War of the Worlds, Superman II, Mars Attacks, The Day the Earth Stood Still, Killer Clowns from Outer Space ... They all share a common premise, namely that Earth is the bee's knees, cosmically speaking.

But really, you've got to wonder what would motivate creatures from other worlds to suffer a journey of hundreds of trillions of miles to visit our planet? It's a trip so relentlessly devoid of scenery, their spacecraft wouldn't need windows. Why bother?

I've been asked this question at least a half-dozen times by Hollywood writers, and the best answer I can muster is "I don't know."

My impoverished reply is clearly disappointing, and the usual response by the filmmakers is to resort to two hackneyed incentives to rope in the aliens, namely (1) a quest for natural resources, and (2) breeding experiments.

Frankly, and not to rain on anyone's parade, neither makes sense.

Consider the idea that the extraterrestrials want materials for their industrial needs. It's nice to imagine that Earth is valuable as a mining claim, but what do we have that they don't?

A frequent suggestion is water. But that's silly: The universe is awash in water, thanks to the abundance of its two atomic ingredients, hydrogen and oxygen. Like Kimye and Kanye, these two elements are everywhere. Heck, there's more water on some of the moons of Jupiter than on Earth, and no one's going to get ruffled if you opt to remove it. But really, you can save the tanker costs by finding water in your own solar system. There's bound to be plenty.

Digging up other minerals and metals is similarly unnecessary and inconvenient. The entire cosmos is made of the same elements (and more or less in the same proportions) as is our local neighborhood. You don't need to import this stuff from light-years away.

Maybe they just need farmland? Like Captain Bligh, perhaps aliens are hoping to find a place to grow breadfruit, or whatever the galactic equivalent might be. Again, this is the kind of incentive that might work if you don't first need to traverse interstellar space. If you do, consider building orbiting greenhouses at home. They'll be cheaper, and the produce will be fresher. And honestly, if Earth's countryside is that attractive, why didn't someone plant a flag (or Klingon breadfruit) millions or billions of years ago? It seems that terrestrial real estate is a dog on the market.

Breeding experiments are even less plausible, even if many movie-goers feel like participating. Anyone who's made it through tenth-grade biology will recognize that breeding with other species here on Earth -- all of whom are card-carrying members of the DNA club, and therefore closely related to you -- is not only difficult, it's guaranteed to be fruitless. And possibly illegal.

Trendy scenarists will often invoke the social concern du jour, and suggest that the extraterrestrials are here to save us from ourselves. Aside from the obvious fact that they don't know of such contemporary problems as climate change or nuclear proliferation (our newscasts haven't reached them yet), why would they be interested? I bet the dinosaurs would have wished for a bit of alien help in giving an asteroid a nudge 66 million years ago, but it seems the extraterrestrials couldn't be bothered. Are we that much more deserving?

No, the bottom line is that the only truly special things about Earth are likely to be our biota and our culture. They could learn a lot about either one by merely analyzing the spectral signature of our atmosphere or tuning in to our TV broadcasts, and they would save a king's ransom on fuel by avoiding actual travel.

Despite the dramas played out at the local cineplex, real aliens won't be itching to visit. What we've got, they've already seen, and the doorbell won't ring. We're not on their bucket list.