According to Hollywood, Earth is surely one of the galaxy's "top places to visit before you die." Cinema aliens come here often enough that the State Department should probably set up passport control.

Of course, that's fiction. But in the last hundred years, Homo sapiens has been flamboyantly belching clues into space that could alert technically savvy extraterrestrials of our presence. Radar and television, odd chemical compounds in the atmosphere, and even the occasional spacecraft sent beyond the heliopause are all messages in bottles that could conceivably wash up on the shores of ET's planet.

When I point this out in talks, a frequent reaction is "Won't they come here and kill us?" I offer this response as proof of the general optimism of 21st century humankind.

Nonetheless, maybe this dystopian view is worth considering. Would the extraterrestrials come here — if not to kill us — then to take our resources or compromise our virtue?

The answer, of course, falls within the discipline of alien sociology — a field in which the data are, shall we say, sparse. Indeed, since we have no idea what the mores or motivations of extraterrestrials might be, you might conclude that, really, there's nothing we can say about whether the aliens would come here or not.

But there's an alternative to this "know-nothing" approach. Let's consider what might conceivably encourage visits by those who've learned that humans are strutting and fretting upon Earth's stage. After all, we've unraveled a few things about astronomy and physics, if not much about alien comportment.

Taking our cue from Tinseltown, I note that most cineplex sentients come to Earth either to solve some sort of ugly reproductive crisis or simply to take over the planet. The former doesn't make any sense whatsoever. You can't breed with creatures at the zoo, despite the fact that most of the base pairs in the inmates' DNA are identical to yours (note that this is a biological incompatibility, and not just zoo regulations). The aliens, needless to say, will have a different biochemistry, and probably no DNA at all. Forget, if you can, the breeding experiments.

Taking over the planet would only make sense if there were something really special about our world. The best guess of the exoplanet specialists is that the number of Earth-size planets in our galaxy exceeds tens of billions. That doesn't sound like our hunk of real estate is terribly privileged.

They won't come here to mine our minerals, either. The entire universe is built of the same stuff, and while the solar system has a higher percentage of heavy elements than found in many stellar realms, it turns out that this is precisely the condition that seems to foster planet formation. In other words, ET's own solar system will be similarly blessed with these useful materials. So why would they come here and incur multi-light-year transport charges?

Colonization? A hunt for additional living space? If the former is something aliens do, then they won't wait to hear from us before doing it. The British, after all, didn't begin their colonization of Australia because they had intercepted some aboriginal communications.

As for getting a bit of lebensraum, well, planets are not great new habitat, because they're spheres. They're cursed with the minimum surface area for their mass. As pointed out three decades ago by Gerry O'Neill, it's both more efficient and enormously cheaper to build artificial habitats in your home star system.

OK, you argue, but Earth is more than just a handy source of gold or molybdenum, more than merely random cosmic acreage aching to be invaded and subdued. It's an exceptional habitat for life. Water, oceans ? it's so gosh-darn good, it's positively rare. The aliens will find our world lovable because it's livable.

Well, that doesn't pass the smell test, either. If the type of world that can support life is rare, then you don't have to worry about nearby extraterrestrials. There won't be any.

Other suggestions about why they might visit include forestalling competition in the Milky Way marketplace, proselytizing, or just learning more about us. It's not clear that any of these goals requires "killing us," of course, but the logic is wobbly anyway. Any beings that actually could come here will be far beyond us in technological accomplishment. Imagine if you could visit the Neanderthals. Would you worry about commercial competition? Would you give them bibles? Remember: these are (nearly) the same species as you are. The aliens won't be. I dare say you wouldn't try convincing porpoises to join your church.

Then again, there's that last point: they just want to learn more about us. Well, perhaps so. Maybe that's really what's interesting about Homo sapiens. Not grabbing our habitat, saving our souls (or our environment), or subverting our industrial output — but assaying our culture. I'm willing to consider that even very advanced beings might find our culture mildly worthy of study.

Keep in mind that if they're near enough to find us, that implies that there are many, many galactic societies (otherwise the distances between any two of them will be enormous). If there are lots of them, then we're just another entry in a big book. Once again, not all that special. Kind of like another weird fish found in the Atlantic. I don't expect mammoth expeditions to be sent our way.

But in any case, if they do pick up our TV signals — or even bother to get in touch — then they can study our society from home. It's a lot cheaper and a lot faster than bridging the light-years.

I guess the State Department doesn't need to set up that passport control.