In movies, when aliens invade our planet, they usually have a ludicrous motivation. Like, they're after our water. Or our gold. (Hint: Asteroids have gold.) Space travel is incredibly difficult and expensive — so why would aliens actually bother to come and invade us? We asked some experts for reasons that actually make sense.


What if space travel is easy?

First off, a caveat. This article basically assumes that space travel is going to be really tough, due to the huge costs of traveling at near-light speeds, shielding yourself from cosmic radiation, and so on.


But what if these aliens have discovered a cheap, easy way to travel to other planets? In that case, all bets are off. A universe where hopping from planet to planet is easy would resemble the recent history of Earth — you'd have invasions for any random reason: "robbery, raids, war, politics, whatever," says Craig J. Rodger, a physics professor with the University of Otago in New Zealand.

At the same time, if they're advanced enough to travel between star systems as easily as driving from Milwaukee to Detroit, then why bother? In that case, they're also advanced enough to make whatever they need, without bothering us.


So if space travel is still difficult for these aliens, what are the reasons they might go to all the trouble of invading Earth?

If habitable planets are rare

Let's say these aliens have high enough technology that they can travel to other star systems, but not enough to build ringworlds, or make barren planets habitable. In that case, taking our planet away might be worth it, says Sean Carroll, a physicist at the California Institute of Technology and a contributor to the Cosmic Variance blog:

I could imagine that the Earth would be useful if habitable planets were hard to come by, and their biochemistry was close enough to ours that they could terraform (exoform?) our planet to make it livable for them. After all, even if half of the stars in our galaxy have habitable planets (unlikely), we're still talking about journeys of light-years, and a fifty-year journey is a much bigger deal than a ten-year journey. So I would be willing to contemplate the idea that there might be a species that is very hungry for habitable planets, and yet not sufficiently impressed by other forms of life that they would mind wiping out the natives.


When you go to build a new house, you don't worry about the ants or small animals that might have built nests or structures on the property, adds Jim Kakalios, professor of physics and astronomy at University of Minnesota and author of The Physics of Superheroes. You just tear down those structures and wipe out the creatures inhabiting them, while you create the foundations for your new building.


Information

Information is the most valuable thing in the cosmos, and it's also the easiest thing to move around between star systems, says Sean Williams, author of the Astropolis series and co-author of the Evergence trilogy with Shane Dix. So aliens might attack, or apply pressure, to get hold of some information that humans have, that the aliens can't just glean from a distance.


For example, what if the aliens have superior weapons and space travel, but humans have made some advances in biotechnology or food production that are superior in some way? In that case, the aliens might turn up to demand our knowledge, says Rodger — but this is, again, more likely in a scenario where space travel is easier.


And anything that we make or have on Earth isn't going to be worth transporting to another planet, adds Williams — much easier just to send the information about how to reconstruct it back to your own planet. Chemistry and physics work the same everywhere, as far as we know, so it's not that hard to replicate anything that you might find on Earth, back home.

Ideology

What if our very existence, or something we've thought up, is threatening — either politically or ideologically, or on religious grounds? Or even scientifically? That could make it worth coming to wipe us out or, to subjugate us, Williams speculates. In his and Dix's novel Geodesica, "war is waged over who owns the definition of "human", so it's possible aliens could start a war over what defines 'life,' or 'intelligence,' or even 'real.'"


Adds Williams:

If we represent an idea that's seriously dangerous to them, that could motivate them to come here and do something about us. For instance, in Greg Egan's Quarantine, they don't invade or wipe us out, but they do stop us from observing the universe, because by just doing that we're destroying the universe. In Ian Watson's The Embedding, it's an obscure experiment in grammar that makes the aliens go batshit for us. In David Brin's Startide Rising (again not really an invasion novel but it could have been) it's some piece of information we stumbled across. Some kind of scenario like that I could totally buy.




Pop culture

What if we're creating some pop culture that they like, and they want to force us to make more of it? Like in that Futurama episode where the aliens show up to demand the final episode of Single Female Lawyer. Could aliens attack and demand that the human race make more Firefly?


But of course, if aliens invade for pop culture, they're inevitably going to change it, notes Williams. In fact, maybe that would be their motivation — not just to force us to make more television, but to make us cast them in key roles? If the aliens want to become part of our pop culture, Williams wonders, is that them invading us, or us invading them?

Revenge

Maybe we did something to these aliens a long time ago, and they want payback? Williams plays with this idea in his novel Evergence. "If we're the devolved remnant of a formerly superior race, then maybe on discovering us another civilisation might invade to find out what secrets we remember," says Williams. "Our fate could rest on the content our myths and legends . . . which is a neat idea, now it comes to me."


Easier to wipe us out from space

There's absolutely no good reason for aliens to invade us, says Phil Plait, creator of Bad Astronomy, author of Death From The Skies!, and a writer for Slate. If the aliens want slaves, they can probably just build robots. If they want food, bear in mind that most of the organic material on our planet is either poisonous or non-nutritious to us, and we evolved on this planet.



But if the aliens hate us, or want to take our biosphere, then they would probably just wipe us out from a distance, says Plait:

If they are just incredibly xenophobic, it's way easier to find a couple of dozen mile wide asteroids and push them into the Earth at high speed. That'll take care of us. If they want to be thorough, make it a hundred. Or a thousand. If they just want the planet to inhabit, then again, invasion is silly. Better to wipe us out entirely to avoid an uprising.


You could also see aliens sending out self-replicating von Neumann machines to wipe out higher-order life on every habitable planet in the galaxy — as a means of getting rid of potential competition, to clear the way for colonization, or to rid the galaxy of inferior life, says Rodger. In that case, it would be nothing personal — just part of a galaxy-wide project of systematic cleansing.


Further reading: Anthropologists explain how to approach aliens parked in Earth orbit