2. ETs maybe contact us, but maybe not When ETI does contact Earth, it may be as unambiguous as a UFO landing in Times Square. Or, it may be a signal that we only receive once and can’t understand or decode. In fact, it’s possible that this has already happened, as with the still unexplained “Wow signal” in the above pic.

8. ETs enslave us Advanced ETI may already have solved problems related to food production and manual labor, but on the other hand, animals slaves are cheap, reliable, and versatile. Enslavement can also take many other forms, such as forcing us to adopt their belief system, using us for entertainment purposes (like trained seals), or simply keeping us as pets.

11. ETs kill us all for being a bad species In greater scheme of things, humans kinda suck. We have a penchant for ruining our environment, we’re currently wiping out other species left and right, and we’re making more of ourselves at a rate that is clearly unsustainable. All of this makes it hard to argue that we’d make for responsible and respectful citizens of the galaxy, and ETI may just decide that everyone (including residents of our own planet) would be far better off if homo sapiens just weren’t around.

1. ETs ignore us Just like in Star Trek, it’s possible that ETI has some kind of Prime Directive or other non-interference policy with other species for whatever reason, so that even if we did know for sure that they were out there, they might not respond to us. They also might not communicate simply because as a species, humans don’t interest them, whether it’s because of our primitive nature or violent tendencies or terrible taste in music or otherwise. Or, they could show up in the form of a giant alien probe, have a pleasant conversation with the whales, and then leave, without sparing us so much as a glance.

3. ETs change our philosophy No matter when or how we detect ETI, if it happens it’s going to cause massive changes to everything from science to religion, even if we find out nothing else besides the fact that ETI do exist. All of a sudden, we’re not alone in the universe, and Earth and everyone on it goes from being a super special place to just another planet covered with self-replicating and tolerably aware biological lifeforms. While most people would probably argue that this is a good thing to know, we’d have to seriously re-evaluate our existence, which has been unavoidably human-centric.

7. ETs eat us Everything has to eat something else, and aliens are no exception. Unfortunately, being high up on the food chain and intelligent enough to take care of ourselves means that we might be an ideal source of nutrition for carnivorous ETIs. Plus, there are lots of us to choose from and we seem to reproduce very rapidly, so the only question is, do we taste good? If the Kanamits from that Twilight Zone episode are to be believed, we sure do.

9. ETs steal our resources It would be hard to develop a galactic civilization without maintaining a society that operates on generally sustainable principles, but that doesn’t mean that Earth might hold some physical value for ETI. Maybe they sprinkle rare-earth metals on their cupcakes or something. It may also be that a rapidly expanding ETI needs to consume the resources of other planets just to sustain their greed for shiny things (think Avatar, but reversed), and while this level of expansion may not be feasible in the long term, that’s not gonna do Earth much good if an expansionist ETI finds us before they burn themselves out.

10. ETs destroy Earth because it’s in the way Even if ETI doesn’t necessarily wish Earth any harm, they may destroy our civilization and planet anyway if they think it would serve a greater good. For example, if Earth happens to be in the way of a Vogon hyperspatial bypass construction project that would lower the commute times of a countless aliens, could our paltry six or seven billion people really object to annihilation in the name of progress?

12. ETs kill us all by accident ETI may be different enough from humans that we don’t have to worry about catching a disease from them. In fact, this is very likely. But it also may be the case that an ETI is just barely not different enough that we catch something that loves humans but that our immune systems are totally unprepared for and it wipes us out. ETI could also destroy us by accident if they unwittingly introduce a tenacious invasive species to Earth that eats all of our quadrotriticale, or if they otherwise negatively modify our ecosystem without intending to.

13. ETs kill us all because they can Consider what happens when we humans encounter an intelligent species, like dolphins or chimpanzees: we hunt them for meat, we hunt them for sport, we lock them up and use them for research, we give them diseases, we dissect them, and we put them in zoos and teach them to do tricks, all because they’re not quite as smart as we are. It’s entirely possible, even likely, that a more advanced ETI might treat humans the exact same way. And if we have a problem with that, well, maybe we should consider how we treat other species.

16. ETs are unintentionally hiding from us It’s big galaxy out there, and an even bigger universe. We’re listening in for ETI signals all the time, but only in a few isolated parts of the spectrum that we think might be useful for communication, so it’s entirely possible that ETI is blaring away on a different spectral band that we’re not paying any attention to. Or maybe ETI is just too far away to detect us or be detected themselves. Or, our assumption that ETI is anything like us is inherently flawed, and it exists as some manifestation of vast hyperintelligent pan-dimensionality (like the little white mice from Hitchhiker’s Guide) and we can’t interact with it.

17. ETs are not there The final scenario to consider is that ETI is simply not out there, and that humanity is completely alone in the ‘verse. At this point, the only thing we have do disprove this is some math that says that the Universe is a frikkin’ huge place and no matter how infinitesimally small a probability life has to exist somewhere besides Earth, the sheer volume of space ensures that it probably does. But that’s just a supposition, and until we actually make contact for the first time, all we can do is watch the skies, and hope.

4. ETs help us solve our problems It’s likely that ETI will be much, much more advanced than we are. And it’s also likely that ETI will be generally cooperative, since it would be hard for a fundamentally uncooperative society to expand into space. This combination (which is the fantasy of everyone interested in alien life) might provide insights that could enable us to learn how to avoid a technological catastrophe like global nuclear war or climate change.

6. ETs are boring and/or annoying While we generally think of ETI as something profound, there’s no certainty of that. Perhaps we’d start receiving unintentional signals from an ETI similar to our own civilization 50 years ago, with no hope for two-way communication, just cryptic military signals and reruns of bad TV shows. Or, a bunch aliens might show up to our planet as needy refugees, requiring a significant amount of resources without offering much in return.

15. ETs are intentionally hiding from us Instead of ignoring us because of some type of Prime Directive noninterference doctrine, it’s possible that ETI are taking it to the next level and actively trying to keep us unaware of their presence. They might be hiding out in the asteroid belt spying on Earth, they might have cloaking devices that allow them to get closer still, or they could be lizard people living among us in disguise. One other intriguing possibility is that ETI have surrounded our entire solar system with a giant “virtual planetarium” that shows the universe as empty, when really it’s full of life.

14. ETs get us to kill ourselves If all ETI wants to do is destroy Earth, the cheapest and easiest way to get us to do it is to simply send a message that provides detailed instructions on how we can go about killing ourselves. This could be in the form of a technology that we are unready for, more deliberate directions telling us how to (say) construct a particle accelerator that will actually create a black hole that will destroy our entire planet, or even simply an alien presence that makes us all go nuts and kill each other.

5. ETs threaten us, we kick their ass As unlikely as it seems, it’s within the realm of possibility that humans could successfully repel an invasion of earf by more advanced ETI, whether it’s because we have some technology that the ETI doesn’t, or because some environmental factor defeats the ETI for us. In addition to forcing Earth to unite as a planet, after it’s all over, we’d be able to steal a bunch of cool new tech from all the crashed flying saucers. It’s also conceivable that a second ETI could come to our rescue, which would be pretty sweet.