The books are fascinating. They involve common subjects, such as rebellion, destruction, fear, insanity, and offer a very dark dystopic image Earth, where 5 teenagers take on an entire race trying to take over the world infesting humans and controlling them with little slug-like Yeerks. Over time, I started to use appeal to my literary side to recognize motifs and understand what these works of fiction say about ourselves. Then my physics brain kicked back in, and questioned how realistic this scenario is. Let's talk about that side.
First of all, do extra-terrestrials even exist?
I think so. The Universe is so unbelievably large, and there are so many more planets than we really know about. Consider this: Each star has its own set of planets. In order for life to exist, there needs to be a planet with an environment that can support living things. One key ingredient comes to mind: liquid water.
Since all living things depend on liquid water, it's only a matter of billions of years (not really a long time considering the life of the Universe) before life can form on its own. All it takes is one star with a planet around 1 AU away, and voila! We will have life. In 1961, Frank Drake considered the projected number of stars, the likely number of planets for each star, and he constructed an equation depicting the probability of extra-terrestrial life, and it does not take an optimist to exceed 99%. So yes, I think it is smart to say that extra-terrestrials exist.
So, where are these extraterrestrials, and why haven't they contacted us, yet?
Enrico Fermi submitted this question in 1951 as the "Fermi Paradox" (which is not really a paradox), but it is an interesting question. With so many possibilities for extraterrestrial life, one would expect some kind of contact with humans at some point, so where are they?
There are a couple of answers to this question. One answer is that they don't know about us, or don't have the technology to reach us. Then you have to ask why they would even consider coming to Earth if they could. Not only are we so far away from other stars in the galaxy, but what about Earth would entice anybody to visit us? This planet might not have anything to offer that more local regions don't already have. If aliens could visit this planet, they could probably access better resources elsewhere.
There has to be at least one extra-terrestrial species that could get to us easily, right?
Conceptually, maybe. This assumes that they are far more advanced than we are, but then you have to ask again why they would want to talk to us. If you are outside listening to music, and you see a group of ants on a hill, do you rush over to the ants, and try to show them the wonders of your technology? Probably not. You don't care about ants because they can't learn anything from you, anyway. You'd probably rather just smush them in the ground or spray them with Raid. In the grand scheme of things, us humans might only be seen as ants in the universe.
What if they want to take over the world and enslave us?
Not likely. The problem is that in most movies, we portray aliens in our image. Hollywood tells us that they walk upright on 2 legs, speak perfect English (and in the American accent), are power-hungry for dominance, and have the same psychological needs as us. Talk about ego. Think about the millions of species just on our own planet, and the differences between ourselves and the rest. Cats don't think like us; nor do dogs. There's nothing that says that aliens have to think like humans. Imagine a mothership landing and out comes a furry 4-legged animal wanting to be petted and fed. Not what Hollywood told you, but no less likely than a big-headed, bipedal sapient.
More likely, an extra-terrestrial species will be something microscopic, like a bacteria or protist. Very boring, and unexciting, but likely. The evolution of animals and plants occurred only through some very lucky circumstances, and not all planets might have the temperate conditions for a large species to come about.
It is overwhelmingly likely that aliens exist, and that they do not exist in ways that we might expect. The reality of it is that we probably do not have to worry about a violent invasion. When we do find extra-terrestrial life, many will probably be disappointed, as it will more likely be something small and insignificant, like a bacteria or algae, but it could pave the road for the future of humanity. There's an entire ocean out there, and we're still stuck on the dock known as Earth.