Aliens are perhaps unique among sci-fi/fantasy creatures for being almost as plausible as you or I. Thirty-four percent of Americans believe that UFOs are real – and 1 in 7 believes they have seen one with their own eyes. Here are the three most popular categories of aliens in sci-fi/fantasy; whether or not they exist beyond novels and movies is for you to decide.

Type 1: The Unidentified Flying Object
Often the result of a suspected government cover-up, this is the kind of alien that people in the real world claim to see most often. Strange objects have been spotted in the skis since ancient times, usually described as angels or supernatural omens by civilizations before the invention of the telescope. UFOs became popular in the modern era during WWII with rumors of European “ghost rockets.” While this kind of alien has many possible applications in fiction, it unusually morphs into one of the following two categories as a story progresses.

Type 2: The Green Bobblehead
The term “little green men” (originally, somewhat ironically, used to describe Martians) became popular during the 1950s, after two rural Kentucky men described an encounter with such a being. With their smooth, grey-green skin and oversized eyes and ears, no human can resist the charm of a green bobblehead (or “grey,” in believers’ circles). E.T. is perhaps the most famous example of this kind of alien, though specimens abound in everything from Indiana Jones to Men in Black. More likely to be friendly to humans than UFOs or AIs.

Type 3: The Apocalyptic Invader
The most feared alien incarnation, the apocalyptic invader is also the most common in popular fiction. Though AIs can take many forms – from plant-like (think “Little Shop of Horrors”) to reptilian to humanoid – they usually share some basic traits. Many times, they want to colonize our world for their own uses (either to replace a home planet that has run out of natural resources or as a weapons base for attacking other species), pack substantial firepower, and possess supernatural abilities like above-human strength and the capacity to read minds. Occasionally, however, an AI is deceptively small – something like a deadly bacteria or nanobot.

Reading assignment for the week: War of the Worlds, by H.G. Wells. Just make sure to remember that aliens aren’t really attacking the earth.