Ridley Scott's Alien is Horror

This apparently contentious topic was brought up when a Twitter user issued a poll on whether or not Alien is a horror movie. Although the vast majority of participants voted that Alien is indeed a horror movie, several prominent voices took to the comments to argue otherwise. 

The original pollster argued that Alien is categorically not horror, because “horror cannot be set in space.” 

Another user furthered this line of reasoning by stating: “No. Horror needs to be grounded in an earthly setting, to give you a sense that it could happen to you. Using Sci Fi as a vehicle for the story, it loses that connection. It’s just a scary futuristic film.”

Others continued on, one saying “No it’s a sci-fi monster movie. Their [sic] are no ghosts, serial killers or zombies. Just because a movie is scary, doesn’t make it a horror film.”