Aliens in movies

Aliens have become some what of an essential part of a good fantasy movie! There have been many movies which are based on creatures and things from other worlds or planets, there also have been many different storylines and ways of portraying these beings! I have tried to pick a few of the most recognisable ones ie: E.T,predator, Alien and Avatar! These movies differ quite a lot from each other. E.T is a children's twist on a alien fantasy film, with the alien being friendly and if I must say cute. E.T gets lost when a space ship leaves him and he basically needs help on returning to his home  but a young boy connects with the alien and becomes good friends. Now let's look at the Predator and Alien movies. These are quite gory and aimed more at teens and adults. These movies consist of a lot of violence, blood and occasionally sex! In the movie "Alien vs Predator" these two creatures basically try to annihilate each other throughout, in the end Predator comes out on top managing to defeat its enemy! Finally Avatar is very unique and different from any other movie to do with other worlds, the movie is about a place called Pandora which is a parallel world where animals and plants have evolved! There are human like beings in the movie which are the actual Avatars who accept a man who gets turned into one and falls in love with a female Avatar. But all goes wrong when humans interfere with Pandora and cause havoc and wreck there sacred land, this then leads to a huge battle between the humans and Avatars! I'd say this movie has a more broader audience being aimed at family's, children and teens! I believe the reason behind all these very different stories are because no one actually know what else is out there and if there is a larger threat than us humans...