This seasons finds the return of ''Gremlins,'' Die Hard,'' and ''Young Guns''

Hollywood was early on this Earth Day business. They started recycling a few years back — their hit movies, that is — and haven’t stopped since. This summer nine sequels are slated to hit theaters. As sequels have become an industry staple, coming up with a movie that can be spun out two, three, four times is like finding that pot of gold at rainbow’s end. It’s called building a franchise — James Bond, Rocky, Ghostbusters all qualify. Batman looks like a very profitable franchise. Expect the caped crusader to make a return to theaters within a couple of summers.

Executives who run large movie companies — and their shareholders — like sequels. So do audiences. Sequels are brand names. They are safe, familiar, fun. And summer movies are all about fun. Last year’s recycled troika of Indiana Jones and the Last Crusade, Lethal Weapon II, and Ghostbusters II combined for nearly half a billion dollars at the box office. Of course, some sequels eventually run out of gas. Karate Kid III took a roundhouse kick to the head last summer, a sign that even blockbusters have a life span.

Here is this year’s crop: Back to the Future Part III, Another 48 HRS., Gremlins 2: The New Batch, RoboCop 2, Die Hard 2, Exorcist III: Legion, Young Guns II, The Two Jakes, and Graffiti Bridge.