Hollywood can't stop, won't stop making science fiction movies.

From "Star Trek" to "Star Wars," studios will boldly go to that galaxy far, far away as long as audiences are willing to pay for it. And while PG-13 genre fare tends to sell more tickets, movie fans also turn out for titles rated "R." (See the "Alien" and "Terminator" franchises.)

As Ryan Reynolds gets his body swap on in the new R-rated "Self/Less," here are 17 must-see films for the must-be-17-or-older crowd.