With Iron Man already established as a movie franchise and his Sherlock Holmes looking like a second surefire series, Robert Downey, Jr. has decided not to star in the comic book-derived Cowboys & Aliens, which will be directed by Iron Man helmer Jon Favreau.  Iron Man writers Mark Fergus and Hank Ostby wrote the original adaptation of the Platinum Comics-based project, which has 19th Century cowboys teaming up with Apaches to thwart an alien invasion of Arizona, and which Dreamworks hopes to turn into a franchise of its own.

According to the L.A. Times, Dreamworks and Favreau intend to move forward on Cowboys & Aliens, which they hope to get into production later this year with an eye toward a summer 2011 debut.  Downey is about as hot as you can get in Hollywood now, and scheduling conflicts, perhaps even one caused by an as-yet-unannounced Sherlock Holmes sequel, are believed to be the cause for his departure from the project.