The Los Angeles Times is reporting that Robert Downey Jr. has dropped out of director Jon Favreau's Cowboys & Aliens, DreamWorks' adaptation of the graphic novel that was scripted by Alex Kurtzman, Roberto Orci and Damon Lindelof.

Cowboys & Aliens will still move forward with Favreau. But Downey has fallen off the project, and studio DreamWorks is seeking a new hero to replace him. Production is tentatively set to begin this year for a planned release in summer 2011.

The sci-fi Western focuses on outer-space creatures who land in Arizona circa the mid-19th century intent on enslaving the local population -- until a group of cowboys and Apache Indians fight back.