The upcoming sci-fi- western, Cowboys & Aliens sounded like a match made in heaven for Iron Man director Jon Favreau and star Robert Downey Jr. The duo was set to work together again on the live action adaptation of the graphic novel that had potential to become another full fledged franchise for the pair, but according to The Los Angeles Times Downey is no longer involved.

Cowboys & Aliens is a graphic novel that centers 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. Downey was set to star as the group’s leader, a gunman named Zeke Jackson. Even though the actor is no longer on board Favreau is still attached to direct and DreamWorks is actively seeking another actor to replace him in the lead role.

The cause behind his decision to leave the film might have something to do with his ridiculous workload. The actor is already at the head of the Iron Man franchise, and the star of a potential Sherlock Holmes sequel. His decision to leave Cowboys & Aliens has sparked rumors that he might be gearing up to work on the Holmes follow up sooner than later. After Iron Man 2, his next major action film will be The Avengers movie and that doesn’t come out until 2012. Therefore, he’s got time to get Sherlock in if he starts early.

What do you think about Downey leaving Cowboys & Aliens? Who could replace the actor?