Robert Downey Jr. has dropped out of Cowboys & Aliens, the Dreamworks/Universal sci-fi project that would have reunited the actor with his Iron Man director Jon Favreau. The news was first reported by the Los Angeles Times but only speculated on why Downey had left the project, which pits Old West cowboys and indians against a group of aliens with superior technology. Now, a source inside the production tells that the star has a scheduling conflict with a Sherlock Holmes sequel. That film has not yet been greenlit by Warner Bros., but it is being fast-tracked based on the success of the first film. (Sherlock’s worldwide gross is fast approaching $300 million.) Warner Bros. commissioned a script by Kieran and Michele Mulroney back in September and is now locking down the cast for a June start date, exactly the time when Cowboys & Aliens is set to begin principal photography. Producers behind Cowboys & Aliens are sticking with original plans and are now looking for a new lead to play cowboy Zeke Jackson, the hero of the 2006 graphic novel from Platinum Studios.