It's official: David Tennant is leaving as the star of Doctor Who, the BBC's hit series about an eccentric time traveler who roams the universe in a phone booth, righting wrongs. Tennant will stay on for four special one-hour episodes, to air in 2009 and early 2010, and then leave. This means his exit will be written (or co-written) by producer Russell T. Davies, who's also leaving after those four specials. And it means new producer Steven Moffat will have a blank slate, including a new lead actor, when he takes over in 2010. Here's a snippet of the video from the BBC site, where Tennant explains why he always intended to leave after three seasons, and why he wanted to leave along with RTD. [BBC]