Fox and the producers of American Idol say Randy Jackson is leaving the show after 13 seasons. In a statement yesterday, the network and producers call Jackson a key part of the singing contest and say he'll be welcomed back as a visitor. Jackson, Simon Cowell, and Paula Abdul made up Idol's original judging panel. Jackson served as a judge for 12 seasons, with "dawg" becoming his trademark word while assessing contestants. He moved to the role of mentor this year. His departure leaves host Ryan Seacrest as the only original cast member. Idol is set to return in January with judges Jennifer Lopez, Harry Connick Jr., and Keith Urban. (More American Idol stories.)