American Idol may have lost Paula Abdul this season, but they have a pretty impressive list of guest judges to take her place. According to a press release, when auditions come to Los Angeles, Katy Perry and Avril Lavigne will be sitting in with Simon, Randy and Kara, listening to aspiring superstars. Other musicians that will also guest judge during the season include Joe Jonas, Shania Twain and Mary J. Blige. The ninth season of American Idol airs on FOX starting in January.