Question: Which season of American Idol featured the headliner of the Hello Katy Tour as a guest judge?

full_context: Document 1: Katy Perry: Part of Me: The film features interviews with Perry and her loved ones documenting the trajectory of her life, containing various clips from her childhood and teenage years as well as her career and personal life. The film is spliced with performances from her worldwide California Dreams Tour, which had 127 concerts from February 20, 2011 to January 22, 2012. Most of the performances were recorded on November 23, 2011, at the Staples Center in Los Angeles, but also included performances in Tokyo and São Paulo. Some of her friends such as Rihanna, Lady Gaga, Adele and Jessie J make cameos in the film. The documentary includes scenes of Perry dealing with the breakdown of her marriage with English actor/comedian Russell Brand.
Document 2: American Idol: Beginning in the tenth season[citation needed], permanent mentors were brought in during the live shows to help guide the contestants with their song choice and performance. Jimmy Iovine was the mentor in the tenth through twelfth seasons, former judge Randy Jackson was the mentor for the thirteenth season and Scott Borchetta was the mentor for the fourteenth and fifteenth season. The mentors regularly bring in guest mentors to aid them, including Akon, Alicia Keys, Lady Gaga, and current judge Harry Connick, Jr..
Document 3: I Kissed a Girl: ``I Kissed a Girl ''Single by Katy Perry from the album One of the Boys Released April 28, 2008 (2008 - 04 - 28) Format CD single digital download Studio Dr. Luke's Studios Legacy Recording Studio (New York City) Conway Recording Studio (Hollywood) Genre Pop rock Length 3: 00 Label Capitol Songwriter (s) Katy Perry Lukasz Gottwald Max Martin Cathy Dennis Producer (s) Dr. Luke Benny Blanco Katy Perry singles chronology`` I Kissed a Girl'' (2008) ``Hot n Cold ''(2008)`` I Kissed a Girl'' (2008) ``Hot n Cold ''(2008) Music video`` I Kissed a Girl'' on YouTube
Document 4: Fearless Tour: The Fearless Tour was the debut concert tour by American singer-songwriter Taylor Swift. Also referred to as the Fearless Tour 2009 and the Fearless Tour 2010, it was launched in support of her second studio album ""Fearless"" (2008). On the tour, she was joined by guests Kellie Pickler and Gloriana. Teen singer Justin Bieber joined her as an opening act for the tour when she went to England in November 2009. During the tour, Swift performed with guests such as John Mayer, Katy Perry, and Faith Hill. It grossed $63,705,590 from 89 reported shows and an audience of 1,138,977.
Document 5: American Idol: With the exception of seasons one and two, the contestants in the semifinals onwards perform in front of a studio audience. They perform with a full band in the finals. From season four to season nine, the American Idol band was led by Rickey Minor; from season ten onwards, Ray Chew. Assistance may also be given by vocal coaches and song arrangers, such as Michael Orland and Debra Byrd to contestants behind the scene. Starting with season seven, contestants may perform with a musical instrument from the Hollywood rounds onwards. In the first nine seasons, performances were usually aired live on Tuesday nights, followed by the results shows on Wednesdays in the United States and Canada, but moved to Wednesdays and Thursdays in season ten.
Document 6: American Idol: The fourteenth season premiered on January 7, 2015. Ryan Seacrest returned to host, while Jennifer Lopez, Keith Urban and Harry Connick, Jr. returned for their respective fourth, third and second seasons as judges. Eighth season runner-up Adam Lambert filled in for Urban during the New York City auditions. Randy Jackson did not return as the in-house mentor for this season.
Document 7: I Kissed a Girl: ``I Kissed a Girl ''is a song recorded by American singer Katy Perry for her second studio album, One of the Boys (2008). It was released on April 28, 2008, by Capitol Records as the lead single from the record. Perry co-wrote the song with Max Martin, Cathy Dennis, and its producer Dr. Luke, with additional production from Benny Blanco.`` I Kissed a Girl'' is a pop rock song with elements of new wave. Perry stated its lyrics are ``about the magical beauty of a woman ''. The song sparked controversy for its homosexual themes.
Document 8: I Kissed a Girl: ``I Kissed a Girl ''is a song recorded by American singer Katy Perry for her second studio album, One of the Boys (2008). It was released on April 28, 2008, by Capitol Records as the lead single from the record. Perry co-wrote the song with its producers Dr. Luke and Benny Blanco, with additional songwriting provided by Max Martin and Cathy Dennis.`` I Kissed a Girl'' is a pop rock song with elements of new wave. Perry stated its lyrics are ``about the magical beauty of a woman ''. The song sparked controversy for its homosexual themes.
Document 9: American Idol: The show had originally planned on having four judges following the Pop Idol format; however, only three judges had been found by the time of the audition round in the first season, namely Randy Jackson, Paula Abdul and Simon Cowell. A fourth judge, radio DJ Stryker, was originally chosen but he dropped out citing "image concerns". In the second season, New York radio personality Angie Martinez had been hired as a fourth judge but withdrew only after a few days of auditions due to not being comfortable with giving out criticism. The show decided to continue with the three judges format until season eight. All three original judges stayed on the judging panel for eight seasons.
Document 10: Hello Katy Tour: The Hello Katy Tour was the debut concert tour by American singer Katy Perry, in support of her second studio album "One of the Boys" (2008). The tour visited North America, Europe, Asia, and Australia.
Document 11: Katy Hudson (album): Katy Hudson is the eponymous debut studio album by American singer Katy Hudson (later known as Katy Perry). It was released on February 8, 2001 by Red Hill Records. The album primarily incorporates Christian rock and contemporary Christian music elements with lyrical themes of childhood, adolescence, and Hudson's faith in God. Before its release, Red Hill went bankrupt, preventing it from marketing and promoting the album, which subsequently sold fewer than 200 copies and received mixed reviews.
Document 12: American Idol: American Idol employs a panel of judges who critique the contestants' performances. The original judges were record producer and music manager Randy Jackson, pop singer and choreographer Paula Abdul and music executive and manager Simon Cowell. The judging panel for the most recent season consisted of country singer Keith Urban, singer and actress Jennifer Lopez, and jazz singer Harry Connick, Jr. The show was originally hosted by radio personality Ryan Seacrest and comedian Brian Dunkleman, with Seacrest continuing on for the rest of the seasons.
Document 13: My World Tour: The My World Tour was the debut concert tour by Canadian recording artist Justin Bieber. It is Bieber's first concert tour which supports his two-part debut album "My World" and "My World 2.0". The tour was for the first time rocky teswin officially announced on March 16, 2010, a week before "My World 2.0" was released. The tour has multiple legs with the supporting acts of Mindless Behavior, Sean Kingston and Jessica Jarrell on North American dates and pop girl group The Stunners also joined the tour for the first twenty dates. Jasmine Villegas joined the tour as opening act for the second leg. The first leg of the tour is estimated to have grossed $35.6 million post inflation. The tour made a total of $53,341,886 worldwide.
Document 14: American Idol: In season eight, Latin Grammy Award-nominated singer–songwriter and record producer Kara DioGuardi was added as a fourth judge. She stayed for two seasons and left the show before season ten. Paula Abdul left the show before season nine after failing to agree terms with the show producers. Emmy Award-winning talk show host Ellen DeGeneres replaced Paula Abdul for that season, but left after just one season. On January 11, 2010, Simon Cowell announced that he was leaving the show to pursue introducing the American version of his show The X Factor to the USA for 2011. Jennifer Lopez and Steven Tyler joined the judging panel in season ten, but both left after two seasons. They were replaced by three new judges, Mariah Carey, Nicki Minaj and Keith Urban, who joined Randy Jackson in season 12. However both Carey and Minaj left after one season, and Randy Jackson also announced that he would depart the show after twelve seasons as a judge but would return as a mentor. Urban is the only judge from season 12 to return in season 13. He was joined by previous judge Jennifer Lopez and former mentor Harry Connick, Jr.. Lopez, Urban and Connick, Jr. all returned as judges for the show's fourteenth and fifteenth seasons.
Document 15: American Idol: Guest judges may occasionally be introduced. In season two, guest judges such as Lionel Richie and Robin Gibb were used, and in season three Donna Summer, Quentin Tarantino and some of the mentors also joined as judges to critique the performances in the final rounds. Guest judges were used in the audition rounds for seasons four, six, nine, and fourteen such as Gene Simmons and LL Cool J in season four, Jewel and Olivia Newton-John in season six, Shania Twain in season eight, Neil Patrick Harris, Avril Lavigne and Katy Perry in season nine, and season eight runner-up, Adam Lambert, in season fourteen.
ground_truth: season nine
sub_queries: Hello Katy Tour >> performer Answer: Katy Perry
Which season of American Idol featured Katy Perry as a guest judge? Answer: season nine