Simon says, "I'm leaving Idol."

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PASADENA, Calif. -- The big news keeps coming, with Fox jolting TV critics Monday with news that the seemingly indispensable Simon Cowell will be leaving American Idol after this season.

But wait, there's more. Cowell will remain with the network as the executive producer and judge of a new U.S. version of The X Factor, which he created and launched in the U.K. in 2004.

Cowell made a surprise appearance in person after reaching a final agreement with Fox just 50 minutes or so before its two top programming executives met with critics. He was typically clad in a gray v-necked sweater, jeans and black boots.

Cowell, whose caustic ways with Idol hopefuls have made him an international brand name, said he wanted a change after eight seasons with America's most popular prime-time attraction. He considered doing both shows, but "when we looked at the practicality of that, it was impossible," he said. "I felt like doing something different. I like a challenge."

Cowell literally signed his new deal on a hotel ballroom stage in the presence of Fox entertainment chairman Peter Rice, entertainment president Kevin Reilly and a gaggle of TV critics whose collective wealth likely couldn't match the Idol star's monthly take-home pay.

The "definitive plan" for X Factor is to premiere it in fall 2011 and retain Idol's annual January start date. Season 9 of Idol launches Tuesday. Taping of the show's Hollywood audition segments began Monday, making it necessary for Cowell to head off to them after spending about 15 minutes taking questions.

"I want to leave Idol this year bigger and better than it's been before," Cowell said before adding that America "needs a second show, a different type of show. I'm going to put my absolute heart and soul into this (X Factor) to try to make it as good as possible."

Idol has a major change of its own this season, with Ellen DeGeneres replacing charter judge Paula Abdul, who quit over a salary dispute. Cowell noted that X Factor has no upper age limit on its contestants.

"You could literally be 100 years old," he said. "So Paula could audition."

But Cowell later tossed a bouquet Abdul's way, saying with seeming sincerity, "I adore Paula. Whatever happens, I will work with her in some capacity, because I miss her."

Fox's Reilly said it's possible that X Factor's presence and Cowell's future absence could result in "some cannibalization" of Idol's audience. But Rice exuded more of an Abdul-esque optimism.

"The American public loves American Idol," he said. "Simon is irreplaceable. So it's going to be incumbent on us to make sure that the show remains vital and entertaining."

In searching for Cowell's replacement, "we're not going to be looking for a Simon clone to do what he does," Rice said.

Still, a possible heir could be the sharply opinionated Piers Morgan, one of three judges on prime-time's most popular summertime show, NBC's America's Got Talent. Cowell's Syco production company, a joint venture with Sony Music Entertainment, owns that show, too. So it would be an easy enough transition.

Cowell also will continue to produce and judge the U.K. version of X Factor, which discovered singing sensation Susan Boyle last season.

"We hope on (the new) X Factor, we're going to find the American equivalent of Susan, Cowell said.