EXCLUSIVE: Hollywood has been wondering exactly when Simon Cowell’s The X Factor debuting on Fox this fall would start adding some celebrity glitz. Now I’ve learned it’s coming soon. The X Factor judges will have one superstar work in each panelist’s respective home to whip their contestants into shape for the show. And Simon Cowell is in final negotiations to have Mariah Carey as his personal pick to mentor his contestants from his house. “He’s still finalizing her deal,” an insider tells me. The X Factor is pulling out all the stops to draw in other superstars as mentors. Contacts are being made now with “the Beyoncés, the Rihannas, all of them.” the insider tells me. The X Factor‘s UK original series reportedly has signed up Usher as a mentor. Mariah’s name has been mentioned in connection with the U.S. show from the beginning but her twins pregnancy prevented her from being one of the judges. This is an interesting way for her and other music pros to be showcased. The result is that The X Factor could outglitz both The Voice and American Idol by combining the best elements from each. The series will air Tuesday and Wednesday nights each week beginning September 21st.