R+B superstar Janet Jackson has cast doubt on her brother MICHAEL's participation in a planned Jackson 5 comeback tour, because she doesn't know if he's ready to go back to singing in a group. Elder brother Jermaine Jackson has voiced plans for the ABC hitmakers to embark on a reunion tour later this year (08), complete with the Thriller superstar - but Janet isn't convinced. She says, "I know it's been kind of tough for Mike. Maybe he just wants to continue to be a solo artist and if that's what he wants, you have to respect his wishes. "But what an impact it (a reunion) would have on the world. (I'd) be the first in line to buy a ticket." Michael is allegedly in talks to revive his pop career with a London residency this summer (08), but reports have as yet been unconfirmed.