I believe it may have been a series so some details may be mixed between the books. I read it maybe 10 years ago.
It starts out with the protagonist waking up in the middle of the night to find his bum missing. He hears some sounds and climbs out of his window and (follows his bum?) to a stadium where a load of bums have come together to have a meeting about taking over the world.
At some point in the story the protagonist leaves Earth to go to Uranus (obviously!) and then comes back to Earth to find it is almost completely taken over by the bums.
I believe they end up stopping the bums from taking over the Earth (doing so by letting out a synchronised fart?).
By bum I mean one's gluteus maximus