Maybe it was really mega-mega-mega Death Star (sans weapons)! Maybe it started as a small artificial satellite that someone built a house on and it kept growing and growing and being built on, etc, etc.... Maybe the core is hollow and contains HUGE water holding/recycling tanks (I mean I assume they pee and stuff so it has to go somewhere) and then occasionally giant artifical geysers errupt 'rain' over areas of the planetoid to clean it up.