Individuals do not own planets. If an individual owns more than 50% of the flats then it will be uncontrolled. In the case of a comet, or other body that is one 1 square, whoever owns the city basically has control.
Be aware that whoever controls the system (or if there's no system control, sector) will still have A/E jurisdiction for the comet if it is uncontrolled (I think. Haven't paid as much attention to the new A/E rules).