Your home planet is known as your Homeworld or home world and is the planet that you start with. All players in most universes start with 163 fields on their home planet. (Plus the bonus amount based on the universe settings).
Many players keep this original planet even though it is smaller than others, since they have developed it before getting a colony. However, some with a long-term view in mind will delete it as soon as they get a colony so as to focus on getting a planet with more fields.