|IAU artist rendering of an exoplanet, seen from its moon.|
The International Astronomical Union is, for this planet anyway, the official body that determines what objects in our solar system are called. They are the ones who revoked Pluto's status as a planet in 2006, to the consternation of school children everywhere. Here you can find the IAU's official explanation of why Pluto got reclassified, and what category it currently fits in. Here is the overview of Astronomical naming conventions, plus the rules for naming exoplanets, which can be helpful to Traveller referees who need a quick name for some remote and uninhabited rock where the PCs are meeting some shady contact.
Remember that regardless of what world generation system you are using, any solar system can have satellites and minor planets in addition to the main world, and any of them can be adventure locations. Get your PCs out there and start exploring.