Currently the planet types are hard-coded, so that modding, expansion, or adjustment are unnecessarily difficult.
Rather the planet type's:
* which other types it can be directly terraformed into,
* what sizes it can have, etc.
... should be definable in a text file like so much of FO's other data.
In addition to changes in the EP wheel (which may not be popular with some developers) this would allow experimentation and easy implementation other of additions to the galaxy such as asteroid belts of varying sizes, and gas giants with different sizes and types.