There is also the "Invalid" group, which surprisingly is a legitimate research class. There's Asteroid, Terran, Desert, Volcanic, Ice, Uncolonizable, and Invalid avaliable, plus the game allows for a single custom planet type.
Sins of a Galactic Empire made the extreme-value worlds like Ecumenopoli a Tier 6 or 7 unlock so a player can't just rush them and use their extreme income and logistic slot counts to achieve an unbeatable lead early on.
Tier 4 (aka. 5 research stations) plus large militias, to be exact. Credit for that goes to NovaCameron; SoGE makes use of a slightly modified Novus Universum.