Class N: Extremely high surface temperature, and an atmosphere inhospitable to most life.

Class D: Planetoid, moon or asteroid. Examples include Regula and Rura Penthe.

Class Y: Demon world. These worlds are not just inhospitable, but are generally lethal to most humanoid lifeforms. If life develops on such a world, it is frequently bizarre. Often the home of the Squalia.

Class I: Gas supergiant