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

Class A: Small Rocky Planetoid - Similar to Pluto

Class M: Earth-like both in its size, core, and age as well as in the advancement of the native creatures. Class M worlds are frequently the home of sapient races capable of traveling in space.

This is the main planet of occupation. Human Beings live here for the most part.

Class L: Habitable planet that supports primitive life. This can be very simple fauna, or even life as advanced as a Bronze Age culture.

Class I: Gas supergiant