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

Class K: Planet that is barren and has no native life, but could be hospitable with terraforming or if buildings were constructed inside habitable domes. Mars is considered a Class K world.

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 T: Gas giant. Second smallest designation of gas giant, one size larger than Class J. Typically, Iffens occur here

Class S: Largest designation for gas giant. Ultra giant, not suitable for Iffens.