Class N: Extremely high surface temperature, and an atmosphere inhospitable to most life.
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 L: Habitable planet that supports primitive life. This can be very simple fauna, or even life as advanced as a Bronze Age culture.
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.