Class H: Uninhabitable desert world. Tau Cygna V is one such planet.

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

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.

Class I: Gas supergiant

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