[ Good, good, questions are the lifeblood of learning. ]

M-class, as we have discussed, is terrestrial. D-class qualifies as a moon, H-class is uninhabitable, J- and T-class are gas giants — such as Jupiter and Saturn in your solar system — K-class is adaptable with pressure-domes, L-class is marginally habitable and N-class is sulfuric.

Y-Class is known as a Demon, inhospitable and dangerous even to passing starships.