It is very likely that this classification scheme is not based on the
English alphabet but instead on the initial letters of the Vulcan
words that describe planet's atmospheres. For example, the Vulcan
word "Min-shara" (which apparently translates to something like
"suitable for humanoid life") has probably been shortened to the class
letter "M". They are alwats talking about class M planets in Star
Trek.
Other clues that the classification is not based on the English
alphabet include:
1. The fact that letters appear in the classification system which
are not even in the English
alphabet. -- (Norcadia Prime is a class Theta planet).
2. Even though the order of letters from H to M in the English
alphabet includes J,
J cannot be included in the habitable range of planets. J refers
to gas giants, and gas giants
cannot reasonably be thought to be habitable.
3. Conversely, even though in English D doesn't fall between H and
M, Class D planets are
considered habitable.
If we arrange the sequence from the least habitable worlds to the most
habitable, I'd guess that
the sequence must go something like:
J T Y C N H D K L Th M
I'd like to think this is a partial representation of the order of
letters in the Vulcan alphabet. At the very least it might give us a
clue to new Vulcan words that describe the habitability of planets.