The choice of L and T is explained in Kirkpatrick et al. (1999) 'Dwarfs Cooler than "M": The Definition of Spectral Type "L" Using Discoveries from the 2 Micron All-Sky Survey (2MASS)'. The principles behind the choice are given at the start of section 5.1:
In choosing a letter designation for the new spectral class,
three important points must be considered : (1) The letter
must be unambiguous, having not been used for any currently recognized spectral type. For example, though "N"
follows "M" in the alphabet, it would be a poor choice of
letter since it is used for a class of carbon stars. (2) The letter
must create a taxonomy that is clearly distinguished from
other types of astronomical objects. In this case, the letter
must be accepted by the entire community, both by
researchers involved in low-mass star and brown dwarf
science and by astronomers in general. Although stellar
spectroscopists might find "E0," "E1," "E2," etc., perfectly
acceptable as new spectral subclasses, extragalactic morphologists already recognize these designations as elliptical
galaxy types. (3) The letter must stand the test of time. For
example, choosing "D" to mean that these objects are
"degenerate" brown dwarfs would be flawed reasoning.
Some of these dwarfs certainly are substellar (see § 7), but
such a designation cannot be tied uniquely to any particular
spectroscopic trait. The designation should apply to spectral features alone and be free of physical interpretation.
Our understanding of the underlying physics may change
with time ; our choice of letter should be impervious to such
changes.
Applying these, they find the letters H, L, T and Y to be ok: the rationale for excluding the other letters is given in their Table 5:
| Letter |
Status |
Notes |
| A |
In use |
Standard spectral class |
| B |
In use |
Standard spectral class |
| C |
In use |
Standard carbon-star class |
| D |
Ambiguous |
Confusion with white dwarf classes DA, DB, DC, etc. |
| E |
Ambiguous |
Confusion with elliptical galaxy morphological types E0–E7 |
| F |
In use |
Standard spectral class |
| G |
In use |
Standard spectral class |
| H |
OK |
|
| I |
Problematic |
Transcription problems with I0 (10, Io) and I1 (11, II, Il) |
| J |
In use |
Standard carbon-star class |
| K |
In use |
Standard spectral class |
| L |
OK |
|
| M |
In use |
Standard spectral class |
| N |
In use |
Standard carbon-star class |
| O |
In use |
Standard spectral class |
| P |
Problematic? |
Incorrect association with planets? |
| Q |
Problematic? |
Incorrect association with QSOs? |
| R |
In use |
Standard carbon-star class |
| S |
In use |
Standard spectral class for ZrO-rich stars |
| T |
OK |
|
| U |
Problematic? |
Incorrect association with ultraviolet sources? |
| V |
Problematic |
Confusion with vanadium oxide (V0 vs VO) |
| W |
Ambiguous |
Confusion with Wolf-Rayet WN and WR classes |
| X |
Problematic |
Incorrect association with X-ray sources |
| Y |
OK |
|
| Z |
Problematic? |
Incorrect implication that we have reached "the end"? |
They go with "L" as it is the closest letter to "M" that is still available. They prefer it to "H" because of the occasional use of the term "hydride dwarf" to refer to M subdwarfs which are dominated by CaH and MgH bands.
After that, the sequence is going alphabetically through the remaining available letters, so they put Gl 229B, which has a notably different spectrum to the other (L-class) brown dwarfs known at the time, into spectral class T. Spectral type Y took a few more years to show up in observations, and was chosen by the same principle.
If observations get to the point where it is worth erecting a new spectral class beyond Y then things will get interesting.
