Sudarsky classification

I have a question about the Sudarsky classification system on hot Jupiters. Why are there two separate classes of hot Jupiters? I do not see much of a difference between a "class IV" and a "class V" that they require two different classes.

Remember that the Sudarsky classification was made when the only reasonably certain transiting planet was HD 209458b. The prediction was that the hottest of the hot Jupiters (class V) would have reflective clouds, while the cooler ones would have dark, absorption-dominated atmospheres (class IV).

The current observations suggest that all hot Jupiters have very low albedos, even the ones that show evidence of clouds, so putting them all in class IV might be the way to go for now... however observations indicate that the presence/absence of clouds is not a very easy thing to determine. There are indications that there is a difference between hot Jupiters at different temperatures (e.g. presence/absence of temperature inversion, how well the planets transport heat to the night side), which has led to a different classification system.

Wikipedia seems having a good page about this.

This classification system would be known as what now lazarus?

((So I can keep info up to date))

There's pM and pL class planets.

In pL class planets, the spectrum is dominated by absorption of H₂O, Na, and K. Photospheric pressures and temperatures prevail such that advective timescales and radiative timescales are similar. Since atmospheric dynamics will be important for the redistribution of energy, the consequences for the structure and thermal emission of these atmospheres will be quite complex. The efficiency of energy redistribution will vary with planetary irradiation levels, surface gravity, and rotation rate. pL Class planets will have smaller day/night temperature contrasts and measurable phase shifts in thermal emission light curves that will be wavelength-dependent. Also, these planets may show variability in secondary eclipse depth. TrES-1 b, HD 179949 b, and HD 189733 b are all consistent with this, and are thus pL class planets.

There's also a classification system based on the equilibrium temperature and the planet's Safronov number, defined as 1/2 * (planetary escape velocity/orbital velocity)^2, see e.g. this paper. The reasons why there should be two distinct groups is not yet clear: it might relate to the mode of migration (scattering versus disk interaction), or the history of mass loss from the planet, or perhaps something else.