It is a broad classification system with the goal of bringing some order to the likely rich variety of extrasolar gas-giant atmospheres.
Gas giants are split into five classes (numbered using Roman numerals) according to their modeled physical atmospheric properties.
[citation needed] The appearance of extrasolar planets is largely unknown because of the difficulty in making direct observations.
Sudarsky et al. listed Epsilon Eridani b, Upsilon Andromedae d, and 55 Cancri d as possible Class II planets.
These planets form cloud decks of silicates and iron deep in their atmospheres, but this is not predicted to affect their spectrum.
The Bond albedo of a class IV planet around a Sun-like star is predicted to be very low, at 0.03 because of the strong absorption by alkali metals.
[2] HD 209458 b at 1300 K (1000 °C) would be another such planet, with a geometric albedo of, within error limits, zero; and in 2001, NASA witnessed atmospheric sodium in its transit, though less than predicted.
[7][8] For the very hottest gas giants, with temperatures above 1400 K (2100 °F, 1100 °C) or cooler planets with lower gravity than Jupiter, the silicate and iron cloud decks are predicted to lie high up in the atmosphere.
[9] Sudarsky et al. listed 51 Pegasi b, Upsilon Andromedae b, HD 209458 b, and Tau Boötis b as possible Class V planets.