Sudarsky's gas giant classification
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.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.