Sudarsky's gas giant classification
Open-knowledge reference entry
Sudarsky's classification of gas giants for the purpose of predicting their appearance based on their temperature was outlined by David Sudarsky and colleagues in the paper Albedo and Reflection Spectra of Extrasolar Giant Planets and expanded on in Theoretical Spectra and Atmospheres of Extrasolar Giant Planets, published before any successful direct or indirect observation of an extrasolar planet atmosphere was made. 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. In the Solar System, only Jupiter and Saturn are within the Sudarsky classification, and both are Class I.
The appearance of planets that are not gas giants cannot be predicted by the Sudarsky system, for example terrestrial planets such as Earth and Venus, or ice giants such as Uranus (14 Earth masses) and Neptune (17 Earth masses).
Background
The appearance of extrasolar planets is largely unknown because of the difficulty in making direct observations. In addition, analogies with planets in the Solar System can apply to few of the extrasolar planets known because most are wholly unlike any of our planets, for example, the hot Jupiters.
Bodies that transit their star can be spectrographically mapped, for instance HD 189733 b. That planet has further been shown to be blue with an albedo greater (brighter) than 0.14. Most planets so mapped have been large and close-orbiting "hot Jupiters".
Speculation on the appearances of unseen extrasolar planets currently relies upon computational models of the likely atmosphere of such a planet, for instance how the atmospheric temperature–pressure profile and composition would respond to varying degrees of insolation.
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