Sudarsky Extrasolar Planet Classification

The Sudarsky extrasolar planet classification system is a theoretical model-based classification system for predicting the appearance of extrasolar gas giants based on their temperature. It was outlined by David Sudarsky et al. 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, HD 85512 b (3.6 Earth masses) and OGLE-2005-BLG-390Lb (5.5 Earth masses); or ice giants such as Uranus (14 Earth masses) and Neptune (17 Earth masses).