Sudarsky's gas giant classification refers to a classification system created by David Sudarsky and colleagues in the paper "Albedo and Reflection Spectra of Extrasolar Giant Planets". The classification's purpose is to determine the appearance of giant extrasolar planets based on their temperature.
The appearance of extrasolar planets is unknown due to the extreme distance of these worlds and the difficulty in making direct observations towards them. It is also difficult to make a direct analogy with planets in the Solar System, as many of the extrasolar planets that have been found are wholly unlike our own - for example, the hot Jupiters.
In the Solar System, Jupiter and Saturn are Class I planets - terrestrial planets such as the Earth or Venus, and ice giants such as Uranus or Neptune, can not be classified or predicted by this system.
Classes[]
Class I: Ammonia clouds[]
A Class I gas giant rendered in Celestia.
Planets in this class have appearances dominated by ammonia clouds. They have temperatures of around 150 K (-120 °C; -190 °F), and are found in the outer regions of a planetary system. The predicted bond albedo of a class I planet around a star like the Sun is 0.57, compared to 0.343 for Jupiter, and 0.342 for Saturn. The temperatures for a class I planet require either a distant orbit or a cooler star. It is hard to detect these planets as the former may mean the orbit is so large that their effect is too subtle to be detected, and the latter may be too dim to be visible. Also, in the case of the former, if a planet has more mass than Jupiter, it would have more internal heating, which could bump it up to a higher class.
The planets Jupiter and Saturn are classified as "Class I" planets. 47 Ursae Majoris c, 47 Ursae Majoris d, and 55 Cancri d may also be Class I planets.
Class II: Water clouds[]
A Class II gas giant rendered in Celestia.
Planets in this class are too warm to form ammonia clouds, and as such would be dominated by clouds made up of water vapor. While the clouds on these would be similar to Earth's, the atmosphere would still mainly consist of hydrogen and hydrogen-rich molecules, such as methane. The planets would be around 250 K (-23 °C; -10 °F), and have slightly more reflective clouds. The predicted Bond albedo for a class II planet around a star like the Sun is 0.81.
Some extrasolar planets that could classify as a "Class II" planet are: HD 45364 b, HD 45364 c, HD 28185 b, and Gliese 876 b.
Class III: Cloudless/No clouds[]
A Class III gas giant rendered in Celestia.
Planets in this class would not have any clouds because they don't have suitable chemicals in their atmosphere. These planets would thus appear as a featureless azure-blue ball due to Rayleigh scattering and absorption by methane in their atmospheres, and would appear essentially like a giant version of Uranus or Neptune. They have temperatures ranging between 350 K (80 °C; 170 °F) to 800 K (980 °F, 530 °C) and would have a low Bond albedo (around 0.12) due to a lack of reflective cloud layer. These would exist in the inner regions of a planetary system, roughly in the same place as Mercury.
Possible "Class III" planets are HD 37124 b, HD 18742 b, 55 Cancri c, Kepler-89e, and COROT-9b. Above 700 K (430 °C; 800 °F), sulfides and chlorides may provide cirrus-like clouds.
Class IV: Alkali metals[]
A Class IV gas giant rendered in Celestia.
In planets of this class, carbon monoxide would become the dominant carbon-carrying molecule in its atmosphere, rather than methane, due to the extreme temperatures of 900 K (630 °C; 1160 °F). In addition, the abundance of alkali metals such as sodium substantially increase, and spectral lines of sodium and potassium are predicted to be prominent in a gas giant's spectrum. 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 this planet would be very low; around a Sun-like star, they would have an albedo of 0.03 due to strong absorption by alkali metals. Gas giants of IV and V type are referred to as "hot Jupiters".
Examples of a class IV planet could be 55 Cancri b and HD 209458 b.
Class V: Silicate clouds[]
A Class V gas giant rendered in Celestia.
The hottest of gas giants, with temperatures of 1400 K (1100 °C; 2100 °F), the silicate and iron cloud decks are predicted to lie high in the atmosphere. The Bond albedo around a Sun-like star is 0.55, due to reflection by the cloud decks. At such high temperatures, a gas giant may glow red from thermal radiation, but the reflected light generally overwhelms thermal radiation.
Examples could include 51 Pegasi b, HAT-P-11b, HAT-P-7b, and Kepler-7b.