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The appearance of extrasolar planets is largely unknown because of the difficulty in making direct observations of extrasolar planets. In addition, many extrasolar planets have no analogues in our solar system, for example the hot Jupiters.

Nevertheless it is possible to make some predictions about the appearances of extrasolar planets, for example by modelling how a model atmosphere would respond to varying degrees of insolation. This involves making assumptions about the composition of the atmosphere (for example, that the chemical abundances are similar to those of Jupiter), so it is possible that a given planet will have a significantly different appearance to that predicted by the model.

Contents 1 Sudarsky planet types 1.1 Class I: Ammonia clouds 1.2 Class II: Water clouds 1.3 Class III: Clear 1.4 Class IV: Alkali metal absorption 1.5 Class V: Silicate clouds 2 Other planet types 3 References 4 See also 5 External links

The Sudarsky classification system is a theoretical classification system for predicting the appearance of extrasolar gas giant planets based on their temperature. It was outlined by David Sudarsky et al. in the paper Albedo and Reflection Spectra of Extrasolar Giant Planets^[1]. The system assumes chemical equilibrium in the planet's atmosphere, which may or may not be a good approximation.

Gas giant planets are split into five classes, numbered using Roman numerals. The system assumes that the general composition of the planet's atmosphere is similar to that of Jupiter. In general, the chemical composition of extrasolar planets is not known, and making the observations necessary to determine this require more advanced detection methods.

There can be significant differences between appearance within a class: for example both Jupiter and Saturn would be designated as class I planets under this system, but Jupiter has much more pronounced bands than Saturn.

This class corresponds to Jupiter and Saturn in our solar system, which have appearances dominated by ammonia clouds. These planets are found in the outer regions of a planetary system. Present-day detection methods are more sensitive towards inner system planets, so very few of the currently known extrasolar planets are likely to fall into this class. They exist at temperatures less than about 150 kelvins (−190 degrees Fahrenheit). The predicted Bond albedo of a class I planet around a star like the Sun is 0.57, compared with a value of 0.343 for Jupiter [2] and 0.342 for Saturn [3]. The discrepancy can be partially accounted for by taking into account non-equilibrium condensates such as tholin or phosphorus, which are responsible for the coloured clouds in the Jovian atmosphere.

Planets in class II are too warm to form ammonia clouds, instead their clouds are made up of water vapor. Class II planets are predicted at temperatures of about 150 to 350 kelvins (−190 °F to +170 °F). Water clouds are more reflective than ammonia clouds, and the predicted Bond albedo of a class II planet around a sunlike star is 0.81. Even though the clouds on such a planet would be similar to those of Earth, the atmosphere would still consist mainly of hydrogen and hydrogen-rich molecules such as methane. Planets in the middle of this range may be able to host Earth-like moons, but the warmest and coldest class II planets receive similar insolation to Venus or Mars. There are no class II planets in our solar system, so their appearance can only be predicted by theory. Possible class II planets listed in Sudarsky's paper include 47 Ursae Majoris b and Upsilon Andromedae d.

Planets warmer than about 350 K (170 °F, 77 °C) do not form global cloud cover, as they lack suitable chemicals in the atmosphere to form clouds. These planets would appear as featureless blue globes because of Rayleigh scattering. Because of the lack of a reflective cloud layer, the Bond albedo is low, around 0.12 for a class III planet around a sunlike star. They exist in the inner regions of a planetary system, roughly corresponding to the location of Mercury. There are no class III planets in our solar system. Exoplanets listed in Sudarsky's paper as being possible class III planets include 55 Cancri b and 70 Virginis b. There might be low-abundance condensates such as Na₂S, ZnS or other salts, but they would only form thin cirrus clouds.

Above 900 K (1160 °F), the equilibrium abundance of alkali metals is increased, and at high temperatures sodium and potassium give broad absorption, which would result in a dark bluish appearance. The Bond albedo of a class IV planet around a sunlike star is predicted to be 0.03. Planets of classes IV and V are referred to as hot Jupiters or "roasters". There are no hot Jupiters in our solar system. Upsilon Andromedae b and 51 Pegasi b are listed as possible class IV planets.

On the very hottest (above 1500 K or 2240 °F) gas giants, or cooler planets with lower gravity than Jupiter, clouds of silicate materials lie above the sodium haze, rather than below, as in cooler planets. The predicted Bond albedo of a class V planet around a sunlike star is 0.55. The planet would also glow visibly from its own heat.

The appearance of planets which are not gas giants cannot be predicted by the Sudarsky system, for example terrestrial planets such as Earth or ice giants such as Neptune.

1. ^  Sudarsky, D., Burrows, A., and Pinto, P. (2000-08-01). "Albedo and Reflection Spectra of Extrasolar Giant Planets". Astrophysical Journal 538: 885 – 903.  (arxiv)

• Extrasolar planet
• Gas giant

• Extrasolar Visions
• The Planetary Classification List (Fourth Edition)
• Celestia Forum: Extended Gas Giant Classification System