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Sudarsky's classification of gas giants for the purpose of predicting their appearance based on their temperature was outlined byDavid Sudarsky and colleagues in the paperAlbedo and Reflection Spectra of Extrasolar Giant Planets[1] and expanded on inTheoretical Spectra and Atmospheres of Extrasolar Giant Planets,[2] 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 usingRoman numerals) according to their modeled physical atmospheric properties. In the Solar System, onlyJupiter andSaturn 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 exampleterrestrial planets such asEarth andVenus, orice giants such asUranus (14 Earth masses) andNeptune (17 Earth masses).[citation needed]
The appearance ofextrasolar planets is largely unknown because of the difficulty in making direct observations. In addition, analogies with planets in theSolar System can apply to few of the extrasolar planets known because most are wholly unlike any of our planets, for example, thehot Jupiters.
Bodies that transit their star can be spectrographically mapped, for instanceHD 189733 b.[3] That planet has further been shown to be blue with analbedo greater (brighter) than 0.14.[4] 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 likelyatmosphere of such a planet, for instance how the atmospheric temperature–pressure profile and composition would respond to varying degrees ofinsolation.
Gaseous giants in this class have appearances dominated byammonia clouds. These planets are found in the outer regions of aplanetary system. They exist at temperatures less than about 150 K (−120 °C; −190 °F). The predicted Bondalbedo of a class I planet around astar like theSun is 0.57, compared with a value of 0.343 forJupiter[5] and 0.342 forSaturn.[6] The discrepancy can be partially accounted for by taking into account non-equilibrium condensates such astholins orphosphorus, which are responsible for the coloured clouds in the Jovian atmosphere, and are not modelled in the calculations.
The temperatures for a class I planet requires either a cool star or a distant orbit. The former may mean the star(s) are too dim to be visible, where the latter may mean the orbits are so large that their effect is too subtle to be detected until several observations of thoseorbits' complete "years" (cf.Kepler's third law). The increased mass ofsuperjovians would make them easier to observe, however a superjovian of comparable age to Jupiter would have moreinternal heating, which could push it to a higher class.
Gaseous giants in class II are too warm to form ammonia clouds; instead their clouds are made up ofwater ice. These characteristics are expected for planets with temperatures below around 250 K (−23 °C; −10 °F).[2] Water clouds are more reflective than ammonia clouds, and the predicted Bond albedo of a class II planet around a Sun-like star is 0.81. Even though the clouds on such a planet would be similar to those ofEarth, the atmosphere would still consist mainly ofhydrogen and hydrogen-rich molecules such asmethane.
Sudarsky et al. listedEpsilon Eridani b,Upsilon Andromedae d, and55 Cancri Ad as possible Class II planets.[2]
Gaseous giants with equilibrium temperatures between about 350 K (170 °F, 80 °C) and 800 K (980 °F, 530 °C) do not form global cloud cover, because they lack suitable chemicals in the atmosphere to form clouds.[2] (They would not form sulfuric acid clouds like Venus due to excess hydrogen.) These planets would appear as featureless azure-blue globes because ofRayleigh scattering and absorption bymethane in their atmospheres, appearing like Jovian-mass versions ofUranus andNeptune. Because of the lack of a reflective cloud layer, the Bond albedo is low, around 0.12 for a class-III planet around a Sun-like star. They exist in the inner regions of a planetary system, roughly corresponding to the location ofMercury.
Sudarsky et al. listedUpsilon Andromedae c,Gliese 876 b, andGliese 876 c as possible Class III planets.[2] Above 700 K (800 °F, 430 °C), sulfides and chlorides might providecirrus-like clouds.[2]
Above 900 K (630 °C/1160 °F),carbon monoxide becomes the dominant carbon-carrying molecule in a gas giant's atmosphere (rather thanmethane). Furthermore, the abundance ofalkali metals, such assodium substantially increase, andspectral lines ofsodium andpotassium are predicted to be prominent in a gas giant'sspectrum. These planets form cloud decks ofsilicates andiron 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. Gas giants of classes IV and V are referred to ashot Jupiters.
Sudarsky et al. listed55 Cancri Ab as a possible Class IV planet.[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. This planet hosts an upper cloud deck absorbing so much heat that below it is a relatively coolstratosphere. The composition of this dark cloud, in the models, is assumed to be titanium/vanadium oxide (sometimes abbreviated "TiVO"), by analogy with red dwarfs, but its true composition is yet unknown; it could well be as per Sudarsky.[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, thesilicate andiron cloud decks are predicted to lie high up in the atmosphere. The predicted Bond albedo of a class V planet around a Sun-like star is 0.55, due to reflection by the cloud decks. At such temperatures, a gas giant may glow red from thermal radiation but the reflected light generally overwhelms thermal radiation. For stars of visual apparent magnitude under 4.50, such planets are theoretically visible to our instruments.[9] Sudarsky et al. listed51 Pegasi b,Upsilon Andromedae b,HD 209458 b, andTau Boötis b as possible Class V planets.[2]
