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Appearance of extrasolar planets

The appearance of extrasolar planets is largely unknown because of the difficulty in making direct observations of extrasolar planets. In addition, analogies with planets in our solar system can apply for few of the extrasolar planets known; because most are wholly unlike any of our planets, for example the hot Jupiters.

Bodies which transit their star may 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 transiting planets are hot Jupiters.

Speculation on the appearances of unseen extrasolar planets currently relies upon models of the likely atmosphere of such a planet, for instance how it 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 - as with HD 189733 b - it is possible that a given planet will have a significantly different appearance to that predicted by the model.

Sudarsky planet types

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. and expanded on in Theoretical Spectra and Atmospheres of Extrasolar Giant Planets.

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. From our solar system: both planets eligible for the Sudarsky classification, Saturn and Jupiter, are Class I.

The appearance of planets which are not gas giants cannot be predicted by the Sudarsky system, for example terrestrial planets such as Earth and OGLE-2005-BLG-390Lb (5.5 Earth masses); or ice giants such as Uranus (14 Earth masses) and Neptune (17 Earth masses).

Class I: Ammonia clouds

Planets in this class have appearances dominated by ammonia clouds. These planets are found in the outer regions of a planetary system. They exist at temperatures less than about 150 kelvins (−120 degrees Celsius/−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 and 0.342 for Saturn. 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, and are not modelled in the calculations.

The temperatures for a class I planet require a cool star or else a distant perihelion for the planet's orbit. The former stars might be too dim for us even to know about them, and the latter orbits might be too unpronounced for notice until several observations of those orbits' "years" (c.f. Kepler's Third Law). Superjovian planets would have mass enough to improve these observations; but a superjovian of comparable age to Jupiter will have more internal heating than said planet, which could push it to a higher class.

As of 2000, the Sudarsky papers could assign no planets to class I except for Jupiter and Saturn. Since then, 47 Ursae Majoris c and Mu Arae e have been found: with minimum mass in the 1-2 Mj range, orbiting with negligible eccentricity further than 5 AU away from a host star of comparable heat emission to the Sun.

Class II: Water clouds

Planets in class II are too warm to form ammonia clouds: instead their clouds are made up of water vapor. This type of planet is expected for planets with temperatures below around 250 K. 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.

Temperature is though to effect these types of planets in somewhat large variations. Cooler planets may have areas of the atmosphere (particularly the poles) that are still frigid enough to support ammonia clouds. Warmer planets may form yellow condensates of sulfurous compounds and may even have clouds of sulfuric acid. These planets (unlike the other two) are thought to be similar to Venus over the Earth, and are often classified as "Sulfur clouded Jovians". It is thought that the planets only have sulfuric clouds in the top layers and water clouds in the lower layers, allowing the idea of this type of planet simply being a "subclass" of type II planets.

The possible class II planets, listed in Sudarsky's original paper, include: 47 Ursae Majoris b and Upsilon Andromedae d (though at periastron, Upsilon Andromedae d may become a Class III planet). The most noted planet though to be in this class is HD 28185 b, because of its circular orbit in the center of its star's habitable zone. For their orbits in habitable zone, Iota Horologii b and Gamma Cephei Ab are the most noted "Sulfur clouded Jovians."

Class III: Clear

Planets with equilibrium temperatures between about 350 K (170 °F, 80 °C) and 800 K (980 °F, 530 °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 and absorption by methane in their atmospheres. 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.

Exoplanets listed in Sudarsky's paper as being possible class III planets include Gliese 876 b and Upsilon Andromedae c. Above 700 K (800 °F, 430 °C), sulfides and chlorides might provide cirrus-like clouds.

Class IV: Alkali metals

Above 900 K (630 °C/1160 °F), carbon monoxide becomes the dominant carbon-carrying molecule in the planet's atmosphere (rather than methane). Furthermore, the abundance of alkali metals, such as sodium substantially increase, and spectral lines of sodium and potassium are predicted to be prominent in the planet's spectrum. These planets form cloud decks of silicates and iron deep in their atmospheres, but this is not predicted to affect the spectrum of the planet. The Bond albedo of a class IV planet around a sunlike star is predicted to be very low, at 0.03 because of the strong absorption by alkali metals. Planets of classes IV and V are referred to as hot Jupiters.

55 Cancri b was listed as a class IV planet.

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 - but less than predicted. This star hosts an upper cloud deck absorbing so much heat that below it is a relatively cool stratosphere. The composition of this dark cloud, in the models, is assumed to be titanium/vanadium oxide (sometimes abbreviated "TiVO"), by analogy with M class dwarf stars, but its true composition is unknown as of yet; it could well be alkaloid as per Sudarsky.

HD 189733 b, with measured temperatures 920-1200 K (650-930 °C), also qualifies as class IV. However it has in late 2007 been measured as deep blue, with an albedo over 0.14 (possibly due to the brighter glow of its "hot spot"). No stratosphere has been conclusively proven for it as yet.

Class V: Silicate clouds

On the very hottest gas giants, with temperatures above 1400 K (2100 °F, 1100 °C) or cooler planets with lower gravity than Jupiter, the silicate and iron cloud decks are predicted to lie high up in the atmosphere. The predicted Bond albedo of a class V planet around a sunlike star is 0.55, thanks to reflection by the cloud decks. At such temperatures, the planet may glow red from thermal radiation. For stars of visual magnitude under 4.50 in our sky, such planets are theoretically visible to our instruments. Examples of such planets might include 51 Pegasi b.

This prediction has fared poorly. Tau Boötis Ab at 1621 K is class V in temperature, but Leigh et al. found that its albedo cannot be higher than 0.39. upsilon Andromedae b (sunward temperature 1700 K) and transiting planet HD 149026 b (2300 K) were discovered to be darker still; like HD 209458 b, they host a dark upper cloud deck shading a stratosphere.

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