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Planetary classes

        Class I: Ammonia clouds
        Class II: Water clouds
        Class III: Cloudless
        Class IV: Alkali metals
        Class V: Silicate clouds
    See also
    References
    External links

Sudarsky's gas giant classification

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Sudarsky classification as used in Celestia.
Class I gas giant
Class I
Class II gas giant
Class II
Class III gas giant
Class III
Class IV gas giant
Class IV
Class V gas giant
Class V

Sudarsky's classification of gas giants for the purpose of predicting their appearance based on their temperature was outlined by David Sudarsky and colleagues in the paper Albedo and Reflection Spectra of Extrasolar Giant Planets[1] and expanded on in Theoretical 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 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 and Venus, or ice giants such as Uranus (14 Earth masses) and Neptune (17 Earth masses).[citation needed]
Background

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

Bodies that transit their star can be spectrographically mapped, for instance HD 189733 b.[3] That planet has further been shown to be blue with an albedo 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 likely atmosphere of such a planet, for instance how the atmospheric temperature–pressure profile and composition would respond to varying degrees of insolation.
Planetary classes
Class I: Ammonia clouds
Jupiter and Saturn, two Sudarsky class I gas giants.

Gaseous giants 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 K (−120 °C; −190 °F). 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[5] and 0.342 for Saturn.[6] The discrepancy can be partially accounted for by taking into account non-equilibrium condensates such as tholins 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 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 those orbits' complete "years" (cf. Kepler's third law). The increased mass of superjovians would make them easier to observe, however a superjovian of comparable age to Jupiter would have more internal heating, which could push it to a higher class.
Class II: Water clouds

Gaseous giants in class II are too warm to form ammonia clouds; instead their clouds are made up of water vapor. 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 of Earth, the atmosphere would still consist mainly of hydrogen and hydrogen-rich molecules such as methane.

Sudarsky et al. listed Ægir, Majriti, and Lipperhey as possible Class II planets.[2]
Class III: Cloudless

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 of Rayleigh scattering and absorption by methane in their atmospheres, appearing like Jovian-mass versions of Uranus and Neptune. 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 of Mercury.

Sudarsky et al. listed Samh, Gliese 876 b, and Gliese 876 c as possible Class III planets.[2] Above 700 K (800 °F, 430 °C), sulfides and chlorides might provide cirrus-like clouds.[2]
Class IV: Alkali metals

Above 900 K (630 °C/1160 °F), carbon monoxide becomes the dominant carbon-carrying molecule in a gas giant'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 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 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 as hot Jupiters.

Sudarsky et al. listed Galileo 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 cool stratosphere. 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]
Class V: Silicate clouds

For 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 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. listed Dimidium, Saffar, HD 209458 b, and Tau Boötis b as possible Class V planets.[2]
See also

    Exoplanet
    List of planet types

References

Sudarsky, D.; Burrows, A.; Pinto, P. (2000). "Albedo and Reflection Spectra of Extrasolar Giant Planets". The Astrophysical Journal. 538 (2): 885–903. arXiv:astro-ph/9910504. Bibcode:2000ApJ...538..885S. CiteSeerX 10.1.1.316.9833. doi:10.1086/309160.
Sudarsky, D.; Burrows, A.; Hubeny, I. (2003). "Theoretical Spectra and Atmospheres of Extrasolar Giant Planets". The Astrophysical Journal. 588 (2): 1121–1148. arXiv:astro-ph/0210216. Bibcode:2003ApJ...588.1121S. doi:10.1086/374331.
"First Map of Alien World". Archived from the original on October 16, 2007. Retrieved November 23, 2007.
Berdyugina, Svetlana V.; Andrei V. Berdyugin; Dominique M. Fluri; Vilppu Piirola (20 January 2008). "First detection of polarized scattered light from an exoplanetary atmosphere" (PDF). The Astrophysical Journal. 673 (1): L83. arXiv:0712.0193. Bibcode:2008ApJ...673L..83B. doi:10.1086/527320. Archived from the original (PDF) on 17 December 2008.
Jupiter Fact Sheet
Saturn Fact Sheet
Ivan Hubeny; Adam Burrows (2008). "Spectrum and atmosphere models of irradiated transiting extrasolar giant planets". Proceedings of the International Astronomical Union. 4: 239. arXiv:0807.3588. Bibcode:2009IAUS..253..239H. doi:10.1017/S1743921308026458.
Ian Dobbs-Dixon (2008). "Radiative Hydrodynamical Studies of Irradiated Atmospheres". Proceedings of the International Astronomical Union. 4: 273. arXiv:0807.4541. Bibcode:2009IAUS..253..273D. doi:10.1017/S1743921308026495.

    Leigh C.; Collier C. A.; Horne K.; Penny A.; James D. (2003). "A new upper limit on the reflected starlight from Tau Bootis b." MNRAS. 344 (4): 1271. arXiv:astro-ph/0308413. Bibcode:2003MNRAS.344.1271L. doi:10.1046/j.1365-8711.2003.06901.x.

External links

    "Behind the speculations". Extrasolar Visions. Retrieved 2008-06-26.
    "Planets Orbiting Other Stars". Harvard University. Retrieved 2008-06-26.

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Exoplanets

    Planet
        Definition
            IAU Planetary science

Main topics	

    Exoplanet Exoplanet orbital and physical parameters Methods of detecting exoplanets Planetary system Planet-hosting stars

	
Sizes
and
types	
Terrestrial	

    Carbon planet Coreless planet Desert planet Dwarf planet Hycean planet Ice planet Iron planet (Super-Mercury) Lava planet Ocean world Mega-Earth Sub-Earth Super-Earth

Gaseous	

    Eccentric Jupiter Mini-Neptune (Gas dwarf) Helium planet Hot Jupiter Hot Neptune Gas giant Ice giant Super-Jupiter Super-Neptune Super-puff Ultra-hot Jupiter Ultra-hot Neptune

Other types	

    Blanet Brown dwarf Chthonian planet Circumbinary planet Circumtriple planet Disrupted planet Double planet Ecumenopolis Eyeball planet Giant planet Mesoplanet Planemo Planet/Brown dwarf boundary Planetesimal Protoplanet Pulsar planet Sub-brown dwarf Sub-Neptune Toroidal planet Ultra-cool dwarf Ultra-short period planet (USP)

Formation
and
evolution	

    Accretion Accretion disk Asteroid belt Circumplanetary disk Circumstellar disc Circumstellar envelope Cosmic dust Debris disk Detached object Disrupted planet Excretion disk Exozodiacal dust Extraterrestrial materials Extraterrestrial sample curation Giant-impact hypothesis Gravitational collapse Hills cloud Internal structure Interplanetary dust cloud Interplanetary medium Interplanetary space Interstellar cloud Interstellar dust Interstellar medium Interstellar space Kuiper belt List of interstellar and circumstellar molecules Merging stars Molecular cloud Nebular hypothesis Oort cloud Outer space Planetary migration Planetary system Planetesimal Planet formation Protoplanetary disk Ring system Rubble pile Sample-return mission Scattered disc Star formation

Systems	

    Exocomet
        Interstellar Exomoon
        Tidally detached Rogue planet Orbits
        Retrograde Trojan Mean-motion resonances Titius–Bode law

Host stars	

    A B Binary star Brown dwarfs F/Yellow-white dwarfs G/Yellow dwarfs Herbig Ae/Be K/Orange dwarfs M/Red dwarfs Pulsar Red giant Subdwarf B Subgiant T Tauri White dwarfs Yellow giants

Detection	

    Astrometry Direct imaging
        list Microlensing
        list Polarimetry Timing
        list Radial velocity
        list Transit method
        list Transit-timing variation

Habitability	

    Astrobiology Astrooceanography Circumstellar habitable zone Earth analog Extraterrestrial liquid water Galactic habitable zone Habitability of binary star systems Habitability of F-type main-sequence star systems Habitability of K-type main-sequence star systems Habitability of natural satellites Habitability of neutron star systems Habitability of red dwarf systems Habitability of yellow dwarf systems Habitable zone for complex life List of potentially habitable exoplanets Tholin Superhabitable planet

Catalogues	

    Nearby Habitable Systems Exoplanet Data Explorer Extrasolar Planets Encyclopaedia NASA Exoplanet Archive NASA Star and Exoplanet Database Open Exoplanet Catalogue

Lists	

    Exoplanetary systems
        Host stars Multiplanetary systems Stars with proto-planetary discs

    Exoplanets
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            1–500 501–1000 1001–1500 1501–2000 K2 Potentially habitable Proper names

    Discovered exoplanets by year
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Other	

    Carl Sagan Institute Exoplanet naming convention Exoplanet phase curves Exoplanetary Circumstellar Environments and Disk Explorer Extragalactic planet Extrasolar planets in fiction Geodynamics of terrestrial exoplanets Neptunian desert Nexus for Exoplanet System Science Planets in globular clusters Small planet radius gap Sudarsky's gas giant classification

    Discoveries of exoplanets Search projects

Categories:

    Astronomical classification systemsExoplanetologyGas giants

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