Appearance of extrasolar planets
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2008-07-12T22:45:53Z
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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, 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 Jupiter]]s.
Bodies which transit their star may be spectrographically mapped, for instance [[HD 189733 b]].<ref>[http://www.spitzer.caltech.edu/Media/releases/ssc2007-09/ssc2007-09a.shtml Image ssc2007-09a<!-- Bot generated title -->]</ref>
That planet has further been shown to be blue with an [[albedo]] greater (brighter) than 0.14.<ref name="Berdyugina">{{cite journal | url=http://www.astro.phys.ethz.ch/papers/berdyugina/exoplanet_hd189733b.pdf | journal=The Astrophysical Journal | volume=673 | month=20 January | year=2008 | title=First detection of polarized scattered light from an exoplanetary atmosphere | first=Svetlana V.| last=Berdyugina | coauthors=Andrei V. Berdyugin, Dominique M. Fluri, Vilppu Piirola | doi=10.1086/527320 | pages=L83}}</ref> 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''.<ref name="sudarsky">{{cite journal |url=http://www.journals.uchicago.edu/ApJ/journal/issues/ApJ/v538n2/50692/brief/50692.abstract.html |author=Sudarsky, D., Burrows, A., Pinto, P.|title=Albedo and Reflection Spectra of Extrasolar Giant Planets|journal=[[The Astrophysical Journal]]|volume=538|year=2000|pages=885 – 903|doi=10.1086/309160|format=abstract}}</ref> and expanded on in ''Theoretical Spectra and Atmospheres of Extrasolar Giant Planets''.<ref name="sudarsky2">{{cite journal|url=http://www.journals.uchicago.edu/ApJ/journal/issues/ApJ/v588n2/56972/56972.html |author=Sudarsky, D., Burrows, A., Hubeny, I.|title=Theoretical Spectra and Atmospheres of Extrasolar Giant Planets|journal=[[The Astrophysical Journal]]|year=2003|volume=588|issue=2|pages=1121 – 1148|doi=10.1086/374331|format={{Dead link|date=May 2008}}}}</ref>
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 planet]]s such as [[Earth]] and [[OGLE-2005-BLG-390Lb]] (5.5 [[Earth mass]]es); or [[ice giant]]s such as [[Uranus]] (14 Earth masses) and [[Neptune]] (17 Earth masses).
=== Class I: Ammonia clouds ===
[[Image:Jupiter.jpg|thumb|200px|left|[[Jupiter]], a Sudarsky Class I planet.]]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 [[kelvin]]s (−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<ref>[http://nssdc.gsfc.nasa.gov/planetary/factsheet/jupiterfact.html Jupiter Fact Sheet<!-- Bot generated title -->]</ref> and 0.342 for Saturn.<ref>[http://nssdc.gsfc.nasa.gov/planetary/factsheet/saturnfact.html Saturn Fact Sheet<!-- Bot generated title -->]</ref> 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_laws_of_planetary_motion#Third_Law|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.<ref name="sudarsky" /> Since then, [[47 Ursae Majoris c]] and [[Mu Arae c]] 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 ===<!-- This section is linked from [[Water Cloud Jovian]] -->
[[Image:Hd28185b.jpg|thumb|right|[[HD 28185 b]] is though to be a Sudarsky Class II planet.]]
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<ref name="sudarsky2" />. 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 though to be similar to [[Venus]] over the Earth, and are often classified as "Sulfur clouded Jovians".<ref name="EV">{{cite web |url=http://www.extrasolar.net/speculations.html |title=Behind the speculations |accessdate=2008-06-26 |work=Extrasolar Visions}}</ref> It is though 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).<ref name="sudarsky2" /> 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.[http://www.extrasolar.net/planettour.asp?StarCatId=normal&PlanetId=158] For their orbits in habitable zone, [[Iota Horologii b]] and [[Gamma Cephei Ab]] are the most noted "Sulfur clouded Jovians."[http://www.extrasolar.net/planettour.asp?StarCatId=normal&PlanetId=32] [http://www.extrasolar.net/planettour.asp?StarCatId=normal&PlanetId=235]
=== 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.<ref name="sudarsky2" /> 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 (planet)|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.<ref name="sudarsky2" />
=== Class IV: Alkali metals ===
[[Image:Osirisplanet.jpg|thumb|200px|right|Artist's impression of a [[hot Jupiter]].]]
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 metal]]s, such as [[sodium]] substantially increase, and [[spectral line]]s of [[sodium]] and [[potassium]] are predicted to be prominent in the planet's [[spectrum]]. These planets form cloud decks of [[silicate]]s 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 Jupiter]]s.
[[55 Cancri b]] was listed as a class IV planet.<ref name="sudarsky2" /> [[HD 209458 b]] at 1300 K (1000 °C) would be another such planet; and in 2001, NASA witnessed atmospheric sodium in its transit - but less than predicted. HD 189733 b, with measured temperatures 920-1200 K (650-930 °C), also qualifies as class IV; 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").
=== 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.<ref>LEIGH C., COLLIER CAMERON A., HORNE K., PENNY A. & JAMES D., 2003 "A new upper limit on the reflected starlight from Tau Bootis b." MNRAS,344, 1271</ref>
Examples of such planets might include [[HD 209458 b]] and [[51 Pegasi b]].<ref name="sudarsky2" /> [[Tau Boötis Ab]] at 1621 K should also be class V, but Leigh et al. found that its albedo cannot be higher than 0.39.
== See also ==
* [[Extrasolar planet]]
* [[Gas giant]]
== References ==
{{reflist}}
== External links ==
{{commonscat|Exoplanets}}
* {{cite web |url=http://www.extrasolar.net/speculations.html |title=Behind the speculations |accessdate=2008-06-26 |work=Extrasolar Visions}}
* {{cite web |url=http://www.sciencedaily.com/releases/2007/09/070924132510.htm |title=Cornucopia Of Earth-sized Planets Modeled By NASA |accessdate=2008-06-26 |work=[[ScienceDaily]] |date=2007-09-25}}
* {{cite web |url=http://cfa-www.harvard.edu/~sasselov/astro201a/A201-061207.ppt |title=Planets Orbiting Other Stars |accessdate=2008-06-26 |work=[[Harvard University]]}}
* [http://j.dollan.home.bresnan.net/ARCPCLIndex4ed.html The Planetary Classification List (Fourth Edition)]{{Dead link|date=June 2008}}
[[Category:Extrasolar planets| ]]
[[Category:Classification systems]]
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