Sudarsky exoplanet classification

Many properties of exoplanets have been studied very poorly, for example, the chemical composition of their atmospheres. The reason for this is the impossibility of direct observation of exoplanets - most of them are studied indirectly. And only a few can be studied by spectral analysis , at the moment of transit in front of their star.

The analogy with the gas giants of the solar system is far from suitable for all exoplanets-giants, since most of the known exoplanets are not similar to Jupiter or Saturn, and belong mainly to the “ hot jupiter ” class. As indicated above, the properties of some exoplanets were studied directly due to their passage (transit) against the background of a star’s disk. ^[4] A study of one of these planets, HD 189733 b , showed that it is blue with albedo greater than 0.14. ^[5] Most open transit planets are also hot jupiters.

In the Solar System, Jupiter and Saturn , according to Sudarsky’s classification, both have class I. Sudarsky’s classification does not apply to ice planets (such as Uranus or Neptune , with 14 and 17 Earth masses respectively), “ super-earths ” and other rocky planets (examples of which are Earth and OGLE-2005-BLG-390L b , which has 5.5 earth masses).

Class I. Ammonia Clouds

In this class, ammonia clouds dominate the planets, and these planets are located in the outer regions of their star system. The condition for the existence of this class of planets is a temperature below −120 ° C. The estimated albedo for class I around the Sun's analogue star is 0.57. This is noticeably higher than the albedo of Jupiter or Saturn (0.343 ^[6] and 0.342 ^[7], respectively). The difference is due to the presence of certain substances in the atmospheres of gas giants in the solar system, such as phosphorus compounds, which are not taken into account in the calculations.

The formation temperatures of the planets of this class of planets depend on the presence of a weak star ( red dwarf ) or a large distance to the star. When circulating around the Sun, the distance to the star must be at least 5 AU so that the giant planet can fall into this class. If the mass of the planet is large enough, it can independently heat up, and thus go to another class.

In 2000, not a single class I planet was known except Jupiter and Saturn. Later, exoplanets that could correspond to class I were discovered. These are 47 Ursa Major c , Mu Altar e , HD 154345 b and many others.

Class II Water Clouds

Since the temperature of the gas giants of the second class is too high for the formation of ammonia clouds, it contains mainly water clouds. The temperature of these planets should be approximately −20 ° C, or lower than this. Water clouds reflect light very well, and the albedo of a water giant can exceed 0.81. The clouds on these planets are in many ways similar to terrestrial ones, but in addition to this, there is a lot of hydrogen and methane in the atmosphere of the planets, which greatly distinguishes the atmosphere of the planets from the earth. Planets of this type are gas giants located approximately or slightly further than the earth's orbit. In the solar system, the water giant would have to be located at a distance of about a little more than 1.2 AU from the sun . There are no planets of this type in the solar system, and among exoplanets in class II they include 47 Ursa Major b and Ypsilon Andromeda d (however, the latter is located at a distance from the star corresponding to class III in perihelion). Also, the planet HD 28185 b is assigned to this class, since the orbit of this planet is in the center of the "zone of life" . ^[eight]

Class III Cloudless

The planets, whose surface temperature varies between 80 ° C and about 530 ° C, are devoid of cloud cover, because there is too much heat for the formation of water clouds, and there is simply nothing for the clouds to form from. ^[3] The appearance of these planets is blue-blue, faceless, similar to Uranus or Neptune . The blue color is due to the presence of methane and Rayleigh scattering in the atmosphere of these planets.

The planets have a relatively small albedo - about 0.12. In the solar system, a gas giant of this type would have to be located roughly on the site of Mercury .

In the upper temperature zone of class III, thin cirrus clouds (above 430 ° C) from chlorides and sulfates appear in the planet's atmosphere. ^[3] Currently, 79 China are considered to be a typical representative of this type. B. Probably the planets of this class are Gliese 876 b and Ipsilon Andromeda c .

Class IV Planets with strong alkali metal spectrum lines

As the temperature of the gas giant rises above 630 ° C, carbon dioxide (rather than methane) becomes the dominant gas in the atmosphere. In addition to carbon dioxide, the atmosphere of these planets consists largely of alkali metal vapors that evaporate at such temperatures, which determines the presence of their strong spectral lines in the atmosphere. There are not very many clouds in this type of atmosphere, and they mainly consist of vapors of iron and silicates, although this does not noticeably affect the spectral lines. The albedo of these planets is very low, and is about 0.03. The record holder is the exoplanet TrES-2 b , the albedo of this exoplanet is less than one percent, and according to the most probable model it is only 0.04% (for comparison, the soot albedo is 1%). ^[9] It is explained by the strong absorption of light by alkali metals in the atmosphere. The color of the planets is gray with a slight shade of pink, since the temperature of the planet is high enough to make it glow . Planets of this class are very close to their luminaries and, as a rule, belong to hot jupiters ; so, for the Sun, the gas giant should be much closer to the Sun than Mercury (at a distance of about 0.1 AU). A typical representative of the planets of this class is 55 Cancer b . ^[3] Also, many well-known hot Jupiters belong to class IV, for example HD 209458 b (Osiris), and another well-known planet of this class is HD 189733 A b (the first planet for which a surface temperature map was compiled). The upper temperature limit for the planets of this class is about a thousand degrees Celsius. ^{[10] [11]}

Class V. Silicon Clouds

Very hot gas giants, whose temperature exceeds 1100 ° C, or less massive and less dense planets at slightly lower temperatures. Planets of this class have continuous clouds, consisting of vapors of iron and silicates. Due to the presence of such clouds, the albedo of the planets is quite high, and is 0.55. Class V includes the well-known short-period hot jupiters. Such planets are so close to their stars that they not only intensely reflect the light of the star, but they themselves glow with a red-orange light. Such planets can be found using terrestrial telescopes, and could theoretically be visually observed if a star containing such a planet has a visible brightness below + 4.5m. However, in practice, the planets will not be visible, since their light will be suppressed by the brilliance of the mother star. ^{[12] The} color of such planets is greenish gray. There are quite a lot of planets of this class, since they are easier to detect. In the solar system, a planet of this class would have to be at a distance of about 0.04 AU. from the sun. The most famous planet (and the first discovered in ordinary, “normal” stars) of this class is 51 Pegasus b . ^[3]

• Gas giant
• Exoplanet
• Hot jupiter
• Loose planet
• Super earth

• Speculation in the classes (neopr.) . Overview of Exoplanets . Date of treatment June 26, 2008. Archived on April 9, 2012. (eng.)
• Models of terrestrial planets (neopr.) . ScienceDaily (September 25, 2007). Date of treatment June 26, 2008. Archived on April 9, 2012. (eng.)
• Planets that revolve around other stars (neopr.) . Harvard University . Date of treatment June 26, 2008. Archived on April 9, 2012. (eng.)