Celestial Body Classification System

Copyright R.M.F. Runyan © 2012

Edited by Aaron Smalley for The Guild Companion

"Jupiter is an exceptional case, having metallic hydrogen near the center, but much of its volume is hydrogen, helium and traces of other gases above their critical points."

Author's Note: Please do not accept this document as an "official" Celestial Body Classification System (CBCS). Although it draws heavily on scientific fact, this list is entirely my work in an attempt to create a simplified CBCS. I am certain that the IAU (International Astronomical Union) has an official CBCS, I just have not found it. The below is a simplified compilation from several websites: JPL, NASA, IAU, even National Geographic, to list but a few. Additionally, I can now look forward to incorporating this system into the Stellar System Generation System I rewrote recently. Joy. :p

  1. Stellar Primary Type – This celestial body type is also called "star."
    1. W Class – Class W (or WR) represents the superluminous Wolf-Rayet stars, notably unusual since they have mostly helium in their atmospheres instead of hydrogen. They are thought to be dying supergiants with their hydrogen layer blown away by hot stellar winds caused by their high temperatures, thereby directly exposing their hot helium shell. It was originally thought that the 60,000°K was the maximum until a Wolf-Rayet star was discovered to be burning at over 70,000°K.
      1. WN Group – These are the nitrogen rich W class stars.
        1. E Category – This is the early category of the WN stars (i.e.- WN0 to WN4).
        2. L Category – This is the late category of the WN stars (i.e.- WN5 to WN9).
      2. WC Group – These are the carbon rich W class stars.
        1. E Category – This is the early category of the WC stars (i.e.- WC0 to WC4).
        2. L Category – This is the late category of the WC stars (i.e.- WC5 to WC9).
    2. O Class – Class O stars are very hot and extremely luminous, being bluish in color; in fact, most of their output is in the ultraviolet range. These are the rarest of all main-sequence stars. About 1 in 3,000,000 of the main-sequence stars in the solar neighborhood are Class O stars. Some of the most massive stars lie within this spectral class. Type O stars are so hot as to have complicated surroundings which make measurement of their spectra difficult. These stars have a surface temperature of 30,000 to 60,000°K.
      1. C Group – These are the carbon rich O class stars.
      2. N Group – These are the nitrogen rich O class stars.
    3. B Class – Class B stars are very luminous and blue. Their spectra have neutral helium, which are most prominent at the B2 subclass, and moderate hydrogen lines. Ionized metal lines include Mg II and Si II. As O and B stars are so powerful, they only live for a relatively short time, and thus they do not stray far from the area in which they were formed. About 1 in 800 of the main-sequence stars in the solar neighborhood are Class B stars. These stars have a surface temperature of 10,000 to 30,000°K.
      1. C Group – These are the carbon rich B class stars.
      2. N Group – These are the nitrogen rich B class stars.
    4. A Class – Class A stars are among the more common naked eye stars, and are white or bluish-white. They have strong hydrogen lines, at a maximum by A0, and also lines of ionized metals (Fe II, Mg II, Si II) at a maximum at A5. The presence of Ca II lines is notably strengthening by this point. About 1 in 160 of the main-sequence stars in the solar neighborhood are Class A stars. These stars have a surface temperature of 7500 to 10,000°K.
    5. F Class – Class F stars have strengthening H and K lines of Ca II. Neutral metals (Fe I, Cr I) beginning to gain on ionized metal lines by late F. Their spectra are characterized by the weaker hydrogen lines and ionized metals. Their color is white. About 1 in 33 of the main-sequence stars in the solar neighborhood are Class F stars. These stars have a surface temperature of 6000 to 7500°K.
    6. G Class – Class G stars are probably the best known, if only for the reason that the Sun is of this class. About 1 in 13 of the main-sequence stars in the solar neighborhood are Class G stars. Most notable are the H and K lines of Ca II, which are most prominent at G2. They have even weaker hydrogen lines than F, but along with the ionized metals, they have neutral metals. There is a prominent spike in the G band of CH molecules. G is host to the "Yellow Evolutionary Void". Supergiant stars often swing between O or B (blue) and K or M (red). While they do this, they do not stay for long in the G classification as this is an extremely unstable place for a supergiant to be. These stars have a surface temperature of 5000 to 6000°K.
    7. K Class – Class K are orangish stars that are slightly cooler than our Sun. Some K stars are giants and supergiants, such as Arcturus, while orange dwarfs, like Alpha Centauri B, are main-sequence stars. They have extremely weak hydrogen lines, if they are present at all, and mostly neutral metals (Mn I, Fe I, Si I). By late K, molecular bands of titanium oxide become present. About 1 in 8 of the main-sequence stars in the solar neighborhood are Class K stars. There is a suggestion that K Spectrum stars are very well suited for biology. These stars have a surface temperature of 3500 to 5000°K.
    8. M Class – Class M is by far the most common class. About 76% (3 in 4) of the main-sequence stars in the Solar neighborhood are Class M stars. Although most Class M stars are red dwarfs, the class also hosts most giants and some supergiants such as Antares and Betelgeuse, as well as Mira variables. The late-M group holds hotter brown dwarfs that are above the L spectrum. This is usually in the range of M6.5 to M9.5. The spectrum of an M star shows lines belonging to molecules and all neutral metals but hydrogen lines are usually absent. Titanium oxide can be strong in M stars, usually dominating by about M5. Vanadium oxide bands become present by late M. These stars have a surface temperature of 2000 to 3500°K.
      1. C Group – Originally classified as R and N stars, these are also known as "carbon stars." These are red giants, near the end of their lives, in which there is an excess of carbon in the atmosphere. A few dwarf (that is, main sequence) carbon stars are known, but the overwhelming majority of known carbon stars are giants or supergiants.
        1. R Category – Formerly a class on its own representing the carbon star equivalent of late G to early K stars.
        2. N Category – Formerly a class on its own representing the carbon star equivalent of late K to M stars.
        3. J Category – A subtype of cool C stars with a high content of 13C.
        4. H Category – Population II analogues of the C-R stars.
        5. Hd Category – Hydrogen-Deficient Carbon Stars, similar to late G supergiants with CH and C2 bands added.
      2. S Group – Class S stars have zirconium oxide lines in addition to (or, rarely, instead of) those of titanium oxide, and are in between the Class M stars and the carbon stars. S stars have excess amounts of zirconium and other elements produced by the s-process, and have their carbon and oxygen abundances closer to equal than is the case for M stars. The latter condition results in both carbon and oxygen being locked up almost entirely in carbon monoxide molecules. For stars cool enough for carbon monoxide to form that molecule tends to "eat up" all of whichever element is less abundant, resulting in "leftover oxygen" (which becomes available to form titanium oxide) in stars of normal composition, "leftover carbon" (which becomes available to form the diatomic carbon molecules) in carbon stars, and "leftover nothing" in the S stars. The relation between these stars and the ordinary M stars indicates a continuum of carbon abundance. Like carbon stars, nearly all known S stars are giants or supergiants.
    9. L Class – Class L dwarfs, 1300--2000°K, get their designation because they are cooler than M stars and L is the remaining letter alphabetically closest to M. L does not mean lithium dwarf; a large fraction of these stars do not have lithium in their spectra. Some of these objects have masses large enough to support hydrogen fusion, but some are of substellar mass and do not, so collectively these objects should be referred to as L dwarfs, not L stars. They are a very dark red in color and brightest in infrared. Their atmosphere is cool enough to allow metal hydrides and alkali metals to be prominent in their spectra. Due to low gravities in giant stars, TiO- and VO-bearing condensates never form. Thus, larger L-type stars can never form in an isolated environment. These dwarfs have a surface temperature of 1200 to 2000°K.
      1. A Group – This is an L class dwarf star in the hotter temperature range (1700-2000°K).
      2. B Group – This is an L class brown dwarf in the colder temperature range (1300-1700°K).
    10. T Class – Class T dwarfs are cool brown dwarfs with surface temperatures between approximately 700 and 1,300 K. Their emission peaks in the infrared. Methane is prominent in their spectra. Class T and L could be more common than all the other classes combined if recent research is accurate. From studying the number of proplyds (protoplanetary discs, clumps of gas in nebulae from which stars and solar systems are formed), then the number of stars in the galaxy should be several orders of magnitude higher than what we know about. It is theorized that these proplyds are in a race with each other. The first one to form will become a proto-star, which are very violent objects and will disrupt other proplyds in the vicinity, stripping them of their gas. The victim proplyds will then probably go on to become main sequence stars or brown dwarf stars of the L and T classes, but quite invisible to us. Since they live so long, these smaller stars will accumulate over time. These dwarfs have a surface temperature of ≅700 to 1300°K.
    11. Y Class – The Y class has been proposed for those substellar objects which are colder than the T class dwarfs, but still hotter than hyperjovians, ranging between 400-600°K. Although such dwarfs have been modelled, there is no well-defined spectral sequence yet with prototypes, and six Y-class bodies have recently (as of August 26, 2011) been detected within 40 light years with the Wide-field Infrared Survey Explorer. The spectra of these objects display absorption around 1.55 micrometers. Delorme et al. has suggested that this feature is due to absorption from ammonia and that this should be taken as indicating the T-Y transition, making these objects of type Y0. However, the feature is difficult to distinguish from absorption by water and methane, and other authors have stated that the assignment of class Y0 is premature. These dwarfs have a surface temperature of ≤ 600°K.
    12. D Class – The class D (for degenerate matter) is the modern classification used for white dwarfs — low-mass stars that are no longer undergoing nuclear fusion and have shrunk to planetary size, slowly cooling down. Class D is further divided into spectral types DA, DB, DC, DO, DQ, DX, and DZ. The letters are not related to the letters used in the classification of other stars, but instead indicate the composition of the white dwarf's visible outer layer or atmosphere.
      1. A Group – Hydrogen-rich atmosphere or outer layer, indicated by strong Balmer hydrogen spectral lines.
        1. B Category – A hydrogen- and helium-rich white dwarf displaying neutral helium lines.
        2. O Category – A hydrogen- and helium-rich white dwarf displaying ionized helium lines.
        3. Z Category – A hydrogen-rich metallic white dwarf.
        4. V Category – These are the variable white dwarfs, sometimes called "pulsars."
      2. B Group – Helium-rich atmosphere, indicated by neutral helium, He I, spectral lines.
        1. Z Category – A helium-rich metallic white dwarf.
        2. V Category – These are the variable white dwarfs, sometimes called "pulsars."
      3. O Group – Helium-rich atmosphere, indicated by ionized helium, He II, spectral lines.
        1. V Category – These are the variable white dwarfs, sometimes called "pulsars."
      4. Q Group – Carbon-rich atmosphere, indicated by atomic or molecular carbon lines.
      5. Z Group – Metal-rich atmosphere, indicated by metal spectral lines (a merger of the obsolete white dwarf spectral types, DG, DK and DM).
      6. C Group – No strong spectral lines indicating one of the above categories.
      7. X Group – Spectral lines are insufficiently clear to classify into one of the above categories.
    13. P Class – Actually, this is not a star, but planetary nebulae.
    14. Q Class – Actually, this is not a star, but novae.

  2. Terrestrial Type – This celestial body type can simply be called "space rocks." They can be boulders smaller than a compact car, to large planetoids actually massing more than some of the smaller jovian types. These celestial bodies are predominantly (98+%) comprised of volatiles, minerals, and ferric-based metals (iron, cobalt, nickel). Contemporary examples include Earth, Mars, Venus, Io, Luna, Ganymede, Enceladus, Titan, Europa, Apophis, Ida, etc. Masses below are listed in comparison to Earth's mass (5.9736 × 1024 kg) and abbreviated as Me. Additionally, mass ranges overlap more so than with jovian types.
    1. Planemo Class – < 0.0000001 Me. From planetoid mass object. Essentially, these are nothing more than pebbles and boulders when compared to the other terrestrial types. These are the remnants that were never captured during the stellar system's early protoplanetary disk (proplyd). These objects can present severe hazards (especially navigational) in their own right, depending upon their size. Although none will ever be large enough to create an extinction level event (ELE), some can be large enough to create a cataclysmic event. The Silicate Group is by far (≥ 50%) the most common. The Metallic Group (due to lack of mass) is the least common.
      1. Volatile Group – These are predominantly composed of volatiles.
      2. Silicate Group – These are predominantly composed of silicates.
      3. Carbon Group – These are predominantly composed of carbonates.
      4. Metallic Group – These are predominantly composed of transition metals (nickel, iron, copper, magnesium, etc).
    2. Asteroidal Class – < 0.0001 Me. From the Greek aster and eidos meaning "star" and "form." These celestial bodies are virtually the same as the planemos class except they are larger and more massive. A few of these objects can cause an ELE, but in a mature or older system, they are very rare (perhaps once in a 100 million years). Although composed mostly of volatiles, comets are grouped within the asteroidal class object. The Silicate and Volatile Groups are by far (≥ 75%) the most common.
      1. Volatile Group – These are predominantly composed of volatiles.
      2. Silicate Group – These are predominantly composed of silicates.
      3. Carbon Group – These are predominantly composed of carbonates.
      4. Metallic Group – These are predominantly composed of transition metals.
    3. Lunan Class – 0.0001 to 0.01 Me. From the Latin luna meaning "moon." This class of celestial body is where a celestial body will begin to achieve hydrostatic equilibrium and form into spheroidal shapes. This type is also called "dwarf planet." Contemporary examples: Luna, Ceres, Eris, Mercury, etc. The Silicate Group is by far (≥ 75%) the most common. It is not known why some objects only in this class will be in the sulfide group. Io is a prime example of a sulfide lunan object.
      1. Volatile Group – These are predominantly composed of volatiles.
      2. Silicate Group – These are predominantly composed of silicates.
      3. Carbon Group – These are predominantly composed of carbonates.
      4. Metallic Group – These are predominantly composed of transition metals.
      5. Sulfide Group – These are predominantly composed of sulfur and metallic sulfides. In fact, this group could be viewed as a Category under the Silicate Group. It was created to account for Io.
    4. Glacial Class – 0.001 to 0.1 Me. From the Latin glacius meaning "ice." Most often, this class of celestial body is only found in the outer and deep system orbits. Although they may have a rocky core, these bodies are covered by thin to very thick layers of volatiles consisting of water, methane, ammonia, hydrogen, oxygen in a frozen and/or slushy and/or liquid forms. Some may even call this class of celestial bodies "ice balls." Contemporary examples include: Europa, Enceladus, Triton, Callisto, etc. The Volatile Group is by far (≥ 75%) the most common.
      1. Volatile Group – These are predominantly composed of volatiles.
      2. Silicate Group – These are predominantly composed of silicates.
      3. Carbon Group – These are predominantly composed of carbonates.
      4. Metallic Group – These are predominantly composed of transition metals.
    5. Terran Class – 0.01 to 1.5 Me. From the Latin terra meaning "dirt." Along with the Pelagic and Oceanic Classes of objects, this class is the most common when one thinks of "terrestrial planet." The only difference between the Terrans and the Pelagics is water (dihydrogen monoxide (H2O)). Venus and Mars are good examples of this class of planet. At most, these bodies will have only up to 15% surface water and will be exceptionally saline in nature. The Silicate Group is by far (≥ 75%) the most common.
      1. Silicate Group – These are predominantly composed of silicates.
      2. Carbon Group – These are predominantly composed of carbonates.
      3. Metallic Group – These are predominantly composed of transition metals.
    6. Pelagic Class – 0.25 to 2 Me. From the Greek pelagos meaning "sea." Earth is a very good example of a silicate pelagic celestial body. Surface water will range from 15% to 90%. Anything over 90% is oceanic class. The Silicate Group is by far (≥ 75%) the most common.
      1. Silicate Group – These are predominantly composed of silicates.
      2. Carbon Group – These are predominantly composed of carbonates.
      3. Metallic Group – These are predominantly composed of transition metals.
    7. Oceanic Class – 1.5 to 4 Me. From the Ancient Greek okeanus meaning "Oceanus," a deity. This class of celestial body is virtually completely covered with water. Surface land only ranges between 0% to 9%. If any land exists, it will invariably be in form of volcanic islands along subduction zones. The Silicate Group is by far (≥ 75%) the most common.
      1. Silicate Group – These are predominantly composed of silicates.
      2. Carbon Group – These are predominantly composed of carbonates.
      3. Metallic Group – These are predominantly composed of transition metals.
    8. Vesuvian Class – 3 to 10 Me. From the Latin vesuvius meaning "volcano." Once a terrestrial type body reaches 3 to 4 times the mass of Earth, it begins to cease behaving like the lower mass terrestrial types. Beginning at 3 Me, these types begin to have an overall mean density that can exceed iron and begin to approach the density of lead (this is also true for the Furian Class below). At this point, when in an Old or younger stellar system, these terrestrial types have two to three times the amount of actinides in the mantle, creating a radioactive furnace which fuels the excessive heat of these planets. The atmosphere tends to be thick and dominated by carbon dioxide, sulfur dioxide, hydrogen sulfide, and hydrogen cyanide. If there is any water on a vesuvian, it is invariably in the form of vapor. Standing water is not impossible on these hellish worlds, just highly improbable. However, some vesuvians may possess some small seas in which only the hardiest primitive life can exist (such as extremophiles around black smokers). It is believed that the metallic and actinide groups would be the most common until the radioactive furnace dies down or burns out. Once the radioactive furnace has burnt out in about 10 to 15 billion years, this class may evolve to become a subjovian planet as the atmosphere cools and becomes hydrogen and helium based.
      1. Silicate Group – These are predominantly composed of silicates.
      2. Carbon Group – These are predominantly composed of carbonates.
      3. Metallic Group – These are predominantly composed of transition metals.
      4. Actinide Group – These have an even higher amount of actinides in the mantle than normal. Enough that about 20-35% of the planet's mantle is composed of actinides.
    9. Furian Class – 9 to 15 Me. From the Latin furia meaning "rage." If the vesuvian class is seen as a nightmare, the furian would make it look pleasant by comparison. If there are any crustal rafts on this type of planet, it is paper thin (1 to 2 km thick) and fairly small in area (a few tens of millions to hundreds of millions of square kilometers). Usually, a furian is nothing more than a ball of blistering and boiling lava. The atmosphere is similar to the vesuvian, except it is denser and thicker and more like a blast furnace with temperatures that could almost approach the melting point of iron (1811 K, 1538°C, 2800°F). No life can survive on a furian unless it is a life form that can exist in the molten environment. It is believed that the metallic and actinide groups would be the most common until the radioactive furnace burns out. Once the radioactive furnace has burnt out in about 10 to 15 billion years, this class may evolve to become a subjovian planet as the atmosphere cools and becomes hydrogen and helium based.
      1. Silicate Group – These are predominantly composed of silicates.
      2. Carbon Group – These are predominantly composed of carbonates.
      3. Metallic Group – These are predominantly composed of transition metals.
      4. Actinide Group – These have an even higher amount of actinides in the mantle than normal. Enough that about 20-35% of the planet's mantle is composed of actinides.

  3. Jovian Type – A jovian planet (sometimes also known as a gas giant) is a large planet that is not primarily composed of rock or other solid matter. The term gas giant was coined in 1952 by the science fiction writer James Blish. Arguably it is something of a misnomer, since throughout most of the volume of these planets all the components (other than solid materials in the core) are above the critical point and therefore there is no distinction between liquids and gases. Fluid planet would be a more accurate term. Jupiter is an exceptional case, having metallic hydrogen near the center, but much of its volume is hydrogen, helium and traces of other gases above their critical points. The observable atmospheres of any of these planets (at less than unit optical depth) are quite thin compared to the planetary radii, only extending perhaps one percent of the way to the center. Thus, the observable portions are gaseous (in contrast to Mars and Earth, which have gaseous atmospheres through which the crust may be seen). The rather misleading term, gas giant, has caught on because planetary scientists typically use "rock", "gas", and "ice" as shorthands for classes of elements and compounds commonly found as planetary constituents, irrespective of what phase the matter may appear in. In the outer solar system, hydrogen and helium are referred to as "gases"; water, methane, and ammonia as "ices"; and silicates and metals as "rock." When deep planetary interiors are considered, it may not be far off to say that, by "ice" astronomers mean oxygen and carbon, by "rock" they mean silicon, and by "gas" they mean hydrogen and helium. The alternative term "jovian planet" refers to the Roman god Jupiter—the genitive form of which is Jovis, hence Jovian—and was intended to indicate that all of these planets were similar to Jupiter. However, the many ways in which Uranus and Neptune differ from Jupiter and Saturn have led some to use the term only for the inner two (Jupiter and Saturn). With this terminology in mind, some astronomers are starting to refer to Uranus and Neptune as "ice giants" to indicate the apparent predominance of the "ices" (in liquid form) in their interior composition (Wikipedia). Otherwise, the class of jovian is delineated by its mass compared to Jupiter (1.8986 × 1027 kg) and abbreviated as Mj. The 13 Mj mass is the generally accepted upper limit for a gas giant. However, the difference between a hyperjovian and a brown dwarf is very subtle and not very easy to determine. Essentially, the main difference between a hyperjovian and a brown dwarf is whether it is fusing deuterium in the core. Also, there are some hyperjovians currently believed to have masses up to 20 Mj (see the Exoplanet Explorer). Thus, I set the maximum at the lower end of the 13 to 20 Mj range at 15 Mj.
    1. Subjovian Class – 0.005 to 0.5 Mj. These are smallest of the jovians, some so small (0.005 to 0.025 Mj) they are sometimes called "failed cores." However, this is a serious misnomer. There is no such thing as a "failed core." They are just subjovians with a rather large solid core in comparison to their gaseous composition. Uranus and Neptune are prime examples of this class of jovian types.
    2. Jovian Class – 0.5 to 3 Mj. Most would call these the normal jovians. Jupiter and Saturn are prime examples of this class of jovian types.
    3. Superjovian Class – 3 to 7 Mj. With no contemporary examples, these planets are still similar to Jupiter, just more massive. Superjovians will only be about up to 2 times the size of Jupiter. Planets tend to become denser with a smaller than expected size when they become more massive.
    4. Hyperjovian Class – 7 to 15 Mj. With no contemporary examples, these are the true giants of the planets. Usually, if a hyperjovian forms in a system, it will be in an eccentric orbit. Additionally, most often, it will be the only planet in the stellar system.

    Furthermore, each class of jovian also has five groups which further complete the description of the Jovian Types.

    • Igni- Group – From the Latin ignis meaning "fire." This group of jovians orbit their star in the epistellar orbits. While Jupiter orbits its parent star (the Sun) at 5.2 astronomical units, these planets referred to as hot Jupiters, orbit between approximately 0.01 and 0.2 astronomical units (adjusted for star's Sol mass factor) of their parent stars. Due to high levels of insolation they are of a lower density than they would otherwise be. They are all thought to have migrated to their present positions because there would not have been enough material so close to the star for a planet of that mass to have formed in situ. Most of these have circular orbits (low eccentricities). This is because their orbits have been circularized, or are being circularized, by the process of libration. This also causes the planet to synchronize its rotation and orbital periods, so it always presents the same face to its parent star — the planet becomes tidally locked to the star. They exhibit high-speed winds distributing the heat from the day side to the night side, thus the temperature difference between the two sides is relatively low. After hot Jupiters get their atmospheres and outer layers stripped away (hydrodynamic escape), their cores may become lunan or terran class terrestrial type planets. However, it is believed for a Jupiter sized ignijovian to take about 8-12 billion years.
    • Cali- Group – From the Latin calidus meaning "hot." This group of jovians orbit their star in the inner orbits region. Adjusting for the star's Sol mass factor, these jovian types will orbit in the 0.2 to 0.8 AUs region.
    • Tepi- Group – From the Latin tepidis meaning "warm." This group of jovians orbit their star in the middle orbits region. Adjusting for the star's Sol mass factor, these jovian types will orbit in the 0.8 to 3 AUs region.
    • Frigi- Group – From the Latin frigidus meaning "cold." This group of jovians orbit their star in the outer orbits region. Adjusting for the star's Sol mass factor, these jovian types will orbit in the 3 to 20 AUs region.
    • Cryo- Group – From the Greek kryos meaning "frozen." This group of jovians orbit their star in the deep system orbits region. Adjusting for the star's Sol mass factor, these jovian types will orbit in the 20 to 40 AUs (to the verge of the Kuiper Belt) region. Normally, jovian types do not form out in this region of a system. Instead, they usually migrate into this region through a phenomenon known as scattering.

  4. Rogue Types – These are the planetoids which usually have been ejected from a stellar system. Some may have even formed in the void. Planetoids of this type will also wildly vary in their masses ranging from 100kg to 10 Me, perhaps even into the mass classes for jovians.
    1. Terran Group – These are predominantly composed of rocks and metals (silicates, carbonates, metals). Unless still very young (< 2 billion years), this planetoid will not have any geomorphology.
    2. Glacial Group – These are predominantly composed of frozen liquids (water, ammonia, methane). Most often, this group will not have any form of geomorphology.
    3. Cryonic Group – These are predominantly composed of frozen gases (hydrogen, carbon dioxide, helium, oxygen). Most often, this group will not have any form of geomorphology.