Can the Sudarsky's gas giant classification be applied to ice giants?

Can the Sudarsky's gas giant classification be applied to ice giants?

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Based on the temperature of a gas giant around another star, I have come to understand that it is possible to guestimate its appearance; a classification scheme for gas giants developed by David Sudarky.

I'm wondering if this scheme could plausibly be used to get an idea of what ice giants - Neptune and Uranus like planets - around other stars might look like? If not, are there any resources concerning this topic?

That classification scheme originates from before we had much in the way of observational data on exoplanetary atmospheres. It's not clear that it's a particularly useful description of how hot Jupiters behave: recent models, e.g. Gao et al. (2020) seem to indicate that silicate clouds and hydrocarbon haze are the main factors at play on hot Jupiters, rather than silicates versus alkali metal absorption.

As regards is applicability to ice giants, these planets have substantially higher atmospheric metallicities than gas giants, so it's not clear that these models would apply. For a study of some warm Neptune atmospheres, there's Fortney et al. (2020), who note that the cooling history of the planet is important, and things like tidal heating at depth caused by an eccentric orbit can have significant consequences. Or for a more experimental approach, see He et al. (2020), who indicate that hazes can form in hydrogen-rich atmospheres even in the absence of methane in the initial gas mix. It's a lot easier to find studies on hot and warm planets as these are the ones that are accessible to observations, so they attract more attention: habitable zone terrestrials are an exception to this pattern.

In summary, it's complicated, and things have moved on quite substantially since the Sudarsky et al. papers.


The following is based on the planetary classification system used in Gregory Mandell's Star Trek Star Charts and Chris Adamek's variant found at The Final Frontier, themselves based on the planetary classes so far named in televised Star Trek (classes D, H, J, K, L, M, N, T and Y). I've tweaked it considerably, though, to hopefully make it more closely match both what we've seen on screen and the types of planets found in reality. It has also incorporated David Sudarsky's gas giant classification scheme.

The classification scheme used on Trek is based around class-M being an Earth-type planet. In the original series we saw numerous class-M planets that ranged from being virtually identical to the Earth to all manner of oddly hued worlds, but all with a breathable atmosphere (except for Arret, which was described as class-M in spite of having lost its atmosphere). Other than the one class-K planet (Mudd), we hear very little about other classes, but the simple rule remained M = habitable.

From the movies and TNG onwards, more classes were introduced, such as the barely habitable class-H, the gas giant class-J and the barren class-D. The latter has been used very inconsistently, applying to a ringed gas planet in Voyager "Emanations" and the arid but habitable planet in Voyager "Gravity." TNG introduced class-L as a planet with a breathable atmosphere but otherwise unsuited to animal life (at least long-term), but Voyager gave us several class-L planets with humanoid civilisations (in the episodes "Muse" and "The 37s," notably). Over the years, the idea that only class-M planets are habitable has been lost, with Mandell's scheme including various classes that would have been included under M in the original Trek. I've tried to centre the scheme back on class-M here.

Enterprise revealed that M stands for "Minshara," a Vulcan term. TNG "The Royale" featured an obscure "Transjovian" class-K with a thick cold atmosphere, that I've tried to incorporate into the below scheme. The hellish Class Y was created for Voyager "Demon" and appeared a couple of times since, and the Class T ultragiant was featured in Voyager "Good Shepherd." Other classes mentioned over the years, such as Theta-class planetoids, class-9 gas giant and the Klingon Q'tahl class don't fit into this scheme. * Updated with a new class to contain Essof IV from Star Trek: Discovery, which I've also presumed is the same class as Elba II from the original series. Had to squeeze it in as Class N3 but that'll have to do unless I start importing more letters.

(Images taken from various sources. Classes F, G, H, L, T, V, X and Y rendered by Chris Adamek at The Final Frontier. Classes A and O nicked from Wookiepedia. Classes B, D, E, I, J, K, M, N1, N2, P and Q are all photgraphs of real planetary bodies. Kudos if you can identify them all. )

Class A

E.g. Gothos
Class A planets are young, rocky planetoids, the surface of which is kept at least 50% molten due to the proximity of the parent star or planet, via direct heating or gravitational effects. The atmosphere is thin, boiled away by the intense heat but replaced by volcanic outgassing. Due to the tenuous nature of the atmosphere, the heat released by the volcanic activity quickly dissipates into space.
Life forms: none

Class B
Ferrous/iron planet

Hot zone/ecosphere
E.g. Mercury, Kepler-10b
Small, mostly metallic rocky planetoids. Class B worlds exhibit a highly iron-rich crust, with a magnetic core and no mantle. Atmosphere thin to negligible, with little to no heat retention. The surface varies from extremely hot to cold dependent on position near star, and can exhibit molten surface areas. The night side of the planetoid will fail to retain the heat exhibited on the day side, left a frigid wasteland. These planetoids are inimical to life.
Life forms: none

Class C
Carbon planet

Hot zone/ecosphere
E.g Janssen (55 Cancri e)
Predominantly carbon-based planet, appearing blackened from orbit due to large deposits of graphite. The pressure within the mantle and outer core produces diamond deposits. The atmosphere is composed primarily of carbon dioxide, rich in hydrocarbons and monoxide smogs. Little to no surface water is to be expected on the surface of a carbon planet.
Life forms: anaerobic carbon-based life may be possible

Class D

Hot zone/ecosphere/cold zone/lunar orbit
E.g. Luna, Ceres, Regula, Paan Mokar
Rocky bodies varying in size from the tiniest planetessimal to planet-sized moons. Common around larger planetary bodies and in asteroid belts. Atmosphere tenuous, although water ice can manifest at the poles. Although naturally lifeless, Class D worlds may be adapted through use of pressure domes or oxygen caverns.
Life forms: none.

Class E
Ice dwarf

Cold zone/outer cloud
E.g. Pluto, Eris, Psi 2000
Small, sub-planetary bodies common in the outer star system, in the orbit of Class I planets, through the scattered disc and out into the Oort Belt. With a rocky crust covered in nitrogen ice, and an atmosphere tenuous in the extreme, Class E worlds are incapable of retaining the limited heat they receive from their distant parent star. There may, however, be subsurface water, heated by mantle activity, which can provide the basis for colonisation through pressure domes.
Life forms: rare, microbial.

Class F

Hot zone/ecosphere
E.g. Excalbia
Young planets that are still developing, Class F planets represent the earliest stage of the formation of a habitable world. With partially molten surfaces, atmospheres rich in reactive gases and heavy vulcanism, Class F planets are inimical to life like ours, but have, on rare occasions, developed inorganic life, when present in the hot zone and continued in their plastic state for long enough. Those further out will cool over billions of years to become Class G, the next step in their evolution.
Life forms: metal-carbon complex (e.g Excalbian)

Class G

Hot zone/ecosphere
E.g. Janus VI
With a primarily silicate-based crust, these planets have cooled and solidified from Class F to form a more stable surface, although vulcanism is still rife. Water has begun to condense to form oceans, amid centuries of constant rainfall. The atmosphere and the life that may develop on the surface are intertwined as the rich carbon dioxide atmosphere allows early photosynthetic life to flourish, these organisms flood the atmosphere with oxygen, pushing towards the next stage in its evolution. Over many millions of years further, these Cambrian-stage planets cool further to become classes H, K, L, M, N, O and P, dependent on various factors.
Life forms: primitive organic or silicon-based life, more rarely advanced silicon-based life (e.g Horta)

Class H
Extreme desert

Hot zone/ecosphere
E.g. Tau Cygna III, Shelia, Delta Vega, Nimbus III
Rocky planets with primarily silicate crusts, Class H planets are true desert worlds. With very limited surface and atmospheric water, and high levels of surface radiation, Class H planets are not conducive to complex ecosystems, although hardy life may develop and flourish. Milder Class H environments may be colonised by humanoids with some adaptation. Class M planets can be reduced to Class H through environmental damage.
Life forms: radiation-resistant carbon-based organisms (e.g Sheliak). Not naturally conducive to humanoid life.

Class I
Ice giant/neptunian

Cold zone
E.g. Uranus, Neptune, Marijne VII
Cold worlds with thick atmospheres of hydrogen, water, methane and ammonia, commonly found in the outer reaches of a solar system. The hydrogen envelope is considerably thinner than on a Class J world, but this is still the dominant element of the planet. Such planets commonly attract a number of moons and impressive ring systems. In spite of the name, ice giants have little solid material and are mostly fluid.
Life forms: unknown

Class J
Gas giant/jovian

Ecosphere/cold zone

E.g. Jupiter, Saturn, Cherela
Huge planets with thick hydrogen and helium-based atmospheres, rich in hydrocarbons. Beneath the gaseous layers lies liquid hydrogen above a metallic hydrogen core. Class J planets commonly support many moons and ring systems, and these moons may themselves be habitable worlds in their own right. Class-J planets dominate a star system in the inner region of the cold zone. With sufficient engineering prowess, habitable Class M environments can be constructed between the cloud layers of a gas giant.
Class J planets correspond to classes I to III on the Sudarsky scale. The coolest are Class I jovians, Jupiter-type planets with ammonia clouds, often with complex and powerful weather systems. Warmer are the Class II jovians, which feature water vapour clouds. Class III jovians have no chemical components that form clouds and appear as featureless blue-white orbs.Those straying closer to the star are captured and are heated to Class-S.
Life forms: Jovian-type, hydrocarbon-based (e.g Lothra)

Class K

Ecosphere/cold zone
E.g. Mars, Mudd
Class K planets are essentially dead terrestrial planets, with a primarily silicate crust, rich mineral deposits and no magnetic field. The atmosphere is thin, predominantly carbon dioxide, and retains little heat, leading to a frigid desert landscape. Nonetheless, there can be some weather systems in a Class K atmosphere, and vulcanism can occur. Water and/or carbon dioxide ice may be found at the poles. Class-K environments can develop from the evolution of Class G, or through the long deterioration of classes G, L or M. Rich in mineral deposits. Although fundamentally lifeless except for the most basic of organisms, Class K planets are readily adaptable through use of pressure domes or oxygen caverns, and are prime targets for terraforming.
Life forms: microbial carbon or silicon-based life.

Class K/T

Cold zone
E.g. Theta-116-VIII
This subclass represents frozen class-K planets that have drifted or been expelled into the outer system, commonly by gravitational perturbation by a larger body. A thick atmosphere of nitrogen, neon and methane accretes and can develop turbulent weather systems. Transjovian-class planets are highly inhospitable and experience phenomenally low surface temperatures.
Life forms: none

Class L

E.g. Phylos, Indri VIII, Briori outpost
Similar to Class M planets, Class L are on the borderline of life-bearing environements. Typically rocky, silicate-crust planets, Class L worlds are commonly arid, but in some cases display oceans or tundra. Surface temperature varies considerably, and the atmosphere is thinner than on a Class M world, with high levels of argon, carbon dioxide, and often other toxic gases. Radiation levels are potentially dangerous. Class L environments may feature basic ecosystems, normally only with plant life. They may, however, be colonised by humanoid life, and are excellent targets for terraforming. (Planets assimilated by the Borg, where the atmosphere has been altered by pollution with carbon monoxide, methane and fluorine, may be considered a variant of Class L).
Life forms: Most have no native animal life. Plant life often abundant on more temperate examples.

Class M

Ecosphere/lunar orbit
Also referred to as "Earth-type," S3 or Minshara-class, Class M planets are the cradles of life. With silicate crusts, those with rotating iron cores can display strong magnetic fields. Rich nitrogen-oxygen atmospheres with some carbon dioxide, water vapour and trace gases are ideal for the development of varied, complex biospheres. Class M planets feature high surface and atmospheric water content, essential for organic life. Surface conditions can vary considerably across the globe, from tundra, to temperate, to desert environments. Class M worlds are found in orbit of stars or larger Class-I, J and U planets, and can vary widely in visual appearance. Class M is divided into subtypes dependent on surface water levels and other features, and these can vary over the course of a planet's lifespan (for instance, Earth was a Type-4 ice-world during one period of its early history, and Exo-III was once a more hospitable Type-2).
Life forms: abundant carbon-based life, including humanoids
M Type-1 Arid. E.g. Vulcan, Cardassia Prime, Deneb IV
Surface water 25-50%
M Type-2 Temperate/varied. E.g. Earth, Bajor, Altamid
Surface water 50-80%
M Type-3 Pelagic. E.g. Argo, Azati Prime, Antede III
Surface water 80-95%
M Type-4 Glacial. E.g. Andoria, Exo-III, Rigel X
Surface ice 50-95%
M Irregular E.g. Ba'ku planet, Gaia, Planet Hell
Class M but with unusual features, such a radiation belts and ring systems.

Class N1

Hot zone
E.g. Venus
Although similar to Class M planets in size and geological make-up, Class N planets are rendered as hugely different environments due to their atmospheric conditions. A thick carbon dioxide atmosphere causes a runaway greenhouse effect leading to extremely high surface temperature and pressure, utterly inimical to humanoid life. Some nitrogen, water and sulphur dioxide exist in the atmosphere, which is dominated by clouds of sulphuric acid, leading to corrosive rainfall. A Class N world may potentially be adapted to class-M by long-term terraforming, but this is a significant undertaking and such planets are usally overlooked in favor of more hospitable worlds.
Life forms: rare microbial organisms may exist in cloud layer.

Class N2

Hot zone/lunar orbit
E.g Tholia, Io
A variation of the Class N planet in which a considerably thinner atmosphere, composed mainly of sulphur dioxide and monoxide, sodium chloride vapours and molecular oxygen. Large deposits of sulphur exist on the surface giving a yellow-green colour from orbit. Temperature is lower than N1 conditions, but still high in comparison to Class M, with significant vulcanism caused by gravitational effects from the host planet or star, or by an unstable core. Unlike on N1 worlds, N2 enviroments may develop complex organic life, although such organisms will rely of sulphur respiration and use hydrogen sulphide as a biological solvent in place of water. This life form type is far rarer than the more common oxygen/water type organisms.
Life forms: sulphurphilic organisms (e.g Tholian)

Class N3

Hot zone/ecosphere
E.g. Elba II, Essof IV
Planets with less severe atmospheric effects than classes N1 and N2, Class N3 worlds are still highly dangerous for organic life forms. Displaying variable surface temperatures and pressures, N3 atmospheres are predominantly carbon monoxide, laced with corrosive and reactive chemicals such as sodium perchlorate, rapidly toxic to oxygen-breathing life. However, they are also rich in useful chemicals such as deuterium, and can be used for small habitats using pressure domes.
Life forms: none

Class O

E.g. The Waters, Megara, Kepler-22 b
True ocean worlds with no surface land area. Oceans on Class O planets are typically thousands of kilometres deep, with phenomenal pressures at the depths. Turbulant atmospheres of nitrogen, oxygen, water vapour and carbon dioxide envelop the planet. On hotter variants of the Class O, the ocean surface may vapourise, giving a continuous fluid surface, rather than a delineated ocean and atmosphere, on the edge of becoming a Class U world.. Cooler Class O worlds can potentially be colonised with artificial habitats, and have considerable scope for food cultivation in the form of plankton and algae.
Life forms: abundant, marine carbon-based organisms.


Cold zone/lunar orbit
E.g. Titan, Breen
Similar in size and structure to Class M planets, but in far colder regions, Class P planetoids display enivronments that are like frigid shadows of terrestrial worlds. With a dense nitrogen-methane atmospheres, and surface rich in hydrocarbons, the seas and oceans on Class P worlds are comprised from short-chain hydrocarbons such as methane and ethane. In place of rock, mountains and landmasses form from water ice cryovolcanism is apparent. These planetoids display a subzero ecosystem. In the later stages of a star's evolution, Class P worlds may be heated to another evolutionary stage, dooming existing ecosystems and pushing the planetoid towards classes K, L or M.
Life forms: hydrocarbon and ammonia-based

Class Q

Cold zone/lunar orbit
E.g. Europa, Ganymede, Enceladus
Ocean worlds in colder regions, these are smaller planetoids enclosed in thick water ice crusts. Atmosphere is tenuous, beneath the ice layer exists an extremely deep ocean. Undersea heating from the planetary core, or gravitational effects from a host planet, can lead to non-photsynthetic ecosystems. Commonly form as moons around planets of classes I, J and U. Can potentially be colonised with artificial habitats, although care must be taken not to damage the existing, submarine environment.
Life forms: marine carbon-based organisms

Class R
Rogue/orphan planet

E.g. Dakala, Omarion
A varied class, containing those bodies that are planet-sized but not tied to a star's gravity. Such bodies, sometimes called planemos, can range from terrestrial to Jovian size the largest are on the borderline with the brown dwarf class. Rogue planets form in the interstellar void from accreted material, while orphan planets are ejected from star systems by gravitational effects. Thick, carbon-rich atmospheres can lead to retained surface heat and non-photosynthetic ecosystems, sometimes displaying very unusual adaptations to their harsh environment.
Life forms: varies, from none to complex carbon or silicon-based

Class S
Hot jovian/pegasid

Hot zone
E.g. Galileo (55 Cancri b), Osiris, 51 Pegasi b
Gas giants, similar to classes I and J but in short, close stellar orbit, maintaining an extremely high temperature. Carbon monoxide is the dominant carbon-carrying molecule. Class S planets correspond to classes IV and V on the Sudarsky scale, with Class IV being the cooler of the two, displaying alkali metal vapour clouds. The hottest planets are Class V, with silicates and even iron forming clouds. These planets glow red due to the high thermal output.
Life forms: none known

Class T
Gas supergiant/ultragiant

Cold zone
E.g. Kappa Andromedae b
Gigantic gaseous planets with thick hydrogen atmospheres and enormous gravitational pull, these planets are on the verge of becoming stars. Supergiants accrue complex systems of moons ranging from planetesimal to planetary size, effectively becoming miniature star systems in themselves. Any such bodies that exceed 13.6 Jupiter masses would begin deuterium fusion and become a brown dwarf or "substar."
Life forms: unknown

Class U

Hot zone/ecosphere/cold zone
E.g, Dulcinea (Mu Arae c), Kepler-10c
Existing in size between the Class I ice giants and the Class V superterrestrials, Class U planets are large enough and with strong enough gravity to retain a thick atmosphere of hydrogen, helium and hydrocarbons. The atmosphere transitions to oceans of semisolid compressed water above a rocky core. Sometimes known as gas dwarfs - something of a misnomer for such large planets.
Life forms: Jovian-type, hydrocarbon-based.

Class V

Ecosphere/cold zone
E.g. COROT-7 b, Gliese 163 c, Persephone
The so-called "super-Earths," large rocky/metallic planets intermediate in size between terrestrial and ice giants. Their higher gravity allows them to retain dense, hydrogen-rich atmospheres. Surface temperature and pressure high and unsuitable for humanoid habitation, but complex high-temperature life can evolve, and they are potentially viable for colonisation using pressure domes.
Life forms: silicon or carbon-based, adapted for higher pressures

Class W

Hot zone/ecosphere/lunar orbit
E.g. Daled IV, Klavdia III, Remus
Rocky planets kept tidally locked to the parent star or sister planet by the intense gravitational interaction of other bodies in their system. One side is overlit and heated, displaying molten areas and a burnt, desert-like surface. The far side is kept in perpetual darkness and cold, sometimes with a more temperate dividing line if the atmosphere is dense enough to mediate the heat. Such planets may be colonised, and some display native life that has adapted to the extreme environment, often in unusual ways.
Life forms: microbes and plants, some display higher organisms.

Class X

Hot zone
E.g. COROT-7b
The dead core of a Class-S or T planet, stripped of its atmosphere by millennia of stellar activity. Dense and metal-rich, these planetoids are rare and valuable. Uninhabitable and ultimately doomed to absorption by their parent star.
Life forms: none

Class Y

Hot zone
E.g. Ha'dara
Exceedingly unfriendly, these planets display thick atmospheres rich in toxic gases, high radiation levels, extreme surface pressure and corrosive conditions, even harsher than Class-N planets.
Life forms: rare, but mimetic life has been discovered.

Can the Sudarsky's gas giant classification be applied to ice giants? - Astronomy

The Non-Luminary World Classification Scheme, or NoLWoCS, is a near standard classification method used to identify the many different forms of planetary bodies, minor worlds, and artificial structures that have evolved naturally or that have been created by the many societies and cultures of the Terragen Sphere. While every world or megastructure is, in its own way, unique, there are certain characteristics that can be used to identify and classify these places. The purpose of NoLWoCS is to provide an easy, "at a glance" platform for the common User, whereby he might find the navigation of the Sphere, virtual or real, a little easier.

  • NoLWoCS is divided into three tiers of classification: Class, Type, and Subtype. The different Classes of worlds are dependent on size, overall characteristics, and status of a planet. For instance, Planetoidal and Terrestrial world Classes are divided according to size, just as Terrestrial and Jovian worlds are different Classes because of their general characteristics, and of course, artificial worlds are different from all of these because they are not naturally occurring.
  • World Types are dependant on a variety of factors, but generally the compositional elements, which often lead to different planetary features and behaviors, are of sufficient difference to separate these worlds. Subtypes are much more specific, and often are the result of what would normally be considered minor planetary features. For instance, Gaian worlds are divided into several different Subtypes based on items such as the amount of surface water, atmospheric composition, and so on.

Planetary Cloud/Haze Types and Temperature Regimes

  • Cryoazurian Type Clarified: Below 60 Kelvin, no visible clouds and only faint organic hazes. Dull blue in color.
  • Methanean Type Clouds: Worlds between 55 and 105 Kelvin, hosting clouds of methane and ethane. Stratospheric cirrus clouds of argon and organic hazes are also commonly found on these worlds. Methanean clouds are usually white-turquoise in color.
  • Tholian Type Haze: Hazy worlds between 60 and 400 Kelvin, sporting various types of organic hazes. These worlds are usually a pale yellow, orange, or brown.
  • Ammonian Type Clouds: Worlds hosting ammonia and ammonium hydrosulfide clouds. They can be found at temperatures between 75 and 190 Kelvin. Ammonian clouds can be cream, beige, or peach in color. Organic compounds can result in an orange, brown, or gold tinge. Ammonia clouds form at colder temperatures than ammonium hydrosulfide, and dominate the appearance below around 100 Kelvin.
  • Aquean Type Clouds: These temperate worlds between 175 and 360 Kelvin have clouds of water vapor. Aquean clouds are predominantly white, but can take on a slightly brown, blue, or grey color.
  • Acidian Type Clouds: Warm worlds between 200 and 570 Kelvin that have high metallicities. Clouds are dominated by sulfuric acid, less commonly phosphoric acid or monoammonium phosphate. Water can also be present on acidian worlds, but is usually restricted to cirrus clouds due to the higher temperatures. Acidian clouds are usually tan, taupe, or beige in color.
  • Mesoazurian Type Clarified: Worlds with no high altitude clouds and minimal organic and organosulfur hazes, between around 400 and 700 Kelvin. Dull blue to teal in color.
  • Silicolean Type Haze: Worlds with hazes of silicone oils, fluorosilicones, and other organosilicon compounds, occurring between 550 and 1000 Kelvin. Light brown or grey in color.
  • Chloroalkalinean Type Clouds: Worlds between 620 and 900 Kelvin with clouds of alkali metal chlorides. Potassium chloride is the most common of these, and may also be accompanied by other metal compounds with similar condensation curves such as zinc sulfide. These clouds can be bronze or pale green in color.
  • Sulfoalkalinean Type Clouds: Worlds between 800 and 1100 Kelvin with alkali metal sulfide clouds. There is significant overlap between sulfoalkalinean and chloroalkalinean clouds, with sulfoalkalinean clouds being significantly hotter. The most common compounds in these clouds are sodium sulfide, lithium sulfide, and lithium fluoride. These clouds are dark brown or grey in color.
  • Pyroazurian Type Clarified: Worlds between around 1100 and 1400 Kelvin with no visible clouds or hazes. Can be dull to deep blue depending on the level of impurities.
  • Erythronian Type Clouds: Worlds between 1150 and 1500 Kelvin, possessing clouds of Manganese oxides and sulfides, as well as metallic chromium. Some erythronian worlds are hot enough to host titanium and vanadium oxide hazes instead of organosulfur compounds. Clouds of this type appear dull red to dark orange in color.
  • Enstatian Type Clouds: Worlds with clouds composed of rocky and metallic compounds, usually iron and magnesium silicates. These worlds occur at temperatures between 1200 and 1900 Kelvin, and are predominantly grey with a green, blue, or brown tinge.
  • Rutilian Type Haze: Hazes of titanium and vanadium oxides occurring between about 1450 and 2000 Kelvin. These worlds are dark grey to black in color.
  • Refractian Type Clouds: These clouds occur between 1700 and 2350 Kelvin. They are comprised of various refractory oxides, such as calcium titanate, calcium aluminate, corundum, as well as vanadium and titanium oxides. These clouds are usually pale red, orange, brown, or pink.


ASTEROID CLASS: non-spherical worlds of extremely low mass

Types of Asteroid

  • Carbonic: Almost exclusively of carbon compound construction, resulting in the formation of bodies from pockets of high carbon material around later generation stars choked with heavier elements common near the galactic core. They may also form in systems where two white dwarfs have spiralled together, and the resulting circumstellar disk coalesces into bodies high in carbon.
  • Metallic: More Information here
  • Carbonaceous: More Information here
  • Silicaceous: More Information here
  • Hydronic: Located close to the system's snow-line, these are silicaceous bodies with high instances of subsurface volatiles, typically in the form of water ice. Polar deposits in permanently shadowed regions may also be present.
  • Gelidic: Located beyond the snow line of their system, these are bodies with high instances of ice, ranging from water to methane to carbon dioxide, and many other compounds besides, surrounding a core of silicate rock. The smaller bodies may be a nearly homogeneous mixture of ice and rock, due to the lack of a mass great enough to have caused layer differentiation early in the formative period. See also Centaurian Type
  • Oortean: More Information here.
  • Vulcanian: More Information here.

Minor Planets (Planetoids)

PLANETOID CLASS: worlds with enough mass to pull themselves into spherical or near-spherical shapes

Types of Planetoid

  • Carbonean type: Carbon worlds of this mass have a chance to form around stars whose proto-stellar disks have developed carbon pockets within them, but they are far more common about late generation, high metal stars or even as the results of secondary planetary formation around high carbon stars such as white dwarfs. However, worlds of this size and mass also experience some differentiation during their formation. The cores of such worlds are dense masses of condensed graphite, though the planetoids at the higher mass range could form cores of partially crystallized diamond.
  • Hadean Type: More Information here.
  • Hygiean: These bodies are typically quite dark, with albedos ranging from 0.03 to 0.1. While there can be deposits of water ice or other volatiles beneath the surfaces of these planetoids, the surfaces are more often marked by craters and large boulders. These worlds are less dense and more easily disrupted by major impacts. The larger bodies, however, will have been differentiated through the formation process, and can have small cores of iron, with a dense mantle or rock and a crust of lighter silicates. The smaller worlds, though, may be a relative even mixture of sparse metals and the far more common silicate rock.
  • Cerean Type: More Information here.
  • Chronian type: Named after the plethora of such bodies orbiting the planet Saturn, these can be a highly varied lot. Typically, these worlds are small and heavily cratered bodies untouched by time, save for the numerous impacts that they have suffered. Their low mass and composition of primarily ice, with small rocky cores, are simply too small for sustained geological activity. As such, there is an absence of atmosphere, or related surface features. However, certain disruptions, such as through tidal flexing or other massive external forces may initiate geological forces that can completely resurface a planetoid, as well as form a minor atmosphere. If such active worlds are positioned properly in a gas giant system, an impressive ring system may even be formed.
  • Vestian Type: More Information here.
  • Kuiperian Type: More Information here.

Terrestrial worlds

TERRESTRIAL CLASS: worlds with an active internal geology that lasts one million years or more: 0.05 to 2.5 x Earth's mass

  • Adamaean: Carbon Worlds. Carbon-rich terrestrials. More information here
  • Ferrinian: Iron-rich, dense worlds. More information here
  • Hermian: Dense, inner system worlds. More information here
  • Selenian: Worlds with little or no metallic core.More information here
  • Cytherean: Hot, greenhouse worlds. More information here.
  • PelaCytherean: Terrestrial sized hot ocean worlds with thick atmospheres. More information here.
  • LithicGelidian: Worlds with a mixed rock and ice composition. More information here.
  • Europan: Icy worlds with a subcrustal ocean. More information here.
  • Titanian: Icy worlds with thick atmospheres. More information here.
  • Ymirian: Worlds made almost entirely of ices. More information here.
  • Vesperian: Tidally locked terrestrial worlds. More information here.
  • Hephaestian: These are the most active of planets, with surfaces that are almost entirely molten and a geology that changes on a yearly basis. The atmospheres of these planets vary greatly according to the world's size and mass, from having thick, Cytherean-like atmospheres to almost non-existent ones, where the feeble gravity loses any elements almost as soon as they are released from the surface. These worlds are generally heated by tidal flexing, by proximity to a star or as a moon of a gas giant. Example: Io.
  • Amunian Type: Cold, dry worlds with high levels of ammonia in the atmosphere but little water. May develop an ammonia-based biosphere (see the Soft Ones xenosophonts for one example).
  • Vitriolic Type: Worlds with lakes, seas or oceans of sulphuric acid often with life, More information here.
  • Arean type: Mars-like worlds where the atmosphere and hydrosphere has largely disappeared due to the cessation of magnetic activity. .More information here.
  • EoArean subtype Young Mars-like Type planet with substantial atmosphere and surface water. More information here
  • AreanLacustric subtype: Young Mars-like worlds with moderate amounts of ocean cover . More information here.
  • AreanXeric subtype Mature, unusually hot and dry Arean type worlds. More information here.
  • AreanTundral subtype Cold Arean type worlds, often with considerable reserves of ice More information here.
  • EuArean Subtype Typical mature Mars-like world with minimal atmosphere and hydrosphere More information here.

Earth-like Worlds

  • Gaian Type: Any Earth-like terrestrial world, of which there are many diverse forms depending on water content, composition and temperature. More information here
  • EoGaian Subtype: Young terrestrial worlds these may develop into Gaian, Cytherean or Arean worlds later More information here
  • MesoGaian Subtype: Earth-like worlds with primitive biospheres More information here
  • Eugaian Subtype A mature Gaian world with life, also known as a Garden World More Information Here
  • GaianTundral Subtype: Cold Gaian worlds with periodic, or persistent, ice ages More Information Here
  • Campian Subtype: Dry Gaian worlds with 25% to 50% ocean cover More Information Here
  • Paludial Subtype: Humid Gaian worlds with 25% to 50% ocean cover More Information Here
  • Lacustric Subtype:Humid Gaian worlds with low topography and 50-80% ocean coverage some of these worlds have extensive rainforest-type biomes More Information Here.
  • Chlorogaian (Halogenic) type: Gaian worlds with high levels of atmospheric chlorine. More: Chlorine Worlds.
  • To'ul'hese Worlds: These worlds are essentially Gaian versions of the Cytherean worlds. Thick and dense atmospheres, as well as a large amount of water, create high surface pressures and high temperatures. Life arises and adapts to these conditions, and can become quite diverse indeed. In one known instance, it has lead to an independent form of sapient life. More: To'ul'hian Worlds.
  • Pelagic Subtype Gaian worlds where oceans cover the surface anywhere from 80 to 100%. More Information here
  • EuPelagic Subtype Gaian worlds where shallow oceans cover the surface anywhere from 80 to 100%. More information here
  • BathyPelagic Subtype: Gaian worlds where deep oceans cover the surface anywhere from 80 to 100%. More information here.
  • PelaGelidic Subtype, ice covered ocean worlds More information here.
  • TundralPelagic subtype, partially ice covered ocean worlds More information here.
  • Xeric subtype Dry worlds with less than 25% ocean cover More information here.
  • HyperXeric subtype Very dry worlds with less than 10% ocean cover More information here.
  • PostGaian subtype: Old Gaian Worlds that are losing their biosphere and hydrosphere More information here.


SUPER-TERRESTRIAL CLASS: worlds that are moderately massive, intermediate in mass between Terrestrial worlds and Neptunian worlds

  • Pyrothalassic Type: Hot superterrestrials More information here
  • Pyrohydrothallasic Type - Hot waterworlds More information here
  • Panthalassic Type: Giant Waterworlds More information here
  • Nebulous Type: Superterrestrials with thick, helium-rich atmospheres. Helium worlds of this kind (generally known as Helian Worlds) often have superrotating atmospheres those which are tidally locked often have wildly assymetric weather patterns.
  • Gas Dwarfs: worlds smaller than 0.03 x the mass of Jupiter which nevertheless have thick atmospheres consisting mostly of hydrogen and helium. Gas-rich worlds with more than 50% water/ice are classified as ocean worlds, either pyrohydrothallassic or panthallassic according to temperature.
  • Other types of superterrestrial planets include certain hyperbarian and chthonian worlds, some of which have very sparse atmospheres indeed.

Neptunian Worlds

Neptunian Worlds

Neptunian worlds, also known as Ice Giants, are large worlds with thick atmospheres and more than 50% water content. These worlds are intermediate in type between ocean worlds and gas giants. They can be classified according to size, composition and temperature in many cases a large ice giant will be larger than a gas dwarf or microjovian, since the difference between these two world classes is dependent on composition, not mass. Ice giants show a full range of cloud and haze subtypes, details of which are given earlier in this article. More information here.

Gas Giants

Gas Giants

Gas Giants (also known collectively as Jovian worlds) mostly consist of hydrogen and helium, and (unlike ice giants) have less than 50% water content. Gas giants form beyond the snow line in a protoplanetary cloud, where volatile elements are abundant. Some rare, and very ancient gas giants formed around the first generation of very low metallicity stars and have almost no rocky or metallic component. Gas giants are classified in two ways - by temperature, which affects the composition of the cloud layers of the giant in a number of significant ways, and by mass. Examples of each of the temperature classes can be found in any of the size classes, and vice versa, although some size classes are more common at certain temperatures and vice versa.

A typical gas giant may be classified using both size and temperature types to create a subtype, so the full classification might be Meso-EuJovian Subtype(This is the full classification for Jupiter) or Super-HyperthermalJovian Subtype (the full classification for Behemoth, Hat-P-1B).

Gas Giant Classes

  • Microjovian Type Jovian worlds with masses from 0.03 to 0.21 that of Jupiter, but less than 50% water More Information here.
  • Mesojovian Type Jovian worlds with masses from 0.21 to 8.0 that of Jupiter More information here.
  • SuperJovian Type Jovian worlds with masses from 8.1 to 13.0 that of Jupiter, the theoretical upper limit of planets. Objects more massive than this are classed as Brown Dwarfs. More information here.
  • See also Gas Dwarfs.

Gas Giant Temperature Types

  • HyperthermalJovian Type: Very hot gas giants, with temperatures above 1400 Kelvin. Includes so=called 'Puffy worlds' and 'Comet worlds'. More Information here.
  • EpiStellar Jovian Type: Hot, dark gas giants with temperatures between 900 Kelvin and 1400 Kelvin. More information here
  • AzuriJovian Type Warm clarified blue gas giants with temperatures between 350K and 800K. More information here.
  • HydroJovian Type: Temperate gas giants with clouds predominantly consisting of water vapour. More information here.
  • EuJovian Type: Cool gas giants, with clouds predominantly consisting of ammonia. More information here.
  • CryoJovian Type: Cold gas giants in the outer reaches of a planetary system, generally too cold for clouds to form at all. More information here.

Other World Classes

  • HyperBarian Class: Very dense planets with cores up to 100 x Earth's mass. More information here.
  • Chthonian Class: Gas giant worlds, formerly HyperthermalJovians, which have lost their volatiles through evaporation. More information here.
  • Stevensonian Class Planetary mass objects which are found in interstellar space. More information here.

World types distinguished by their orbital and rotational parameters

Eccentric worlds and Tilted Worlds

  • Skolian Type Worlds Worlds with axial tilts greater than 45 degrees any class of world can have Skolian characteristics. More information here
  • Janusian Type Worlds Worlds in resonant orbits which regularly exchange momentum. More Information here
  • Ikarian type worlds Worlds with eccentric orbits, with an eccentricity greater than 0.35. Any class of world can have an Ikarian type orbit. More Information here.


The artificial worlds found within the Terragen Sphere have almost all been constructed by humans and their mind-children some, such as the Black Acropolis, are much older. Artificial worlds fall into two broad categories- those that rotate to produce artificial gravity, and those that do not.

What does the colour of gas giant planets, like Jupiter's Red Spot come from?

For the primary whites and browns that cover most of the planet, note that almost everything you see when you look at Jupiter is ammonia clouds, which on their own are bright white. Some latitudes are regions of upwelling (zones), and have high ammonia cloud-tops, while other latitudes are regions of downwelling (belts), and have low ammonia cloud-tops, as shown in this diagram. In between these high and low heights sits a thick brown hydrocarbon haze (not shown in the diagram), very chemically similar to smog. The cloud-tops in the zones are sticking up above most of the haze and thus appear fairly white. The cloud-tops in the belts, though, lie below the haze layer, and thus appear colored brown by the overlying haze.

For the occasional bluish regions seen just to the north and south of the equator (as indicated by the arrow here), these are some of the rare cloud clearings that occur in very strong downwelling regions. We're actually peering through the ammonia top cloud layer, and perhaps even down through the ammonium hydrosulfide middle cloud layer and the bottom water cloud layer. So, in those regions we're looking at just clear air, which has the exact same color as it does one Earth, blue. This is entirely due to Rayleigh scattering, the same reason that Earth's sky is blue.

Then there's the reds, notably in Jupiter's Great Red Spot, although also occasionally seen in another big vortex here and there. As of right now, we don't actually know what makes the Great Red Spot red - this is generally known as the Jovian chromophore problem. Since this color is only seen in very large vortices, it's believed to be caused by some mixture of compounds already present on the planet getting pushed very high in the atmosphere by these vortices. In three dimensions, the Great Red Spot is essentially shaped like a wedding cake, so the cloud-tops at the center of the spot are at very high altitudes where there's a lot more ultraviolet light. You can end up producing all kinds of odd substances through UV photochemistry of trace substances in the atmosphere, and the working hypothesis at this point is that it's some kind of imine or azine.

Worldbuilding in Practice [ edit | edit source ]

Hyrja is a class II gas giant of 1.99 MJ and 0.90 RJ orbiting the blue giant Hrimnir at an enormous distance of 1,437 AU. At such a distance, the planet completes an orbit every 6,515 Earth years, but still receives more sunlight from its incredibly luminous star than Earth does from the Sun. Hyrja has over 40 moons, many of which are exploited by ESIC mining teams. The moons alone supply a steady income of mineral wealth and water, supplemented by a vast agricultural habitat on Hyrja's moon Ari, to keep the inhabitants of the Hrimnir system self-sufficient.

What Makes Something A Planet, According To An Astrophysicist?

The Solar System formed from a cloud of gas, which gave rise to a proto-star, a proto-planetary . [+] disk, and eventually the seeds of what would become planets. The crowning achievement of our own Solar System's history is the creation and formation of Earth exactly as we have it today, which may not have been as special a cosmic rarity as once thought. Our planet will persist for a very long time, but just like everything else in this Universe, we won't last forever.

Ever since 2006, when the International Astronomical Union (IAU) officially defined the term planet — introducing the term 'dwarf planet' to classify Pluto, Eris, Ceres and others — the scientific community has been split in two. Only you have enough mass to pull yourself into a spheroid, orbit the Sun and no other body, and can clear your orbit within Solar System timescales, can you be classified as a planet.

On the one hand are astronomers, mostly planetary astronomers, who largely like the IAU's definition, but want to extend it to more general cases, including exoplanetary systems. On the other hand are planetary scientists and planetary geologists, who look at intrinsic properties only, and argue that if you can pull yourself into a spheroidal shape, you deserve to be a planet. But to an astrophysicist, both definitions are insufficient. Here's why.

Although we now believe we understand how the Sun and our solar system formed, this early view is an . [+] illustration only. When it comes to what we see today, all we have left are the survivors. What was around in the early stages was far more plentiful than what survives today.

Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute (JHUAPL/SwRI)

An astrophysicist looks at the objects in the Universe from a different point of view than other types of scientists. We're not just interested in what the objects we find throughout outer space are like, where they are, and how they behave. Instead, we're interested in the physics behind their intrinsic and extrinsic properties. We ask questions like the following:

  • How did these objects form?
  • How is their composition related to their formation history?
  • What processes were at play that led to them having the physical and chemical properties they have today?
  • And what are the dynamics that drive the evolution of these objects throughout our cosmic history?

When you start asking questions like this, you begin arriving at very general stories that describe planetary formation in general. If you follow these lessons, they'll lead you in directions that most astronomers and planetary scientists would never have assumed.

The same three-dimensional molecular cloud is responsible for all three of the star-forming nebulae . [+] shown here, along with much more. The cloud extends for thousands of light years in all directions in space, and will eventually lead to the creation of tens-to-hundreds of thousands of new stars.

Most stars — and therefore, most solar systems and most planets — form under the same circumstances: in a large, massive, collapsing molecular cloud. When a large enough gas cloud collapses, it fragments into smaller components, where the most overdense regions accrue greater and greater amounts of matter. In our galaxy alone, dozens of these regions are known, giving rise to new stars with new solar systems around them.

These star-forming regions, like the ones found in the Orion Nebula (below), are the locations where new stars and planets form most copiously throughout the Universe. About 50% of all the stars that form will be like our own Solar System, with one central star surrounded by a protoplanetary disk, while the remaining stars will form as part of multi-star systems.

30 protoplanetary disks, or proplyds, as imaged by Hubble in the Orion Nebula. Forming a star with . [+] rocky planets around them is relatively easy, but forming one with Earth-like conditions in subtle-but-important ways is far more challenging.

Most of the matter in these newly forming systems will either fall onto the central star(s) in the system or, failing that, will be blown away back into the interstellar medium. However, within these protoplanetary disks, small imperfections begin to grow by gravitationally attracting more and more matter to them.

What ensues, therefore, is a great cosmic race: between the radiation from the stars that evaporates and blows off the nearby matter, and the gravitational growth of these imperfections. The overdense clumps that grow the fastest are the cosmic winners, as gravitation is a runaway force. These lead to the largest planets of all: the gas giants and ice giants of the Universe, with hydrogen and helium envelopes surrounding them.

20 new protoplanetary disks, as imaged by the Disk Substructures at High Angular Resolution Project . [+] (DSHARP) collaboration, showcasing what newly-forming planetary systems look like. The gaps in the disk are likely the locations of newly-forming planets, with the largest gaps likely corresponding to the most massive proto-planets.

S. M. Andrews et al. and the DSHARP collaboration, arXiv:1812.04040

But, at least according our best understanding, it will take some time to get there. Even with one or more central stars (or proto-stars), there are complicating factors.

First off, the protoplanetary disk is going to undergo segregation of its elements. Just as the heaviest, densest elements sink to the centers of planets (or fall to the bottom of a centrifuge), the heaviest elements will preferentially segregate towards the center, while the lighter elements will be found more abundantly progressively farther out.

As these gravitational perturbations grow, the race intensifies: between planets trying to grow and accrue matter, and the nearby star(s) that evaporate these protoplanetary disks with their high-energy radiation.

An illustration of a protoplanetary disk, where planets and planetesimals form first, creating . [+] 'gaps' in the disk when they do. As soon as the central proto-star gets hot enough, it begins blowing off the lightest elements from the surrounding protoplantary systems. A planet like Jupiter or Saturn has enough gravity to hold onto the lightest elements like hydrogen and helium, but a lower-mass world like Earth does not.

This leads to a few separate zones around a newly forming star.

  1. An interior region where only metals, minerals, and heavy elements and compounds can exist. Organic, aromatic carbon bonds are destroyed by the intense radiation this close to the star.
  2. A "soot line" that defines the barrier between this interior region and the next one out.
  3. A temperate region, where these carbon bonds can persist but ices — such as water-ice, methane-ice, and carbon dioxide-ice — are sublimated/evaporated/boiled away.
  4. A "frost line" that defines the barrier between this temperate region and the next one out.
  5. A colder region, where ices can form and remain stable.

The location of these lines will change over time, as the star will evolve in temperature and luminosity over its lifetime.

A schematic of a protoplanetary disk, showing the Soot and Frost Lines. For a star like the Sun, . [+] estimates put the Frost Line at somewhere around three times the initial Earth-Sun distance, while the Soot Line is significantly further in. The exact locations of these lines in our Solar System's past is hard to pin down.

NASA / JPL-Caltech, annonations by Invader Xan

Now, planets and proto-planets don't simply stay where they first form, but interact with one another over time, leading to a great many interesting possibilities for what can happen. These gravitational interactions will typically lead to planetary migration, where these young planets can move inwards or outwards depending on the dynamics of the Solar System: they will not necessarily remain in the same approximate location to where they formed.

In addition, these planets or proto-planets can collide and merge this may be the mechanism that created our modern Earth-Moon system.

They can also gravitationally interact, either hurling planets into the Sun or ejecting them from a Solar System entirely.

In the early Solar System, it's very reasonable to have had more than four seeds for giant planets. . [+] Simulations indicate that they are capable of migrating inwards and outwards, and of ejecting these bodies as well. By time we reach the present, there are only four gas giants that survive.

K. J. Walsh et al., Nature 475, 206–209 (14 July 2011)

Outside of the frost line, meanwhile, the largest, most massive planets can form. Far enough away from the high temperatures and radiation of their parent star, atoms and molecules of all types can grow into their own miniature solar system. The central planet will accrue most of the mass and matter, enough that they should have a core and mantle like the rocky planets, but enclosed by an enormous gas envelope.

Meanwhile, the matter surrounding them forms a circumplanetary disk, which will break up into rings and moons and moonlets: something we see around all four of the gas/ice giants found in our Solar System at present. These gravitationally dominant bodies — the most massive ones at their location in their solar system — are a product of their own star system's unique evolutionary history.

As solar systems evolve in general, volatile materials are evaporated, planets accrete matter, . [+] planetesimals merge together or gravitationally interact and eject bodies, and orbits migrate into stable configurations. The gas giant planets may dominate our Solar System's dynamics gravitationally, but the inner, rocky planets are where all the interesting biochemistry is happening, as far as we know. In other solar systems, the story can be vastly different, depending on where the various planets and moons wind up migrating to.

Wikimedia Commons user AstroMark

Sometimes, though, we find gas giant or ice giant planets nearby their parent stars: interior to the frost line or even the soot line!

Migration. Gravitational interactions. Via the ejection of other planets or proto-planets. Or even from forming outside of the frost line, and then having the frost line evolve outwards with time.

We think you have to be outside of the frost line to first form a gas/ice giant, but that migration is quite normal. These hot Jupiters (or hot Neptunes) are not at all uncommon, and are some of the easiest planets to find with our current techniques. From the combination of metal-rich material (which forms planetary cores), mantle-like silicates (which can form all throughout a proto-solar system), and ices, gases, and other volatiles (which are more abundant beyond the frost line), we see a general picture begin to emerge.

The planetesimals from the portions of the Solar System beyond the Frost Line came to Earth and made . [+] up the majority of what is our planet's mantle today. Out beyond Neptune, these planetesimals still persist as the Kuiper belt objects (and beyond) today, relatively unchanged by the 4.5 billion years that have passed since then.

NASA / GSFC, BENNU’S JOURNEY - Heavy Bombardment

Interior to the frost line, we'd expect to find a mix of rocky and gas/ice giant planets. Some of them will have formed in situ there, others will have migrated into that region. They may have moons or not.

Right around the frost line, there should be a belt of planetesimals, assuming they haven't been cleared out by migrating planets, that failed to grow into a full-borne planet. This corresponds to the asteroid belt in our Solar System, and there should be an analogue of this belt in most solar systems.

Exterior to the frost line, there will be additional planets: gas giants, ice giants, and in many systems (but not our own), terrestrial-sized planets. There will continue to be planets, moving outward, until some limit is reached. Beyond that, there will be the icy bodies akin to the ones we find in the Kuiper belt and Oort cloud: interesting in their own right, but composed almost entirely of ices and volatile materials, with comparatively minuscule cores.

A logarithmic view of our Solar System, extending out all the way to the next-nearest stars, shows . [+] the extent of the asteroid belt, Kuiper belt, and Oort cloud. What we know as the 8 planets, today, have definitively different formation histories than any of the other rocky or icy bodies found in the Solar System.

This is an accurate descriptor of what we expect to find around any singlet star. Multi-star systems will have certain components removed: tight binaries should have a significant region close to both stars where planetary orbits are unstable. Wide binaries should have interior regions where planet formation is fine, then an intermediate region where no stable planetary orbits are possible, followed by a region well outside of the stellar orbits where planets (or Kuiper belt/Oort cloud objects) are fine again.

But there's an additional type of planet that we're missing if we only look at the bodies that remain in orbit around full-blown stars: rogue planets.

Rogue planets may have a variety of exotic origins, such as arising from shredded stars or other . [+] material, or from ejected planets from solar systems, but the majority should arise from star-forming nebulae, as simply gravitational clumps that never made it to star-sized objects. There is no name for these objects that doesn't have 'planet' in their title.

Christine Pulliam / David Aguilar / CfA

These are planets that either were ejected in the early days of their solar system's history, or that formed in isolation, without a parent star at all, from the collapse of a molecular cloud. The first type of planet could be a full-grown planet like any of the ones found in nature, or they could be proto-planets that were not yet finished growing up before they were ejected.

The second, on the other hand, could range from small, rocky/icy worlds all the way up to gas giants or even brown dwarfs (failed stars), complete with their own pseudo-planetary systems. As our telescopic power and the surveys we conduct with these instruments continue to increase, we fully expect to find large populations of all of these bodies: around stars, in interstellar space, and all throughout the galaxy and Universe.

TRAPPIST-1 system compared to planets of the solar system and the moons of Jupiter. Although it . [+] might seem arbitrary how these objects are classified, there are definitive links between the formation and evolutionary history of all of these bodies and the physical properties that they have today.

From an astrophysicist's perspective, the types of objects we find throughout the Universe are inextricably linked to their composition and formation, and that's the only sensible way to classify them. Non-stellar objects that are massive beyond a certain threshold are like animals: the broadest category that we can classify them into.

Objects that win their gravitational race against radiation and that don't become the failed planets of the asteroid belt, Kuiper belt, or Oort cloud are more like a narrow category like mammals: where they have certain properties and histories that link them together, independent of the other classes. Similarly, asteroids within a solar system are all similar, as are Kuiper belt objects and Oort cloud objects. They are like birds, reptiles, and amphibians: all animals, but of a different class than mammals.

Europa, one of the solar system's largest moons, orbits Jupiter. Beneath its frozen, icy surface, a . [+] liquid water of ocean is heated by tidal forces from Jupiter. Its properties are governed by its history and location in the Solar System. Even though it is large, massive, and may harbor life beneath its surface, its properties would be vastly different if it were a planet instead of a moon.

NASA, JPL-Caltech, SETI Institute, Cynthia Phillips, Marty Valenti

A dolphin may look like a fish, but it's really a mammal. Similarly, the composition of an object is not the only factor in classifying it: its evolutionary history is inextricably related to its properties. Scientists will likely continue to argue over how to best classify all of these worlds, but it's not just astronomers and planetary scientists who have a stake in this. In the quest to make organizational sense of the Universe, we have to confront it with the full suite of our knowledge.

Although many will disagree, moons, asteroids, Kuiper belt and Oort cloud objects are fascinating objects just as worthy of study as modern-day planets are. They may even be better candidates for life than many of the true planets are. But each object's properties are inextricably related to the entirety of its formation history. When we try to classify the full suite of what we're finding, we must not be misled by appearances alone.

Terminology [ edit | edit source ]

The term gas giant was coined in 1952 by the science fiction writer James Blish. Arguably it is something of a misnomer, because throughout most of the volume of these planets the pressure is so high that matter is not in gaseous form. ⎛] Other than solid materials in the core, all matter is 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 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 planets similar to the latter two.

With this terminology in mind, some astronomers have started referring to Uranus and Neptune as "ice giants" to indicate the apparent predominance of the "ices" (in liquid form) in their interior composition. ⎜]

Objects large enough to start deuterium fusion (above 13 Jupiter masses for solar composition) are called brown dwarfs, and these occupy the mass range between that of large giant planets and the lowest-mass stars. The 13-Jupiter-mass (Template:Jupiter mass) cutoff is a rule of thumb rather than something of precise physical significance. Larger objects will burn most of their deuterium and smaller ones will burn only a little, and the Template:Jupiter mass value is somewhere in between. ⎝] The amount of deuterium burnt depends not only on the mass but also on the composition of the planet, especially on the amount of helium and deuterium present. ⎞] The Extrasolar Planets Encyclopaedia includes objects up to 25 Jupiter masses, and the Exoplanet Data Explorer up to 24 Jupiter masses.

What does the colour of gas giant planets, like Jupiter's Red Spot come from?

For the primary whites and browns that cover most of the planet, note that almost everything you see when you look at Jupiter is ammonia clouds, which on their own are bright white. Some latitudes are regions of upwelling (zones), and have high ammonia cloud-tops, while other latitudes are regions of downwelling (belts), and have low ammonia cloud-tops, as shown in this diagram. In between these high and low heights sits a thick brown hydrocarbon haze (not shown in the diagram), very chemically similar to smog. The cloud-tops in the zones are sticking up above most of the haze and thus appear fairly white. The cloud-tops in the belts, though, lie below the haze layer, and thus appear colored brown by the overlying haze.

For the occasional bluish regions seen just to the north and south of the equator (as indicated by the arrow here), these are some of the rare cloud clearings that occur in very strong downwelling regions. We're actually peering through the ammonia top cloud layer, and perhaps even down through the ammonium hydrosulfide middle cloud layer and the bottom water cloud layer. So, in those regions we're looking at just clear air, which has the exact same color as it does one Earth, blue. This is entirely due to Rayleigh scattering, the same reason that Earth's sky is blue.

Then there's the reds, notably in Jupiter's Great Red Spot, although also occasionally seen in another big vortex here and there. As of right now, we don't actually know what makes the Great Red Spot red - this is generally known as the Jovian chromophore problem. Since this color is only seen in very large vortices, it's believed to be caused by some mixture of compounds already present on the planet getting pushed very high in the atmosphere by these vortices. In three dimensions, the Great Red Spot is essentially shaped like a wedding cake, so the cloud-tops at the center of the spot are at very high altitudes where there's a lot more ultraviolet light. You can end up producing all kinds of odd substances through UV photochemistry of trace substances in the atmosphere, and the working hypothesis at this point is that it's some kind of imine or azine.


A gas giant is a massive planet with a thick atmosphere of hydrogen and helium. They may have a dense molten core of rocky elements or the core may have completely dissolved and dispersed throughout the planet if the planet is hot enough. [ 4 ] The hydrogen and helium in "traditional" gas giants like Jupiter and Saturn constitutes most of the planet, whereas the hydrogen/helium only makes up an outer envelope on Uranus and Neptune which are sometimes called ice giants, as they are mostly composed of water, ammonia, and methane molten ices.

Among extrasolar planets, Hot Jupiters are gas giants that orbit very close to their stars and thus have a very high surface temperature. Hot Jupiters are currently the most common form of extrasolar planet known, perhaps due to the relative ease of detecting them.

Gas giants are commonly said to lack solid surfaces, but it is closer to the truth to say that they lack surfaces altogether since the gases that make them up simply become thinner and thinner with increasing distance from the planets' centers, eventually becoming indistinguishable from the interplanetary medium. Therefore landing on a gas giant may or may not be possible, depending on the size and composition of its core.

Belt-zone circulation

The bands seen in the Jovian atmosphere are due to counter-circulating streams of material called zones and belts, encircling the planet parallel to its equator. The zones are the lighter bands, and are at higher altitudes in the atmosphere. They have an internal updraft, and are high-pressure regions. The belts are the darker bands. They are lower in the atmosphere, and have an internal downdraft. They are low-pressure regions. These structures are somewhat analogous to high- and low-pressure cells in Earth's atmosphere, but they have a very different structure—latitudinal bands that circle the entire planet, as opposed to small confined cells of pressure. This appears to be a result of the rapid rotation and underlying symmetry of the planet. There are no oceans or landmasses to cause local heating, and the rotation speed is much faster than it is on Earth. There are smaller structures as well: spots of different sizes and colors. On Jupiter, the most noticeable of these features is the Great Red Spot, which has been present for at least 300 years. These structures are huge storms. Some such spots are thunderheads as well.

Planetary composition classes

I’m starting this thread as a spin off to a question raised in a different thread. The question involved classifying planets by the composition types:

The IAU definitions for planet and dwarf planet are not based upon composition. The definitions are based upon two dynamical traits and one geophysical trait. The two dynamical traits are (1) that the body must have primary orbit around the Sun (or extending to exoplanetary systems a star) and (2) that the body is a “planet” if it is dynamically dominant in its orbit and it is a “dwarf planet” if the body is not dynamically dominant in its orbit. The geophysical trait is that the body must have sufficient mass to self-gravitate into a spheroidal shape. This geophysical trait is significant because bodies that are massive enough to be spheroidal will generally undergo internal differentiation and chemical/physical alteration of composition from the raw material state.

Obviously, these definitions have been controversial. I think there would have been very little controversy if the IAU had done a more complete job on the dynamical side of the definitions. Basically spherical sub-stellar bodies formed in a proto-planetary disk have 4 general orbital dynamical situations. The IAU ignored two of them. The 3rd and 4th are: (3) the spherical body orbits a larger spherical body – what we call “moons”. So these bodies should be identified as the class “satellite planets”. (4) Some spherical bodies are ejected from their stellar orbit and are free floating. These bodies should be called “rogue planets”.

So when it comes to composition types, it turns out there are more composition classes than typically named in the traditional Terrestrial, Gas Giant, and Ice giant system that has been utilized. The purpose of this thread is not to debate the IAU planet definitions, but rather to explore how we might classify the numerous planetary composition types that are present both in the Solar System and in exoplanetary systems.

Before looking at specific planetary composition types it is important to note that there are three general compositions for the materials that make up planets: Rock, Ice, and Gas. Rock is primarily what are called “refractory” materials that maintain a solid state at higher temperatures. This is primarily silicate minerals and iron/nickel, but can also include carbon rich minerals. Astrophysical ices primarily include water, methane, ammonia, and various other C-H based compounds. These compounds are considered “volatile” because they can be gases at much lower temperatures than the refractory materials. Final gas is hydrogen and helium. For astronomers, heavy elements or the “metallicity” refers to elements heavier than helium. Since helium as an atomic number of 2 (Z=2) the metallicity refers to the Z>2 composition of a star. Note that the astrophysical ices need not be in a cold solid state, but can be in high temperature, high pressure liquid or solid state in the interior of a massive planet. So “ices” simply is referring to a composition, not a state.

So in our Solar System there are the following composition types for planets, dwarf planets, and spherical moons:

1. Terrestrial: Mercury, Venus, Earth, Mars, the Moon, Io. These are “rock” planets with the mass composed entirely of various silicate and iron/nickel based minerals. The interior composition of a terrestrial planet includes an iron core, silicate mantle, and silicate crust. It is important to note that the iron fraction can vary quite significantly. Mercury is mostly iron by mass whereas the others listed are mostly silicates by mass.

2. Gas giant: Jupiter & Saturn: Gas giants are theorized to have form by the “core accretion” process in which a 6-10 Earth mass core of rock & ice forms and then the body rapidly accretes H/He gas from the solar nebula. The gas giant interior models include the rock/ice core surrounded by a metallic hydrogen envelope and finally an outer hydrogen envelope.

3. Ice giant: Uranus & Neptune: Ice giants are the only planet type in the Solar System that contain significant mass fractions of all three composition components: rock, ice, and gas. The general model for an ice giant is a rock core or rock/ice core of roughly 20-25% of the planetary mass, overlain by a liquid ionic ice mantle that comprises roughly 65% of the planetary mass. The outer envelope of the ice giants is H/He with enough methane to give the blue color and comprises about 10-15% of the planetary mass.

So these three planetary composition types have specific names, but there are other types:

4. Dwarf planets and satellite planets that are mostly rock by mass with a non-negligible contribution from ices. Examples of these rock/ice planets include Ganymede, Europa, Triton, Eris, Pluto, Enceladus. Pluto, for example, has a composition that is

70% rock and 30% ices by mass. The interior structure of these bodies are varied but generally there is either a metallic iron core or a rock core, then a rock mantle, and finally ice layers that include solid ices and typically a liquid ocean layer and finally an ice crust. So Pluto’s surface is made from nitrogen and other ices, but its interior is mostly rock.

5. Dwarf planets and satellite planets that are mostly ice by mass with a non-negligible contribution from rock. Examples of these bodies include: Callisto, Iapetus, Tethys, Mimas and quite a few of the other moons of the outer planets. Tethys is an almost pure ice body with a rock fraction of only

These ice/rock planets are similar to the rock/ice planets described above but the higher composition of ices results in important differences. The lower rock composition means that the core is more likely to be primarily silicates rather than iron. In addition, the liquid ocean layers in the interior are going to be lying on top of solid ice layers rather than a rock mantle. This is one of the reasons why Europa and Enceladus are considered good location to search for life whereas others are less often mentioned. The interior oceans of Europa and Enceladus sit upon a rock mantle which can provide raw materials that will support life. Moons with a liquid ocean sandwiched between solid ice layers are less likely to have the energy and raw materials for life.

It is unclear how Ceres should be classified as. It is rocky in nature, but may have enough water that, while it does not form a pure ice crust and liquid interior layers, it could be said to be “hydrated”. So technically this might be yet another composition class. The important thing is that if bodies like Ceres are found, they would be terrestrial with a solid, mostly rock surface, but they would have a different minerology due to the larger fraction of water and other ices.

There are some composition types that are only seen in the exoplanet population:

6. Exoplanets around stars with a carbon to oxygen ratio much higher than the Sun’s may form carbon rich terrestrial planets with a graphite crust overlying deeper silicate mantle layers.

7. The “super-Earth” and “sub-Neptune” population includes a class of planets that is almost entirely rock by mass, but contains a

0.5% to 5% by mass H/He envelope that significantly increases the planetary radius. These bodies would have a solid terrestrial surface but the envelope of gas that expands the planetary radius.

8. The “super-Earth” and “sub-Neptune” population also likely includes planets that are mostly rock by mass but with an envelope of ices (mostly) water in the gas phase that again increases the radius. These bodies are similar in bulk composition to bodies like Ganymede but with much greater mass and higher temperatures.

9. The “super-Earth” and “sub-Neptune” also likely includes a population of planets that are similar to ice giants in that they have non-negligible mass fractions of rock, ice, and gas, but instead of most mass as ices, most mass is rock with much smaller fractions of ices and gas. In fact, this class likely also extends up into the “super-Neptune” population which may grow as large as

10. While the IAU definition for a brown dwarf is based upon the deuterium burning limit (

13 Jupiter masses), many researchers prefer a definition based upon formation mechanism. When I was growing up it was sometimes mentioned that Jupiter might be a “failed star”. However, this is incorrect. Jupiter formed in a proto-planetary disk most likely via the core accretion process and therefore has a rock core.

Brown dwarfs as originally defined in the 1960’s when proposed by Kumar were true failed stars. Kumar proposed the gas collapse mechanism that forms stars could form a hydrogen core body that did not gain sufficient mass to enable sustained core hydrogen fusion. These bodies would not have a rock core.

So further research has uncovered that gas giants formed in a proto-planetary disk could exceed the deuterium burning limit and thus have a mass in the “brown dwarf range”. Note that the stellar gas collapse process can form brown dwarfs below the deuterium burning limit (as small as

4 Jupiter masses). There is therefore an overlap between the gas giant planet formation process and the brown dwarf formation process. Gas giants that exceed the deuterium burning limit are referred to as “deuterium burning planets”.

So I’ve just listed 10 distinct planet composition types that are known to exist or likely to exist. It would be difficult to provide names for all of these classes. And in the future more may be discovered.

What I have proposed is a composition classification system that instead of exclusively using names, uses codes. The code is simple. It starts with R, I, or G for planets that are > 50% rock, ice, or gas respectively. Subscripts are then added to further define the composition as most planets have a mixed composition.

Here are the codes:
RM: 100% rock composition with >50% iron (Mercury)
RS: 100% rock composition with > 50% silicates (Venus, Earth, Mars, Moon, Io)
RC: 100% rock composition with a carbon enriched composition (possibly certain exoplanets)
RI: >50% rock with remaining mass as ices (Europa, Ganymede, Pluto, Titan, Triton … numerous super-Earth exoplanets)
RH: Almost 100% rock but composition altered by a few % water by mass (possibly Ceres)
RG: Nearly 100% rock, but with a few % H/He envelope that increases the planetary radius (numerous super-Earth exoplanets).
RIG: > 50% rock with non-negligible mass percentage of both ices and H/He gas (exoplanets and actually this composition cannot be completely ruled out for Uranus and Neptune).
I: Almost 100% ices with <10% rock core. (Tethys)
IS: >50% ices with a non-negligible rock (silicate) fraction.
ISG: Ice Giant composition
GZ: Gas giant composition. This would be more specific where the Z would be subscripted numbers indicating the estimated mass percentage of Z>2 elements in the core. For example Jupiter would be a G01 because the core mass fraction is estimated to be between 0 and 10% whereas Saturn would be a G23 because the core mass fraction is modeled to be between 20 and 30%. You could have any number of ranges for this such as G13, G24, G12
GD: Gas giant exceeding the deuterium burning limit.

With this system you could apply the names we already use. Terrestrial planets are RM and RS and you could in theory expand that to RC and RH planets. Ice Giant are ISG and Gas Giants are GZ. If you wanted to create names for other classes you could but certainly for identifying planet composition in databases the codes would be an easy way to search them. The other thing about the system I have proposed is that it is adaptable to future discoveries. If a composition type is discovered that is not included above, a new code could be added to the list.

I have a paper posted on arXiv that describes a version of this system, but it was not accepted for publication. The composition codes were not the area of disagreement . The reviewer took issue with other classification ideas the paper also discussed. I haven’t bothered to try submitting again but I might in the future. I’ve been busy with many other things in my life.

Watch the video: So sehen die Planeten innen aus (May 2022).