--- title: Planet classification canonical_url: https://paxabyssi.com/wiki/Planet_classification markdown_url: https://paxabyssi.com/wiki/Planet_classification.md type: wiki-page revision_id: 237 revision_view: stable last_updated: 2026-09-27 license: CC BY-SA 4.0 license_url: https://creativecommons.org/licenses/by-sa/4.0/ science_status: - observed - model - sim summary: How astronomers sort planets by size, mass, temperature and composition, and how Pax Abyssi files every world it generates under one of 37 coded types. categories: - Planets - Planet classes - Reference aliases: - Planet types - Planet classes - Planet type codes - Planet taxonomy - 37 planet types - Planet codes infobox: type: physics_concept name: Planet classification image: File:Planet_classes_sim_montage.avif formulae: - "Bulk density: rho = 3M / (4 pi R^3); in Earth units rho = 5.51 g/cm^3 x (M/M_Earth) / (R/R_Earth)^3" - "Equilibrium temperature: T_eq = T_star sqrt(R_star / 2a) (1 - A_B)^(1/4), about 278 K x (1 - A_B)^(1/4) (L/L_Sun)^(1/4) (a/1 AU)^(-1/2)" validity: Classes built on radius or minimum mass alone cannot tell rock from water or gas; a composition class needs both mass and radius, and an atmosphere class needs a spectrum. sim_types: note: in 9 series; about 100 physics-derived subtypes value: 37 definition: "Sorting planets into classes by what can be measured: radius, mass, bulk density, equilibrium temperature and, where spectra exist, atmospheric chemistry. No single scheme is official beyond the IAU's definitions of what a planet is." sim_source: "Type registry and pipeline: the sim's planet type reference and type-code mapping; physics engines per type" last_verified: 2026-09-27 sim_frequency: 8,742 planets in the 5,159 generated systems of the naked-eye sky (committed system sheets, as of 2026-09-27) worked_example: "Earth: 1 M_Earth, 1 R_Earth, 5.51 g/cm^3, T_eq 255 K at Bond albedo 0.3. Sim class TMW (mixed world)." sim_code_levels: type (TOW), subtype (TOW-AR), sub-subtype (a visual variant, e.g. TBB-FB) literature_schemes: - IAU planet and dwarf planet (Solar System, 2006) - "IAU working definition of an exoplanet (2018): below about 13 Jupiter masses, mass ratio to host below about 1/25" - "Kepler size classes: Earth-size below 1.25 R_Earth, super-Earth 1.25 to 2, Neptune-size 2 to 6, Jupiter-size 6 to 15" - "Mass-radius regimes: Terran below about 2 M_Earth, Neptunian to about 0.4 M_Jup, Jovian to about 0.08 M_Sun" - Sudarsky classes I to V for giant-planet atmospheres (2000, theoretical) confirmed_exoplanets: as_of: 2026-09-25 value: 6372 source: NASA Exoplanet Archive related: - https://paxabyssi.com/wiki/Arid_world.md - https://paxabyssi.com/wiki/Barren_rock_world.md - https://paxabyssi.com/wiki/Greenhouse_world.md - https://paxabyssi.com/wiki/Dry_habitable_world.md - https://paxabyssi.com/wiki/Super-Earth.md - https://paxabyssi.com/wiki/Ice_world.md --- # Planet classification > Source: https://paxabyssi.com/wiki/Planet_classification > > Licence: [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0/). Text by Pax Abyssi Wiki contributors; history at https://paxabyssi.com/wiki/Planet_classification/history > > Revision 237, 27 September 2026 **Planet classification** is the sorting of planets into kinds by what can be measured about them: size, mass, density, temperature and, where a spectrum exists, the chemistry of the atmosphere. Astronomy has no single official scheme. There is a formal definition of what a planet is, and a set of working categories (super-Earth, mini-Neptune, hot Jupiter) that grew out of the instruments that found them. Pax Abyssi needs something stricter, because its simulation has to decide what every generated world is before it can compute an atmosphere or draw a surface, so it files each planet under one of 37 coded types. This page explains both: the categories astronomers use, and how the sim's codes map onto them. ## What counts as a planet For the Solar System the rule is the one the International Astronomical Union (IAU) voted on in Prague on 24 August 2006. A planet orbits the Sun, is massive enough for its own gravity to pull it into a nearly round shape, and has cleared the neighbourhood around its orbit. A body that meets the first two tests but not the third, and is not a moon, is a dwarf planet, which is why Pluto and Eris are now classed as dwarf planets [1]. Planets around other stars need a different test, because nobody can check whether a world 100 light years away has cleared its orbit. The IAU's working definition of an exoplanet, amended in 2018, sets an upper mass: below about 13 Jupiter masses, the point at which an object of solar composition can fuse deuterium in its core. It must orbit a star, a brown dwarf or a stellar remnant, and its mass must be less than about one twenty-fifth of its host's [2]. The 13 Jupiter-mass line is a convention more than a physical wall; the deuterium-burning threshold moves between about 11 and 16 Jupiter masses with composition [3]. Objects above it are [brown dwarfs](https://paxabyssi.com/wiki/Brown_dwarf.md). As of 25 September 2026 the NASA Exoplanet Archive listed 6,372 confirmed exoplanets [4]. ## How astronomers sort planets ### By size and mass Most known exoplanets were found by one of two methods, and each measures one number. A **transit**, the small dip in starlight as a planet crosses its star, gives the planet's radius. A **radial-velocity** survey, which detects the star's wobble, gives a minimum mass (the true mass times the sine of the orbit's unknown tilt). The first working classes were therefore classes of size. The Kepler mission sorted its candidates into Earth-size (below 1.25 Earth radii), super-Earth-size (1.25 to 2), Neptune-size (2 to 6) and Jupiter-size (6 to 15 Earth radii) [5]. NASA's public categories follow the same logic: terrestrial planets, super-Earths, Neptune-like planets (including mini-Neptunes) and gas giants [6]. The surprise of the Kepler era was that the most common planets around Sun-like stars, at least on orbits shorter than about a year, fall between Earth and Neptune in size, a kind the Solar System lacks [7] [8]. Inside that range sits the [Radius valley](https://paxabyssi.com/wiki/Radius_valley.md), a shortage of planets between about 1.5 and 2 Earth radii that divides rocky [super-Earths](https://paxabyssi.com/wiki/Super-Earth.md) from gas-wrapped [mini-Neptunes](https://paxabyssi.com/wiki/Mini-Neptune.md) [9]. Where both mass and radius are known, the pair itself defines classes. Fitting the mass-radius relation of 316 well-measured objects from dwarf planets to low-mass stars, Chen and Kipping (2017) found the slope changes at three places: near 2 Earth masses, where solid "Terran" worlds give way to "Neptunian" worlds with thick envelopes; near 0.41 Jupiter masses (about 130 Earth masses), where "Jovian" worlds begin and radius stops growing with mass; and near 0.08 solar masses, where hydrogen fusion turns a body into a star [10]. ### By density and composition Mass and radius together give the bulk density, $$ \rho = \frac{3M}{4\pi R^3} \approx 5.51\ \mathrm{g\,cm^{-3}} \times \frac{M/M_\oplus}{(R/R_\oplus)^3}, $$ and density is the first clue to composition. Earth averages 5.51 g/cm³, Mercury 5.43, water ice about 1, and Saturn 0.69. Planets are compressed by their own weight, so the comparison is made against model curves for pure iron, Earth-like rock, rock mixed with water, and rock with a hydrogen envelope rather than against a single number [11]. The curves overlap: a planet of a given mass and radius can often be matched by more than one recipe, a problem called **compositional degeneracy**. That is why the same measured world can be argued to be a water world or a rocky core under a thin hydrogen layer (see [Ocean world](https://paxabyssi.com/wiki/Ocean_world.md) and [Mini-Neptune](https://paxabyssi.com/wiki/Mini-Neptune.md)). ### By temperature The second axis is warmth. A planet's **equilibrium temperature** is the temperature a black body would reach if it absorbed the starlight falling on it and radiated the same power back to space, with the heat spread over the whole sphere: $$ T_\mathrm{eq} = T_\star \sqrt{\frac{R_\star}{2a}}\,(1 - A_B)^{1/4} \approx 278\ \mathrm{K}\,(1 - A_B)^{1/4} \left(\frac{L_\star}{L_\odot}\right)^{1/4} \left(\frac{a}{1\ \mathrm{AU}}\right)^{-1/2} $$ Here $T_\star$, $R_\star$ and $L_\star$ are the star's temperature, radius and luminosity, $a$ is the orbital distance and $A_B$ is the Bond albedo, the fraction of all incoming light the planet reflects. For Earth, with $A_B \approx 0.3$, the formula gives 255 K, well below the measured mean surface temperature of about 288 K; the 33 K difference is the greenhouse effect. It is still the one temperature that can be computed for any planet with a known orbit, so it is the standard axis for words like "hot", "temperate" and "cold". A dayside that cannot share its heat with the night side runs hotter than $T_\mathrm{eq}$; see [Lava world](https://paxabyssi.com/wiki/Lava_world.md). ### By atmosphere For giant planets, temperature controls which clouds can condense, and so how the planet looks. Sudarsky, Burrows and Pinto (2000) turned that into five theoretical classes, from ammonia-cloud giants below about 150 K to silicate-cloud giants above about 1,400 K [12]. The scheme predates any measured spectrum of a cool exoplanet giant and is best read as a model; see [Sudarsky classification](https://paxabyssi.com/wiki/Sudarsky_classification.md). For small planets, spectra from the James Webb Space Telescope are only now starting to separate bare rock from atmosphere, so atmospheric classes of rocky exoplanets remain provisional. ## The Pax Abyssi scheme The sim's taxonomy is built on the same axes (mass, radius, temperature, composition, atmosphere), but it has to be complete: every generated world must land in exactly one class. It has three levels. The **type** (for example `TOW`, an ocean world) decides which physics engine runs. The **subtype** (`TOW-AR`, a young Archean-style ocean world) is a physics-derived refinement that every planet carries. Some subtypes have a third level, the **sub-subtype**, a visual variant that decides what the planet looks like (for example `TBB-FB`, a flood-basin basalt world). Codes are mnemonic: `T` for terrestrial, `B` for barren, and so on. Older codes such as `T2-O` survive in the sim's internals and are listed as aliases on each class page. There are 37 types in nine series: | Series | Types (code: name) | Solar System analogue | | --------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------- | | Barren | TBR [Barren rock world](https://paxabyssi.com/wiki/Barren_rock_world.md); TBI iron-rich; TBS anorthosite "sandy"; TBB basalt; TBM metallic; TPI pure ice | Mercury, the Moon, Tethys | | Terrestrial with atmosphere | TAW [Arid world](https://paxabyssi.com/wiki/Arid_world.md); TAC arid clouded; TGW [Greenhouse world](https://paxabyssi.com/wiki/Greenhouse_world.md); TNH nitrogen-hydrocarbon; TDH [Dry habitable world](https://paxabyssi.com/wiki/Dry_habitable_world.md); TMW [Mixed world](https://paxabyssi.com/wiki/Mixed_world.md); TOW [Ocean world](https://paxabyssi.com/wiki/Ocean_world.md); TSO [Subsurface ocean world](https://paxabyssi.com/wiki/Subsurface_ocean_world.md); TIW [Ice world](https://paxabyssi.com/wiki/Ice_world.md); TVO [Volcanic world](https://paxabyssi.com/wiki/Volcanic_world.md) | Mars, Venus, Titan, Earth, Europa, Pluto, Io | | Lava | LWN hot lava world; LWTL tidally locked lava world (both [Lava world](https://paxabyssi.com/wiki/Lava_world.md)) | none | | Super-Earth | SET temperate; SEV volcanic; SVT volcanic, tidally locked; SEI ice ([Super-Earth](https://paxabyssi.com/wiki/Super-Earth.md)) | none | | Mini-Neptune | MNT temperate; MNC cold; MNH hot ([Mini-Neptune](https://paxabyssi.com/wiki/Mini-Neptune.md)) | none | | Gas giant | GGC cold; GGT temperate; GGH hot ([Gas giant](https://paxabyssi.com/wiki/Gas_giant.md), [Hot Jupiter](https://paxabyssi.com/wiki/Hot_Jupiter.md)) | Jupiter, Saturn | | Ice giant | IGC cold; IGH hot ([Ice giant](https://paxabyssi.com/wiki/Ice_giant.md)) | Uranus, Neptune | | Exotic | XCB carbon; XFE iron; XHE helium; CHT chthonian ([Exotic worlds](https://paxabyssi.com/wiki/Exotic_worlds.md)) | none | | Other | BRD brown-dwarf companion; RGT rogue terrestrial; RGG rogue gas giant | none | Most types split further. The gas giants divide by chemistry and temperature, the lava worlds by dayside temperature and whether they are tidally locked, the mini-Neptunes by temperature bands, and the habitable-zone worlds by atmosphere and ocean cover, giving roughly 100 subtypes in all. The class pages give each type's subtypes and the numbers that trigger them. ### How a planet gets its type A generated system is built outward from its star. The system's architecture (compact, resonant, hot-Jupiter and so on) lays out orbital slots, each slot is labelled by how much starlight it receives, and a type is drawn from that zone's probability table. The table is tilted by the chemistry of the star's disc: a carbon-rich disc makes carbon planets more likely, an iron-rich one allows iron planets, and a volatile-rich one favours ocean and ice worlds. Temperature guards then catch impossible draws, so an ice type placed too close to its star becomes a barren or lava world. The type's physics engine then computes the planet: mass, radius, atmosphere, albedo, surface temperature. The engine may **reclassify** the world if the physics disagrees with the label. A world whose rock is melting becomes a lava world; a greenhouse world that ends up below the boiling point of water becomes a mixed world; an ocean world that freezes over becomes a subsurface ocean world. Real exoplanets in the sim's catalogue are typed differently, because for them the mass is known and the architecture is not. A decision tree works down from mass: rocky branches below about 2 Earth masses, lava worlds and mini-Neptunes from 2 to 10 depending on temperature and radius, hot mini-Neptunes or ice giants to 50, and gas giants beyond, split into hot, temperate and cold by equilibrium temperature. ### How the codes relate to the literature The sim's types are physically motivated but they are the game's own, and most of their names are too. A planetary scientist will not recognise "Phosphine-Ruddy" as a gas-giant class, and the IAU has no category called "dry habitable world". Where a sim class matches a literature class, the class page says so and cites it (the [Super-Earth](https://paxabyssi.com/wiki/Super-Earth.md) and [Mini-Neptune](https://paxabyssi.com/wiki/Mini-Neptune.md) pages follow the literature's definitions closely; the [Sudarsky classification](https://paxabyssi.com/wiki/Sudarsky_classification.md) page explains how the sim's temperate giants relate to that scheme). Where the sim invents a subtype, the page says that too. Every class page carries a science-status banner: observed, model, sim or speculative. (Image pending: A log-log plot of planet mass against radius, with confirmed exoplanets as dots, four composition curves, and shaded regions for the sim's planet series.) *Figure 1.* Diagram: confirmed exoplanets on the mass-radius plane, with curves for iron, rock, water-rich and gas-enveloped compositions and the regions each Pax Abyssi series occupies. > **In Pax Abyssi** > > The type decides everything downstream: which physics engine runs, which atmosphere classes are possible, how many moons a planet may keep, and which texture plate the game draws. In the game build, generated systems are committed as system sheets, and each planet is drawn as a prebaked colour map on a lit sphere, picked from a library by its three-level code. The 5,159 generated systems of the naked-eye sky hold 8,742 planets (sheets as of 27 September 2026). The most common types there are cold gas giants (900), tidally locked lava worlds (814), cold ice giants (749) and arid worlds (685). These stars are mostly bright, luminous ones, so the mix is not a census of a typical patch of the Galaxy. ## See also - [Radius valley](https://paxabyssi.com/wiki/Radius_valley.md) - [Super-Earth](https://paxabyssi.com/wiki/Super-Earth.md) and [Mini-Neptune](https://paxabyssi.com/wiki/Mini-Neptune.md) - [Habitable zone](https://paxabyssi.com/wiki/Habitable_zone.md) - [Sudarsky classification](https://paxabyssi.com/wiki/Sudarsky_classification.md) - [Gas giant](https://paxabyssi.com/wiki/Gas_giant.md) and [Ice giant](https://paxabyssi.com/wiki/Ice_giant.md) - [Star system generation](https://paxabyssi.com/wiki/Star_system_generation.md) - [Planetary system archetypes](https://paxabyssi.com/wiki/Planetary_system_archetypes.md) - [Planet occurrence](https://paxabyssi.com/wiki/Planet_occurrence.md) - [Sol](https://paxabyssi.com/wiki/Sol.md) ## References 1. International Astronomical Union (2006). IAU 2006 General Assembly: Result of the IAU Resolution votes. IAU press release iau0603. 2. Lecavelier des Etangs, A. and Lissauer, J. J. (2022). The IAU working definition of an exoplanet. New Astronomy Reviews 94, 101641. 3. Spiegel, D. S., Burrows, A. and Milsom, J. A. (2011). The deuterium-burning mass limit for brown dwarfs and giant planets. The Astrophysical Journal 727, 57. 4. NASA Exoplanet Science Institute (2026). NASA Exoplanet Archive. Caltech/IPAC. 5. Borucki, W. J. et al. (2011). Characteristics of planetary candidates observed by Kepler. II. Analysis of the first four months of data. The Astrophysical Journal 736, 19. 6. NASA Science. Exoplanet types. NASA Science: Exoplanets. 7. Fressin, F. et al. (2013). The false positive rate of Kepler and the occurrence of planets. The Astrophysical Journal 766, 81. 8. Petigura, E. A., Howard, A. W. and Marcy, G. W. (2013). Prevalence of Earth-size planets orbiting Sun-like stars. Proceedings of the National Academy of Sciences 110, 19273-19278. 9. Fulton, B. J. et al. (2017). The California-Kepler Survey. III. A gap in the radius distribution of small planets. The Astronomical Journal 154, 109. 10. Chen, J. and Kipping, D. (2017). Probabilistic forecasting of the masses and radii of other worlds. The Astrophysical Journal 834, 17. 11. Zeng, L. et al. (2019). Growth model interpretation of planet size distribution. Proceedings of the National Academy of Sciences 116, 9723-9728. 12. Sudarsky, D., Burrows, A. and Pinto, P. (2000). Albedo and reflection spectra of extrasolar giant planets. The Astrophysical Journal 538, 885-903. ## Infobox (physics concept) | Field | Value | | --- | --- | | Name | Planet classification | | Image | File:Planet_classes_sim_montage.avif | | Formulae | Bulk density: rho = 3M / (4 pi R^3); in Earth units rho = 5.51 g/cm^3 x (M/M_Earth) / (R/R_Earth)^3, Equilibrium temperature: T_eq = T_star sqrt(R_star / 2a) (1 - A_B)^(1/4), about 278 K x (1 - A_B)^(1/4) (L/L_Sun)^(1/4) (a/1 AU)^(-1/2) | | Validity | Classes built on radius or minimum mass alone cannot tell rock from water or gas; a composition class needs both mass and radius, and an atmosphere class needs a spectrum. | | Sim types | 37 | | Definition | Sorting planets into classes by what can be measured: radius, mass, bulk density, equilibrium temperature and, where spectra exist, atmospheric chemistry. No single scheme is official beyond the IAU's definitions of what a planet is. | | Sim source | Type registry and pipeline: the sim's planet type reference and type-code mapping; physics engines per type | | Last verified | 2026-09-27 | | Sim frequency | 8,742 planets in the 5,159 generated systems of the naked-eye sky (committed system sheets, as of 2026-09-27) | | Worked example | Earth: 1 M_Earth, 1 R_Earth, 5.51 g/cm^3, T_eq 255 K at Bond albedo 0.3. Sim class TMW (mixed world). | | Sim code levels | type (TOW), subtype (TOW-AR), sub-subtype (a visual variant, e.g. TBB-FB) | | Literature schemes | IAU planet and dwarf planet (Solar System, 2006), IAU working definition of an exoplanet (2018): below about 13 Jupiter masses, mass ratio to host below about 1/25, Kepler size classes: Earth-size below 1.25 R_Earth, super-Earth 1.25 to 2, Neptune-size 2 to 6, Jupiter-size 6 to 15, Mass-radius regimes: Terran below about 2 M_Earth, Neptunian to about 0.4 M_Jup, Jovian to about 0.08 M_Sun, Sudarsky classes I to V for giant-planet atmospheres (2000, theoretical) | | Confirmed exoplanets | 6372 | ## Related pages - [Arid world](https://paxabyssi.com/wiki/Arid_world.md): A rocky planet with a thin, cold atmosphere and no stable liquid water at the surface, only ice and the traces of ancient rivers. Mars is the Solar System's example. - [Barren rock world](https://paxabyssi.com/wiki/Barren_rock_world.md): A rocky planet with no real atmosphere, whose surface lies bare to starlight, cosmic rays and meteorites. Mercury is the Solar System's example, and JWST has found several around other stars. - [Greenhouse world](https://paxabyssi.com/wiki/Greenhouse_world.md): A rocky planet smothered by a thick carbon dioxide atmosphere whose greenhouse effect bakes the surface far above the boiling point of water. Venus is the Solar System's example, a near-twin of Earth in size. - [Dry habitable world](https://paxabyssi.com/wiki/Dry_habitable_world.md): A rocky planet with liquid water on its surface but only a little of it, in lakes, brines and polar seas rather than oceans. Climate models suggest such land planets can stay habitable over a wider range of distances than an Earth-like world. - [Super-Earth](https://paxabyssi.com/wiki/Super-Earth.md): A planet more massive than Earth but lighter than Neptune, usually rocky, with no counterpart in the Solar System. Super-Earths are among the most common planets found around other stars. - [Ice world](https://paxabyssi.com/wiki/Ice_world.md): A cold world whose surface is made of frozen water, nitrogen, methane and carbon monoxide, like Pluto and Triton, where ices behave like rock and even the atmosphere can freeze out. Categories: [Planets](https://paxabyssi.com/wiki/Category:Planets.md), [Planet classes](https://paxabyssi.com/wiki/Category:Planet_classes.md), [Reference](https://paxabyssi.com/wiki/Category:Reference.md)