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The Astronomical Classification Index (ACI) is an indexing system used by the Final Frontier Project and, to an extent, the United Nations government to classify astronomical bodies into clear classes for cataloguing and definitions. It is currently the international standard for astronomical body classification and has been adopted by the International Standards Organization (ISO) for use.
The ACI organizes bodies using alphanumeric codes. These codes are laid out in the following order: Temperature (T), Atmosphere (A), Composition (CMP), Habitability (H), Mass (M), and Special Flags (SP), in the format of T-ACMP/H-M (SP). In addition to the code, an expanded readout can be provided next to the code for fast reading. For example, Earth has an ACI code of T-MRN/H-1E, spelled out as a "temperate marine, habitable Earth-sized planet" with no special flags.
| Planet | ACI Code | Readout |
|---|---|---|
| Earth | TTMRN/H-1E | Temperate N2/O2 atmosphere marine habitable, Earth-sized |
| Mars | CCDST-sE | Cold CO2 atmosphere desert sub-Earth-sized |
| Luna | FOSLC-1L | Frigid no-atmosphere silicate, Luna-sized |
Temperature (T)
Temperature is classified by the planet's average surface temperature over the course of its orbital period (year).
| Code | Meaning | Mean surface temperature (°C) |
|---|---|---|
| F | Frigid | -273°C to -180°C |
| C | Cold | -180°C to -20°C |
| T | Temperate | -20°C to 40°C |
| W | Warm | 40°C to 100°C |
| H | Hot | 100°C to 500°C |
| S | Scorched | >500°C |
Atmosphere (A)
| Code | Meaning | Atmospheric composition |
|---|---|---|
| O | None | Atmosphere is nonexistent or extremely low in volume to be negligible. |
| T | Terran | A nitrogen-oxygen atmosphere with trace other elements. If at sufficient pressure, it can be breathable by humans. |
| C | Carbonic | Atmosphere dominated by carbon oxides (CO or CO2.). |
| N | Nitrogen | Atmosphere dominated by nitrogen. |
| H | Hydrogen / Helium | Atmosphere composed primarily of hydrogen and helium, with trace amounts of other gases like ammonia. |
| S | Sulfuric | Atmosphere dominated by sulfur or compounds of sulfur, such as sulfur dioxide. |
| W | Water / Ice | Atmosphere dominated by water in any of its states of matter. |
Composition (CMP)
Composition codes determine the dominant material composition of a planet, sorted into the following classes.
| Code | Classification | Description | Examples |
|---|---|---|---|
| LCT | Lacustrine | Planets where liquid water is concentrated primarily into lakes. | Homestead |
| MRN | Marine | Planets where liquid water makes up between 40% and 90% of the planet's surface. | Earth |
| OCL | Oceanic, Liquid | Planets dominated by a liquid water ocean. Islands may or may not be present. | Ballad |
| OCF | Oceanic, Frozen | Planets dominated by a water ocean underneath an ice shell. | Jupiter |
| OCT | Oceanic, Toxic | Planets covered in an ocean of a different kind of liquid, usually methane, that is toxic to human life. | |
| MLT | Molten | Planets covered in significant amounts of lava or other molten material. | |
| DST | Desert | Planets possessing an atmosphere and limited water (usually frozen) but otherwise devoid of life. | Mars |
| SLC | Silicate | Planets possessing no atmosphere and a dominantly silica-based surface composition. | Mercury |
| ICE | Ice | Planets dominated by an ice shell but have no liquid water underneath. | |
| FER | Ferric | Planets dominated by an iron composition with no mantle. | Heze VII |
| CBN | Carbonic | Planets composed primarily of carbon-based compounds such as graphite. | |
| SUL | Sulfuric | Inhospitable planets with a dominant sulfur composition. | Venus |
| PTO | Protoplanet | Planets that are still in the active process of forming, usually in a protoplanetary disk. | |
| CTH | Cthonian | Cores of former gas giants, usually stripped by being too close to a parent star. | |
| JOV | Jovian gas giant | Gas giants composed primarily of hydrogen and helium. | Jupiter |
| ICG | Ice giant | Gas giants composed primarily of some kind of ice, such as water. | Neptune |
Habitability (H)
The habitability flag is an optional indicator to mark if a planet is conventionally habitable for human or otherwise Earth-based animal and plant life.
A planet is considered "habitable" if humans, Earth-based animals, and plants can all respirate normally in the atmosphere without usage of special apparatus or structures and without experiencing discomfort or irritation. A habitable planet, therefore, must ideally have a nitrogen-oxygen (T-type) atmosphere of between 80 - 130 kPa, at least 5% humidity, no toxic gases or particulates, and a sufficient amount of carbon dioxide in the atmosphere for Earth-based plant life to photosynthesize.
The Habitability flag does not take into account the proximity of liquid water, nor the quality of the soil for natural plant growth, as these can be substituted or extracted during colonization.
| Flag | Classification | Description |
|---|---|---|
| /H | (Fully) habitable | Planet meets all the atmospheric prerequisites for habitability. |
| /P | Partially habitable | Planet meets several but not all criteria for habitability. |
| /L | Marginally habitable | Planet only meets one or two critical criteria (N-O atmosphere) for habitability, but nothing else. |
Mass (M)
The mass index is composed of two characters: a comparative character and a reference object in Sol. The comparative character shows how much larger or smaller an ACI-classed body is compared to its reference. For example, a Mass code of -sN means that an object is 0.3 - 0.8 times the mass of Neptune.
| Code | Meaning | Relative mass to reference R |
|---|---|---|
| m | Micro | <0.3 MR |
| s | Sub | 0.3 - 0.8 MR |
| 1 | Equivalent | 0.9 - 1.2 MR |
| S | Super | 1.2 - 3.0 MR |
| M | Mega | >3 MR |
| Code | Reference body | Reference body mass (M🜨) |
|---|---|---|
| L | Luna (Moon) | 0.01 M🜨 |
| M | Mercury | 0.055 M🜨 |
| E | Earth | 1 M🜨 |
| N | Neptune | 17.1 M🜨 |
| J | Jupiter | 317.8 M🜨 |
| S | Sol (Sun) | 332,946 M🜨 |
Stars
Stars and other stellar-mass objects use a variant of the ACI coding system, as Type (T), luminosity (LMN), and Mass (M) in the format of T-LMN-S.
Luminosity
The Morgan–Keenan (MK) classification is used for a star's luminosity. For example, Sol has an ACI code of M-G2V-1S, indicating it is a main aequence G2V-type star the mass of 1 Sol.