Planetary classification is a scheme for classifying planets according to mass, surface, and composition.
It is estimated that there are 820 billion planets in the Milky Way Galaxy orbiting 180 billion stars, corresponding to the average of 4.55 planets per star. Based on the probability of being shut out in baseball scoring 4.55 runs per game, 95% of all stars would have planets. There are ~524 billion terrestrial planets (639‰) and ~296 billion giant planets (361‰) in the Milky Way Galaxy. There are nearly 6000 known planets as of 2024, which is seven billionth of all planets in our galaxy.
Classifications[]
Mass[]
Planetary mass classification is the classification of planets by mass. The mass ranges from 1 Lunar mass to 13 Jupiter masses. This classification uses the letter codes in order of decreasing mass. There are three broad types of planets by mass: Jupiter-mass planet (class J), Neptune-mass planet (class N), and Earth-mass planet (class E) with subclasses a (super-), b (mid-) and c (sub-) in order of decreasing mass.
Planets are non-stellar objects that revolve around stars including the Sun that have suitable mass range that clear the neighborhoods without fusing deuterium in their cores. Brown dwarf (hyperjovian) and dwarf planet (hypoterrestrial) were originally included. It was removed because these are not planets. Brown dwarf is not considered a planet because they are massive enough to fuse deuterium in their core, somewhat like stars, and dwarf planet because it is not massive enough to clear the neighborhood.
Of all known non-Kepler planets, super-Jupiters are most abundant despite it is the rarest mass class of planet in the galaxy due to the fact that super-Jupiters are most easily detected. Midplanets are the most common type of known planets overall. There are only few dozen known mid-Earths and sub-Earths due to difficulties in detecting them. Mid-Earth is the Milky Way's most abundant mass class of planet with sub-Earth close behind. Planets massing less than 20 M⊕ make up about 73% of all planets with remainder 27% are more massive than 20 M⊕.
| Class | Name | Symbol | Mass range | Composition | Abundance in our galaxy | Examples |
|---|---|---|---|---|---|---|
| I | Super-Jupiter | Ja | 2–13 MJ | Gaseous | ~46 billion | PSR B1620-26 b, Tau Boötis b |
| II | Mid-Jupiter | Jb | 0.5–2 MJ | Gaseous | ~69 billion | Jupiter, HD 209458 b |
| III | Sub-Jupiter | Jc | 0.1–0.5 MJ | Gaseous or icy | ~65 billion | Saturn, HD 164922 b |
| IV | Midplanet | N | 0.03–0.1 MJ 10–32 M⊕ |
Icy, watery, gaseous or rocky | ~123 billion | Neptune, Gliese 436 b |
| V | Super-Earth | Ea | 2–10 M⊕ | Rocky, watery, icy or gaseous | ~155 billion | 55 Cancri e, Kepler-22b |
| VI | Mid-Earth | Eb | 0.5–2 M⊕ | Rocky, watery or icy | ~185 billion | Earth, Alpha Centauri Bb |
| VII | Sub-Earth | Ec | 0.01–0.5 M⊕ | Rocky | ~177 billion | Mars, Kepler-37b |
Orbit[]
Planetary orbit classification is the planet classification according to their orbital distance (semimajor axis) from the parent star.
| Class | Name | Symbol | Orbit range (AU) | Abundance in our galaxy | Examples |
|---|---|---|---|---|---|
| 0 | Helian | H | <0.05 | ~17 billion | KELT-9b, Kepler-70b |
| I | Dimidian | D | 0.05-0.1 | ~42 billion | 51 Pegasi b, HD 209458 b |
| II | Themian | T | 0.1–0.25 | ~54 billion | Gliese 581 d, Gliese 667 Cc |
| III | Hermian | Hr | 0.25–0.5 | ~67 billion | Mercury, PSR B1257+12b |
| IV | Cytherean | C | 0.5–0.8 | ~82 billion | Venus, Kepler-62f |
| V | Gaian | G | 0.8–1.25 | ~81 billion | Earth, Kepler-22b |
| VI | Martian | M | 1.25–2.5 | ~124 billion | Mars, 47 Ursae Majoris b |
| VII | Alphian | A | 2.5–5 | ~135 billion | Epsilon Eridani b, HD 154345 b |
| VIII | Jovian | J | 5–10 | ~105 billion | Jupiter, Saturn |
| IX | Uranian | U | 10–25 | ~76 billion | Uranus, Kappa Andromedae b |
| X | Plutonian | P | 25–50 | ~29 billion | Neptune, 2M1207b |
| XI | Eridian | E | 50–100 | ~11 billion | HR 8799 b, HD 95089 b |
| XII | Sednian | S | 100-300 | ~13 billion | Fomalhaut b, FW Tauri b |
| XIII | Farfarian | FF | 300-600 | ~2 billion | Farfarout, Planet X |
| XIV | Oortian | O | >600 | ~719 million | COCONUTS-2 b |
Surface[]
Planetary surface classification is the classification of planets about what's on the surface, such as deserts, forests, and oceans. More than one surface class can be identified for one planet.
| Class | Name | Symbol | Life-bearing status | Color | Abundance in our galaxy | Examples |
|---|---|---|---|---|---|---|
| -II | Plasma | P | Very Poor | Red, orange, yellow | ~64 million | None |
| -I | Acid | A | Very Poor | Various | ~40 million | Venus (sorta) |
| 0 | Gas | G | Poor | Various | ~296 billion | Jupiter, 51 Pegasi b |
| I | Lava | L | Poor | Red, orange | ~133 billion | COROT-7b, Kepler-10b |
| II | Crater | Ct | Poor | Various | ~169 billion | Mercury, Kepler-42d |
| III | Barren | B | Fair | Brown, gray | ~233 billion | Venus, HD 10180 j |
| IV | Desert | D | Fair | Tan, brown, peach, yellow | ~141 billion | Mars, Gliese 581 c |
| V | Mountain | Mt | Fair | Brown, gray | ~23 billion | Kepler-54c, Kepler-102f |
| VI | Ice | I | Good | White, gray | ~113 billion | OGLE-2005-BLG-390Lb, MOA-2007-BLG-192Lb |
| VII | Ocean | O | Good | Blue | ~52 billion | Earth, Gliese 581 d |
| VIII | Forest | F | Excellent | Green | ~620 million | Dagobah (Star Wars) |
| IX | City | E | Excellent | Gray | ~16 million | Coruscant (Star Wars) |
Composition[]
This scheme is about what the planet is dominantly made of, such as carbon, methane, or water. Like the surface classification, more than one composition class can be identified for one planet, albeit more rarely.
| Class | Name | Symbol | Life-bearing status | Color | Abundance in our galaxy | Examples |
|---|---|---|---|---|---|---|
| 0A | Hydrogen | H | Poor | Various | ~296 billion | Jupiter, Saturn |
| 0B | Helium planet | He | Poor | Various | ~301 million | None |
| 0C | Uranium | U | Poor | Green | ~15 billion | None |
| I | Iron planet | Fe | Poor | Reddish brown | ~9 billion | Kepler-57b, Kepler-57c |
| II | Chlorine | Cl | Fair | Green | ~3 million | None |
| III | Ammonia | A | Fair | Brown | ~24 billion | None |
| IV | Phosphorus | P | Fair | White | ~427 million | Kepler-55c |
| V | Methane | M | Fair | Turquoise | ~43 billion | None |
| VI | Sulfur | S | Good | Yellow | ~13 billion | Kepler-52c |
| VII | Carbon planet | C | Good | Brown, black, gray | ~86 billion | PSR B1257+12c, 61 Virginis b |
| VIII | Silicon planet | Si | Good | Various | ~429 billion | Earth, Mars |
| IX | Water planet | W | Excellent | Blue | ~77 billion | GJ 1214 b, Kepler-10c |
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