PlanetStar Wiki

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 4000 known planets as of 2018, which is five billionth of all planets in our galaxy.



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


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
I Bellerophian B <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 H 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 ~13 billion Fomalhaut b, FW Tauri b


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
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 planet B Fair Brown, gray ~233 billion Venus, HD 10180 j
IV Desert planet 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 planet I Good White, gray ~113 billion OGLE-2005-BLG-390Lb,
VII Ocean planet O Good Blue ~52 billion Earth, Gliese 581 d
VIII Forest F Excellent Green ~620 million None
IX City E Excellent Gray ~16 million None


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
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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