OTHER | PLANET CLASSIFICATION GUIDE

PLANET CLASSIFICATION GUIDE

We've for long referred to planets by their types here in this blog, using words like Terran or Terrestrial (derived from the Latin for Earth, Terra), or Gas Giant / Jovian (as relating to Jupiter), and so on, some terms are analogous, others rely on the knowledge of the reader to catch on, wouldn't it be good to have table with known, or, theoretically possible planet types?

Fans of the Star Trek series may recall have heard a couple times mr.Spok read on a planet's stats after scanning, coming up with planet classes as M, when referring to Earth-like worlds.

There isn't much of a real life parameter to pinpoint planet types as of today, it may happen in the future though, what we have are some base models applied to our planets and known exoplanets.

When I started this blog, I presented to you a couple planet types, but they come in even more flavours than that.

I'll build a planet classification chart along this post, more of as a baseline for future references.

THE FOLLOWING TEXT REFERS TO HOW I WOULD CLASSIFY PLANETS ACCORDING TO WHAT I HAVE READ ABOUT THEORETICAL AND OBSERVED PLANET TYPES

COMPLEMENTARY GRAPHS ARE FOUND AT THE END OF THE POST 

When we first look at our solar system we tend to classify it in rocky planets and gaseous planets, which is pretty fine, but we need to get more detailed.

Composition can vary a lot even between the 'uniform' gas giants, as they can be made of several different compounds such as light and heavy volatiles.

We can already expand our classification from 2 boxes (rocky and gaseous) to a lot more if we include mean composition, mass, and density alone - some planet types are also characterized by their temperatutes, orbits, or atmospheric conditions.

Me ~ Earth Mass

Mj ~ Jupiter Mass

Re ~ Earth Radius

Rj ~Jupiter Radius

I will use a concatenation system of Class-Subclass-Type/Subtype, or for clarity, UPPERCASE-Number-lowercase.

Potentially habitable for complex-life and microorganisms only types depending on atmospheric conditions color-coded GREEN and RED,  respectively, with an uncertainty midterm being YELLOW.

Class S - Subterran

A planet which mass and radius are substantially inferior to that of Earth and Venus.

A Subterran planetary mass range from 0,001 Me to 0,5 Me, and depending on composition, are usually smaller than Earth.

Subclass 1 - Dwarf Planet

A body which is massive enough to molded in a round form by it's own gravity, but not enough to cleanse it's orbital vicinity, and it's also not gravitationally bound to a particularly larger body than itself (like a main planet).
The mass of a Dwarf Planet is less than 0,01 Me, but more than most common large asteroids. Commonly made of rock and ice and can form beyond a star's terrestrial accretion limit.

Such a body around 0,01 Me in mass - made of Lunar regolith, would have a diameter of 0,254 Re.

Type a - Plutine
A body that's roughly 70% rock and 30% water ice by mass, may contain a thin atmosphere of methane and nitrogen, and some ammonia, the body most likely formed and still orbits beyond the Frost line.

Such world with 0,01 Me would be 3.058km wide - or 0,24 Re.

Type b - Cererian
A body that's 60~70% rock and 30~40% water ice by mass, the main difference between a Selenes and Hygieans is that those form before the Frost line, likely to inhabit the system's asteroid belt, it's tenuous atmosphere is composed of water vapour outgassing and sublimation and likely extends as a cloud around the object rather than a layer near the surface - which is composed of a silicate, piroxene, and ice mixture, and presents heavy createring.

Type b - Selene
A dwarf planet that is almost entirely rock, covered by piroxene, silicates and basaltic rock, having little to no water ice, much probably the leftovers of protoplanet formation of the system.


Type d - Hygiean
A dwarf planet that is not nearly as spherical as the others, but it's not an asteroid too, having some significant assimetry of it's axis for some reason - too little mass, low density and high rotation, or had a chunk of itself tore apart during impact events, like Haumea and Hygiea.

Subclass 2 - Minor Planet

A planet which mass is greater than 0,01 Me and less than 0,1 Me, can be made of metals, silicates, and ices depending on specific composition, due to low mass - these planets aren't suitable for holding on a long lasting atmosphere.

Type a - Mercurian

A planet which mass is within the minor planet subclass, however, it's found in the inner part of it's star system, atmosphere is nearly or completely absent and one remarkably characteristic is the extreme temperature difference between the night and day side of the planet.

A planet like Mercury would have a density similar to that of Earth's, about 5,4 g/cm³.

Type b - Super-Mercurian

A planet which mass is not quite within the minor planet subclass, however, it is made of dense materials such as metals, making the planet heavier than it appears to be by sheer size - as it would still be smaller than Earth, typically, these planets tend to have large core, surrounded by a thin mantle and rocky crust. Could also be more commonly found on the inner part of their systems.

A planet composed of 75% metals and 25% silicates would have density of ~7 g/cm³.

Type c - Martian

A planet which mass within the minor planet subclass, likely around the outer boundary of 0,1 Me, having a rocky composition and a metallic core, being substantially less dense than Earth, these planets tend to be rather large relative to their mass but still solid, though most probably geologically dead due lack of strong internal pressures.

Is probable that these worlds form with such a low density by either inhabiting the outer edge of the terrestrial accretion disk, or by having it's mass 'stolen' by a more massive planet nearby.

A planet of such type would have densities between 3,5 ~ 4,5 g/cm³, martian regolith density is about 3,93 g/cm³.

Type e - Vulcanoid

A rocky planet that is close to the star's innermost stable orbit, having no atmosphere at all, may be tidally-locked, if it's close enough to it's star, tidal forcing may heat up the planet to the point it's surface bears strong volcanism.

Classes T and TT - Terran and Superterran

A planet which mass is greater than 0,1 Me and less than 2 Me, or between 2 Me and 10 Me as a Superterran, can be made of metals, silicates, and ices depending on specific composition and location.

Planet's in these classes would have densities between 4,5 ~ and 7,0-ish g/cm³.

The term of this classification is mean atmospheric nature and distance from the star.

Subclass 1 - Dry

A planet with nearly or completely absent liquid water on it's surface.

Type a - Radiated

A planet is so close to it's star that is heavily radiated by it's emission, having no atmosphere at all, may be tidally-locked, if it's close enough to it's star, tidal forcing may heat up the planet to the point it's surface bears strong volcanism.

 

Type b - Barren

A planet that has little to no atmosphere, and has a rocky surface.

 

Type c - Dune

A planet that has some atmosphere, but little to no water, it surface is largely covered by dry deserts.

 

Type d - Desert
A planet that has some atmosphere, and little water, it surface is largely covered by deserts.

 

Type e - Carbonic

A planet that has a considerable atmosphere, largely composed of nitrogen and carbon dioxide, enough to maintain it's mean temperature above the melting point of water. Carbonic planets can be found on the outer habitable zone, but also between the Venus zone and the tropical habitable zone. Such planets would also require active volcanism to cycle the carbon in the atmosphere beyond the absorption capacity of it's soil. Water on Carbonic planets is rather acidic.

 

Type f - Venusian

A planet that has ran into a carbon-cycle runaway greenhouse effect, and has essentially lost all it's water, temperatures are evenly scorching throughout the planet, and pressures are up to hundreds of atmospheres.

Subclass 2 - Moist

A planet with little liquid water on it's surface.

Type a - Moisty Carbonic

A planet has enough water to be a global swamp, too close to it's star to favor other kinds of climates - think 20th century's view on Venus.

Type b - Steppe

A planet has enough water to bear sparse savannah or steppe environments, but too far away from it's star to favor other kinds of climates.

Subclass  3 - Wet

A planet that has an amount of water comparable or superior to that of Earth's.

Type a - Moisty Venusian

A planet that ran into a runaway moist greenhouse effect, has a dense water vapour and carbon dioxide atmosphere, has a rocky surface.

Type b - Steam World

A planet that has a considerable amount of it's mass formed by water, and it's close enough to it's star that a great portion of it is in vapour form, between it's solid surface and the dense water vapour atmosphere, lies an ocean either as a compresible or supercritical fluid depending on the exact pressures and temperatures.

Water may account for up to 5~15% of the planet's mass.

Type c - Water World

A planet that has a considerable amount of it's mass and own radii formed by water. A water world commonly has a hydrogen, water and carbon dioxide atmosphere, after some point into the water mantle, it's now a supercritical fluid, and further down, a layer of ice VI, VII or X depending on the mean temperature, bellow the ice layer, lies a rocky envelope and a metallic or ice core depending on where it formed in the inner system.

Type d - Oceanic

A planet that has a comparable amount of water to that of Earth's, however, don't have as much landmass, having more than 75% of the planet's surface covered by water.

Type e - Tropical
A planet that has a comparable amount of water to that of Earth's, and similar ratio of landmass to ocean ratio (about 1:3) of the planet's surface covered by water.

Type f - Snowball

A planet that has a comparable or superior amount of water to that of Earth's, however, most of it is in ice form, such a planet has either large polar caps, or is mostly frozen.

Some may have an atmosphere and temperature similar to Saturn's moon - Titan, ie, are cold enough to support a methane-cycle analogous to Earth's water-cycle, or other volatile that can exist in liquid form at these temperatures such as Chlorine, and other hydrocarbons.

Type g - Super-Sized Titan
A planet that's 30~50% water ice by volume, having an Earth-sized silicate lower-mantle and an iron core, the upper-mantle is composed mostly of pressurized water, methane, and ammonia ices, it's distance from the star and atmosphere allows a small portion of the surface to be liquid - forming an ocean that's hundreds of kilometers deep. The atmosphere is mostly water vapor, hydrogen, ammonia, methane. and carbon monoxide/dioxide.

Such worlds have densities around 3~4g/cm³, and topping masses comparable to those of Ice Giants, ~15% water and ~85% silicates and metals by weight.

Tectonic activity is locked by the sheer amount of water ice above the rocky material, so some form of cryovolcanism may be the predominant driving force of pseudo-tectonics with pressures over the ice and radiogenic heating involved.

Bodies like these could have formed early in the star system and due perturbations in those early days acquired eccentric orbits, so while they would have wandered far enough from the star to collect volatiles, they also wandered close enough to star so atmospheric escape take away most of free hydrogen.

Type h - Super-Sized Super-Mercury
A planet that has a metal-like density (more than 5 g/cm³ and less than 9 g/cm³), a Terran/Super-Terran size, a mass comparable to that of an Ice Giant - without having a thick atmospheric envelope as one, if it does have one at all depends o where exactly it did formed.

 

Type i - Cold Venusian
or Cold Primordial

A rocky planet with a dense atmosphere - mainly composed of nitrogen, carbon dioxide, methane, other volatiles and trace gases, can contain hydrogen.

This type of planet could actually be common in young star systems, with orbits on the outer regions of the habitable zone, the atmosphere is thick enough so the planet doesn't freeze but it's not hot enough to trigger runaway greenhouse effect.

Marked by extreme temperature differences between daytime and night-time on the order of ~100K, the thick cloud cover would make little to no light get through the atmosphere. I would say this world-type is an Acheron-analogue.

 

Class N - Mini-Neptune

A planet which is composed mostly of volatiles other than water, like ammonia and methane, are usually found to be short period planets around their stars, forming before the frost line.

A Mini-Neptune mass range from ≳1 Me to 20 Me, the major definition turns around it's size and composition. Mini-Neptunes are identical in composition to Ice Giants (about 20% Hydrogen/Helium and 80% Volatiles), however they are smaller than 3,9 Re and greater than 1,6~1,7 Re.

Type 1 - True Mini-Neptune

A planet that more or less attends the general rule of composition for an Ice Giant.

By mass, 60% mix of supercritical Water, Ammonia and Methane, ≲20% mix of Hydrogen, Helium and Methane gas, ~20% icy/rocky core.

Type 2 - Super-Sized Water World (or Wet-Neptune)

A planet that DOES NOT attends the general rule of composition for an Ice Giant, nor is as big as one, however, the amount of volatiles is so considerably large that it falls in between a TT3c planet and a N1-class planet, having a rocky/metallic bulk mass, a supercritical/liquid water envelope, and thick hydrogen/helium atmosphere - no pressurized Ice, being roughly 2~3 Re wide, with an atmospheric envelope no thicker than 0,5 Re.

Class J - Ice Giant

A planet which is composed of mostly of volatiles other than water, like ammonia and methane.

An Ice Giant mass range from ≳10 Me to 130 Me, the major definition turns around it's size and composition. Ice Giants are made of about 20% Hydrogen/Helium and 80% Volatiles, and attain to sizes greater than 3,9 Re and less than 8,4 Re, densities range on about 1~2 g/cm³.

An Ice Giant's general azure~cyan appearance is due to methane traces in it's atmosphere. Ice Giants aren't restricted to form beyond the frost line, however when they form before the frost line it is very close to their parent stars, those are called Hot-Neptunes then.

Type 1 - True Ice Giant

An Ice Giant that sits beyond the frost line, like Neptune and Uranus.

Type 2 - Hot-Neptune

A short-period (year is less than 10 days and longer than 1 day) planet rich in volatiles, with too much atmosphere to be a Steam World, too big to be Mini-Neptune, and too heavy to be Gas Dwarf.

Hot-Neptunes with radii greater than 2 and less than 10 Re are relatively scarce and the most common type of planet when looking at orbital distances of less than 0,1 AU, creating what's called a Hot-Neptune Desert, planets within these radii, are rarely found between the following lines - most exoplanets with these radii cram just around the border of this zone.
In case your planet fall inside the Hot Neptune Desert, increase the orbital period / distance to an acceptable degree of at least 0,5 log10 into the desert.

Upper Limit Log10(MJ) = [1,07 * Log10(period in days)] - 2
Lower Limit Log10(MJ) =  - Log10(period in days) + 0,1

Class G - Gas Giant

A planet which is composed of mostly of Hydrogen and Helium, having some amount of volatiles like water, ammonia and methane.

A Gas Giant can be as light as ≳10 Me or as heavy as 4.134 Me (beyond that they're classified as Brown Dwarfs), Gas Giants have their atmospheres made up of ≳90% Hydrogen/Helium, nas less than 10% volatiles, a small amount of their mass comes from a solid rocky/metallic core, Gas Giants have strong magnetic fields and make perfect magnetic shields for habitable moons around them.

Type 1 - Lesser Gas Giant

A planet which is mostly made of Hydrogen alone (about ≳95%), ≲5% Helium, and less than 1% trace volatiles.

Lesser Gas Giants are very light like Saturn, having densities from anywhere less than 1,32 g/cm³ and around 0,7 g/cm³ (about as light as Saturn), and at first seem featureless, depending on how low is the concentration of volatiles and tholins in the atmosphere.

Bellow a relatively thin gaseous atmosphere (compared to their size), lies a mantle of liquid supercritical hydrogen, and at the center a rocky core about the size and mass of a Superterran.

Type 2 - Jovian Gas Giant

A planet that is similar in composition to Jupiter, 90% Hydrogen, 10% Helium, and less than 1% trace volatiles.

Jovian Gas Giants are rather heavy when compared to Lesser Gas Giants, their density wander above 1 g/cm³ and less 2 g/cm³, and they might have or not a solid Superterran-sized core depending on where and how it formed.

Jovian Gas Giants have a higher concentration of tholins and other chromophores (like ammonia and sulfur) than it's lighter counterpart, then, it's atmospheric bands appear to be tinted in red/orange/pink depending on the concentration of the materials in it.

Planet's with a mass higher than that of Jupiter aren't much larger than the planet, as the pressures involved kick electron degeneracy in the core, allowing atoms to pack more tightly, so instead of growing in size, they get denser until when they reach 13 Jupiter masses, at start to fuse deuterium.

Type 3 - Inflated Gas Giant

A planet that can be even lighter than a Lesser Gas Giant, due thermal expansion of it's atmosphere for being so close to it's star.
It's worth noting that planets with considerably less gravity or mass than Jupiter (around 24,8m/s²), when exposed to temperatures around 1800K or more will have their  atmospheres evaporated in the timescale of 1~2 billion years.

Gas giants in general (up to 10 Mj) seem to have similar sizes (0,85 to 1,15 Rj) at T <1000K, once the temperature

A reasonable radius estimate for any gas giant at a given temperature is given by:

RJupiter = 0,915 * ( 1,00027 ^ T )

Where T is the temperature in K.
Of course, aspects such as distance from the star, albedo and exact composition are ignored in this case, but it seems to offer quite a solid baseline in that aspect - further bellow you can sort out your gas giant by mass range and calculate it's radius within a 0,2~0,7 Rj margin for variance based on actual exoplanet catalog work - but that's also under the same limitations.

Type 4 - Gas Dwarf
A rocky planet that has a considerable amount of it's mass and volume due to high concentrations of Hydrogen and Helium in a thick atmospheric envelope, however it's atmosphere also contains high levels of volatiles such as ammonia, water, and methane - a Gas Dwarf also isn't as massive or large as a Gas Giant, being similar in size to Hot-Neptunes and Super-Earths.

Due their low density and low gravity - is rather probable that most Gas Dwarfs are in the most part featureless planets, the hotter ones would have a hyper-inflated haze-like atmosphere envelope, while the colder ones could have pale colored bands according to it's rotational period and chromophores in the atmosphere.

Their radii vary in between 1,2 ~ 3,9 Re, and minimum mass of 0,6 Me.
(Kepler 138d is the smallest known Gas Dwarf with 1,2 Re).

A Gas Dwarf with 1,0 Me and a density comparable to that of Saturn would have a radius of 2,0 Re.

Type 5 - Helium Titan

A planet that has a considerable amount of it's mass and volume due to high concentrations of Helium, it would be mostly made of supercritical helium and hydrogen, being ~4x as dense and ~4x as heavy as an usual Gas Giant.

Such a planet could compact 318 Me within only 7,75 Re, presenting a nearly featureless pale or gray appearance due lack of methane, that would otherwise give it a bluish tint.

In nature, the more likely way these planets form is via the Hydrogen evaporation of Hot-Neptunes and Hot-Jupiters, as a late stage of these planets life, a Helium Titan can form within a couple million years, but take up to 10 billion years to lose all of it's hydrogen, so it is possible that most of these planets still contain a large portion of hydrogen, thus, appearing as either pale jovians or a jovian-sized ice giant due methane scattering of light.

SUBTYPES (Sudarsky Classification)

S I

The planet is beyond the frost line, and it's atmosphere color is defined chemically rather than physically, temperatures are less than 150 K, and albedo range from 0,57 w/o tholins and 0,34 w/ tholins.

S II

The planet is close enough to it's star that it's clouds are made of water vapour and methane, giving it a more featureless and pale color, temperatures are less than 250 K, albedo is around 0,81.

S III
The planet is so close to it's star that the incoming solar radiation decompose cloud formations, giving it a more featureless and azure~cyan appearance due sheer Rayleigh scattering, temperatures are less than 800 K but more than 250 K, above 700 K the presence of sulfides and chlorides favor the formation of thin cirrus-like clouds, with an albedo as low as 0,12.

S IV

The planet is close enough to it's star that the main component of the upper atmosphere changes to carbon monoxide and other alkali metal impurities, which darkens the planet, having an albedo low as 0,03, temperatures are above 900 K. Those are commonly called Hot-Jupiters.

S V

The planet is either a super Hot-Jupiter, with temperatures above 1400 K, or cooler and less heavy than Jupiter, with silicate and iron clouds, and albedo at 0,55.

POTENTIALLY HABITABLE PLANET TYPE SUMMARY

• S2c type
• T1 and TT1, c through f types
• T2 and TT2, a and b types
• T3 and TT3, a through f types
• G4, subtype S-III

Other Multi-Class Types

Eyeball Planet

A planet within the Subterran-Superterran mass range, granted it does have an atmosphere and some amount of liquid water, and sufficiently close to it's star, usually a Red Dwarf, it will be tidally-locked, creating a desert hemisphere and a water ice hemisphere, having a thin temperate zone around it's terminator. Such planets are bombarded with radiation and charged particles from it's parent star, however, if it does have a sufficiently strong magnetic field, it could harbor complex life.

Rock/Ice Puffy Planet

A planet that has a density comparable to that of the Moon or an asteroid (less than 3,5 g/cm³), generally nearly or completely absent of metals and composed mainly of silicates and ices.

Puffy planets of this kind have a weak gravity when compared to Sub, Terran, and Super Terran counterparts of the same size, because of that, are probably geologically dead on their own. This kind of body is found in the solar system as the icy moons of Jupiter, and tidal heating could make them habitable to some extent.

Classifying Terran-Sized planets by Water Content

A planet within the S throughout TT classes could be also classified by it's water content, proportionally speaking - because it doesn't help much having as much water as Earth if the planet has 4x as much area, making for a lower water content, or if the planet has half the water on Earth, but have 1/3rd of the area - making for a relatively larger water content.

In this classification, I will be referring to the Mars water proportion to it's mass, ie how much of the planet's mass is made of water.

Color-codes for habitability assumes the planet that much liquid water in a stable temperature and pressure for life as we know it.

DRY

Dry - < 0,0000024%

Martian-dry - around 0,0000032%

MOIST

Moist I - up to 0,00032%

Moist II - up to 0,002%

WET

Oceanic I - up to 0,01%

Oceanic II - around 0,023%

Earth goes here.

Oceanic III - 0,04%

SOAKED

Water World I - 0,07%
Water World II - 0,2%

Water World III - 5~15%

At this point the planet has at least 5% of it's mass composed by water, and above 15% we can consider it a True-Water-World (a T3c or TT3c planet).

Earth-like biochemistry is difficult with 0,1% of the planet's mass worth of water.

Tectonic activity stops once the water amount is above ~1,1% of the planet's mass, so chemosynthesis would also be difficult, if not, non-existant beyond this point.

RELATIONSHIP GRAPHS

Here is a graph of planetary mass in Earths x Volatile content in percent.

Earth posess 0.02% of its mass in water, and so we get -1.7 in the vertical axis, and zero in the horizontal axis since the log of 1 is 0 (0.0,-1.7).

Mars has coordinates (-0.97, -5.49)

 

Planetary Density as a function of it's distance from the Sun

 

Minor Planet Density Zones

Dwarf Planet Mass/Size Relationship

 Gas Giant planet Radius x Temperature

Gas Giant planet Radius x Temperature - but it's my attempt to find a relationship

Metal/Silicate Ratio and respective representative densities - metallic material is assumed to be 8,05g/cm³ and silicate material to be 2,75gcm³

- M.O. Valent, 15/09/2020

- M.O. Valent, updated 27/09/2020