Planetary Classification

Notice: Pictures and texts are used with the kind permission of Chris Adamek (sttff.net) for non-commercial purposes. 
Some texts and details were altered to better fit the intended RPG setting.

A planet is a celestial body in orbit around a star or stellar remnants, that has sufficient mass for self-gravity and is nearly spherical in shape. A planet must not share its orbital region with other bodies of significant size (except for its own satellites), and must be below the threshold for thermonuclear fusion of deuterium.

If a celestial body meets those requirements, it is considered a planet; at that point, the planet is further classified by its atmosphere and surface conditions into one of twenty-two categories.

Class A
Geothermal
Class B
Geomorteus

Class C Geoinactive

Class D
Dwarf

Class E
Geoplastic

Class F
Geometallic

Class G
Geocrystalline

Class H
Desert

Class I
Ice Giant

Class J
Jovian
Class K
Adaptable
Class L
Marginal
Class M
Terrestrial
Class N
Reducing
Class O
Pelagic
Class P
Glaciated

Class Q
Variable

Class R
Rogue

Class S
Super Giant

Class U
Ultra Giant

Class X
Chthonian

Class Y
Demon

Class A - Geothermal

Age
Diameter
Location 
Surface
Temperature
Composition
Atmosphere
Evolution
Life Forms
Habitability

less than 2 billion years
1,000 – 10,000 km
Hot/Ecosphere/Cold Zone
partially molten, very hot
carbon dioxide, hydrogen
up to 1000° C
igneous silica; basalt
cools to become Class C
none
uninhabitable

Class A planets are generally young, rocky worlds that are rife with volcanic activity. This volcanic activity spews vast amounts of sulfur and carbon dioxide into the atmosphere, causing a greenhouse effect that keeps temperatures relatively hot. Planets in a star system’s hot zone may have worldwide temperatures in excess of 1500° C. Due to the tenuous nature of the atmosphere, planets in the cold zone do not retain as much heat, and while temperatures might approach 1000° C near volcanoes, the average temperature may be as low as -130° C. This combination of extreme temperature and tenuous, toxic atmosphere make Class A worlds highly unfavorable to life of any kind.

When this extreme volcanic activity eventually ceases, the planet “dies” and becomes a Class C: Geoinactive world.

Class A planets are common in the universe; Jupiter’s moon Io is a prime example.

Class B - Geomorteus

Age
Diameter
Location 
Surface
Temperature
Composition
Atmosphere
Evolution
Life Forms
Habitability

less than 10 billion years
1,000 – 10,000 km
hot zone
barren, partially molten
-170° – 430 °C
iron, potassium, silicon
oxygen, sodium, hydrogen
none
none
uninhabitable

Class B planets are generally very small, very rocky worlds located within a star system’s hot zone, such as Mercury. In the harsh daylight, these planets are scorched by their parent star, often to the point of rock becoming molten. Because Class B worlds have little to no atmosphere, this heat quickly radiates away at night, leaving the dark side of the planet a frigid wasteland. As a result, these planets are highly unsuitable for humanoid life.

Despite their small size, Class B planets are often extremely dense, with a large inner core, up to 55% of the planet’s volume, that is made of molten iron.

Class B planets are fairly common in the universe.

Class C - Geoinactive

Age
Diameter
Location 
Surface
Temperature
Composition
Atmosphere
Evolution
Life Forms
Habitability

2-10 billion years
1,000 – 10,000 km
hot / eco / cold zone
barren and cratered
none
-150° to 120° C
anorthosite and basalt
cools to become Class C
none
generally uninhabitable

Class C planets are small, barren worlds, utterly devoid of just about everything: no atmosphere, no water, no geological activity, and absolutely no life.

While the vast majority of geoinactive worlds are actually moons in orbit of a larger planet, make mo mistake, they are planets in all other regards. In fact, most have a semi-molten iron core, and many are far larger than the bona fide terrestrial planets that inhabit their star system.

Moon or not, some geoinactive worlds are born into the classification, and spend their entire existence within the boundaries of that definition. Others begin life as a highly volatile Class A: Geothermal planet; once the volcanic activity eventually ceases, those worlds cool off and become geoinactive.

Class C planets are often rich in minerals, and while they are not suitable for life, they can be transformed into mining colonies via the use of pressure domes. Some Class C worlds, such as Earth’s moon, are colonized in that manner just for the heck of it.

Class D - Dwarf

Age
Diameter
Location 
Surface
Temperature
Composition
Atmosphere
Evolution
Life Forms
Habitability

2-10 billion years
100 – 1,000 km
hot / eco / cold zone
barren and cratered
-200° C
rock, ice
none or very tenuous
none
none
habitable via pressure domes

A Class D planetoid is a tiny world that generally does not meet the criteria for a planet. This includes many moons, asteroids, and small planet-like objects. Many are not even spherical, and have eccentric orbits that are cluttered with various smaller rocks and asteroids.

Dwarf planetoids generally lack the internal structure of larger worlds; instead of having an inner-core, mantle, and crust, many dwarf planetoids are simply composed of solid rock and ice (however, some larger dwarfs, such as Pluto, do have a more planet-like internal structure).

They are often rich in natural resources, making them ideal for mining operations. However, most dwarf planetoids either have a very tenuous atmosphere (usually composed of water vapor and nitrogen) or more commonly, no atmosphere at all, making pressure domes a requirement for colonization.

Class E - Geoplastic

Age
Diameter
Location 
Surface
Temperature
Composition
Atmosphere
Evolution
Life Forms
Habitability

less than 2 billion years
10,000 – 30,000 km
eco zone
molten
hydrogen compounds
1500° C
igneous silica, basalt
none
none
generally uninhabitable

Class E planets represent the earliest stage in the formation of a habitable world, and are commonly found in newly formed star systems, often within a few thousand years of the parent star’s formation.

Though similar in appearance to the volcanic Class A worlds, Class E planets are actually molten due to the heat generated by the collision of the many millions of rocks and asteroids that came together in the formation of the planet. Additionally, Class E worlds are many times larger than Class A, and typically have much longer and more varied lives.

Over the course of several billion years, the crust will cool and solidify, and the hellish geoplastic world will slowly transition into a Class F: geometallic one.

Class F - Geometallic

Age
Diameter
Location 
Surface
Temperature
Composition
Atmosphere

Evolution
Life Forms
Habitability

1-3 billion years
10,000 – 30,000 km
eco zone
volcanic, barren and cratered
95° C
silicate rocks, iron, nickel
carbon dioxide, ammonia, methane
Class G: Geocrystalline​
none
uninhabitable

As a Class E: Geomorteus planet cools, the crust and mantle slowly begin to solidify and the planet is reborn as a Class F: Geometallic world. These barren worlds are witness to much geologic activity. Steam expelled from volcanic eruptions condenses in the atmosphere and falls to the earth as rain.
Over the eons, this precipitation will form the planet’s first shallow seas, in which primitive bacteria may develop and ultimately thrive.

While geometallic worlds are extremely rich in natural resources, mining these treasures is generally frowned upon, as the bacteria that inhabit the shallow seas might one day evolve into plants, animals, or even fully sentient life
forms. For a young geometallic world, such grandiose developments are still millions, if not billions of years away. In the interim, the planet’s crust will continue to solidify, and the barren surface will eventually cool to the point where the Class F: Geometallic world transitions into the cradle of life, a Class G: Geocrystalline world.

Class G - Geocrystalline

Age
Diameter
Location
Surface
Temperature
Composition
Atmosphere

Evolution
Life Forms
Habitability

3-4 billion years
10,000 – 30,000 km
eco zone
rocky, mostly barren
50° C
silicate minerals, iron
carbon dioxide, oxygen, nitrogen
several*
vegetation, simple orgasms
habitable with restrictions

Once the core of a Class F: Geometallic planet sufficiently cools, volcanic activity lessens and worldwide temperatures continue to fall. The planet eventually transitions into a Class G: Geocrystalline world. Oxygen and nitrogen are present in some abundance in these “Cambrian” worlds, giving rise to increasingly complex organisms, such as primitive vegetation similar to lichen and algae, and animals akin to sponges and jellyfish. Over the course of the coming eons, these primitive life forms will eventually flood the atmosphere with enough oxygen to support highly complex flora and fauna, and potentially sentient life.

Meanwhile, the planet itself begins to inch ever closer to its final stage of evolution during the geocrystalline phase. Class G planets closer to their parent star might become arid wastelands; those further away could transition into snowbound lands; yet others could blossom into a tropical paradise. *In fact, there are any number of potential evolutions for a Class G planet, including Class H: Desert, Class K: Adaptable, Class L: Marginal, Class M: Terrestrial, Class O: Pelagic, or Class P: Glaciated.

Class H - Desert

Age
Diameter
Location 
Surface
Temperature
Composition
Atmosphere
Evolution
Life Forms
Habitability

4-10 billion years
8,000 – 15,000 km
eco zone
silicate rocks
30° C
less than 20% surface water
oxygen, nitrogen, argon
none
drought resitant flora and fauna
habitable with restrictions

An extremely arid planet with less than twenty percent of its surface covered in water is considered a Class H: Desert world. While many of these worlds are very hot and covered in a blanket of glittering sand, these conditions are not requisite for a desert classification. In fact, a desert world may be both cold and rocky.

Though precipitation is rare and the climate harsh, drought-resistant plants and animals are actually quite common on desert worlds, and many support humanoid populations.

Class I - Ice Giant

Age
Diameter
Location
Surface
Temperature
Composition
Atmosphere
Evolution
Life Forms
Habitability

2-10 billion years
30,000 – 100,000 km
cold zone
rocky and barren
-55° C
iron and other silicate rock
thin, mostly carbon dioxide
none
simple single cell organisms
can be colonized via terraforming or pressure domes

Also known as Uranian planetoids, Class I: Ice Giants are massive, frozen worlds that lurk on the outskirts of a star system. They are also dramatically different from their gaseous brethren; while Class J: Jovian planets are massive spheres of hydrogen and helium, Class I worlds are typically composed of tenuous layers of water, ammonia, and methane, surrounding a small core of ice and iron. Many ice giants also have magnetic fields that are sharply inclined to the axis of rotation.

Class J - Gas Giant

Age
Diameter
Location 
Surface
Temperature
Composition
Atmosphere
Evolution
Life Forms
Habitability

2-10 billion years
50,000 – 500,000 km
cold zone
liquid metallic hydrogen
-145° C
hydrogen and helium
hydrogen and helium
none
none
uninhabitable

Class J Jovian planets are among the most common and familiar sites in star systems across the universe. These giant spheres of liquid and gaseous hydrogen, boast extremely dense atmospheres, and are able to support powerful storms that might endure for hundreds of years and produce winds in excess of 600 kph.

Many Class J planets also possess impressive ring systems,composed primarily of rock, dust, and ice. They form in the coldzone of a star system, though typically much closer than Class I, S, or U planets.

Class K - Adaptable

Age
Diameter
Location
Surface
Temperature
Composition
Atmosphere
Evolution
Life Forms
Habitability

4-10 billion years
5,000 – 10,000 km
eco zone
rocky and barren
-55° C
iron and other silicate rocks
thin,mostly carbon dioxide
none
simple single cell organisms
can be colonized via terraforming or pressure domes

Class K: Adaptable planets represent an unfortunate part of planetary development: a failed world. Over the course of a terrestrial planet’s long and arduous evolution (from Class E to Class F to Class G), something, somewhere goes wrong, and the blossoming young planet fails to reach its full potential. Volcanic activity slows to a halt, the tenuous atmosphere begins to disperse, any liquid on the surface evaporates, and the rocky young world essentially dies.

Though rare, simple single cell organisms can still thrive on these barren worlds, but more complex forms of life never evolve. As a result, Class K planets are easily colonized via the use of pressure domes, and are often prime candidates for terraforming. Average temperatures are quite cold by humanoid standards, but a warm summer day on a terraformed Class K planet might creep as high as 20°C.

Class L - Marginal

Age
Diameter
Location 
Surface

Temperature
Composition
Atmosphere

Evolution
Life Forms
Habitability

4-10 billion years
10,000 – 15,000 km
eco zone
rocky and forested, very little water
15° C
silicate materials
argon, oxygen, trace elements
none
abundant native plant life
habitable

Class L: Marginal planets are typically rocky planets with a nominal ecosystem consisting almost entirely of plants. It is extremely uncommon for higher forms of life, such as animals and humanoid life, to evolve naturally on marginal worlds. A contributing factor in this lack of fauna is a limited supply of
surface water (generally more than 20% of the surface is covered in water, otherwise the planet is Class H: Desert).

While the limited water supply prevents widescale colonization, marginal planets can easily support modest humanoid populations, and a small amount of terraforming can easily transform them into robust and completely habitable worlds.

Class M - Terrestrial

Age
Diameter
Location
Surface
Temperature
Composition
Atmosphere
Evolution
Life Forms

Habitability

4-10 billion years
10,000 – 15,000 km
eco zone
abundant surface water
-89° to 58° C
iron and other silicate rocks
nitrogen, oxygen, argon
none
abundant native plants and animals
prime conditions for vast populations of plant, animal, and humanoid life

Class M: Terrestrial planets are robust and varied worlds composed primarily of silicate rocks. Located
in a star system’s habitable zone, most are temperate worlds with vast blue oceans and wide swaths of verdant forest. However, conditions can vary greatly between worlds and still be considered Class M; as long as the surface is between 20 and 80 percent water, the climate is generally temperate, and the atmosphere made of oxygen and nitrogen, even dry rocky worlds and cold snowy planets can be Class M.

Class N - Reducing

Age
Diameter
Location 
Surface

Temperature
Composition
Atmosphere


Evolution
Life Forms
Habitability

3-10 billion years
3,000 – 10,000 km
N1: eco zone, N2: cold zone
N1: rocky and barren, N2: liquid methane lakes, icy mountains
N1: 462° C, N2: -179° C
N1: iron, N2: ice / nitrogen
N1: very dense; carbon dioxide, sulfides, N2: very dense; nitrogen, methane
none
none
uninhabitable

Class N: Reducing planets have a reducing atmosphere in which oxidation is prevented by removal of oxygen and other oxidizing gases or vapours, and which may contain actively reducing gases such as hydrogen, carbon monoxide, and gases such as hydrogen sulphide that would be oxidized by any present oxygen. There are two sub-classes: Class N1: Hot reducing and Class N2: Cold reducing.

CLASS N1: HOT REDUCING
Though frequently found in the ecosphere of a star system, Class N1 planets are not conducive to life. The terrain is barren and blazing hot, with a surface pressure more than 90 times that of a Class M: Terrestrial world. The atmosphere is extremely dense, and composed of carbon dioxide. Water exists only in the form of thick, vaporous clouds that shroud the entirety of these worlds.

CLASS N2: COLD REDUCING
While Class N1 planets are among the hottest planets known to exist, at the opposite end of the temperature spectrum is the frigid Class N2. Though similar in appearance to N1 thanks in part to the dense atmosphere and characteristic veil of thick clouds, this secondary class is structurally very different. Temperatures are so cold that liquid methane can pool into lakes, and vast mountain ranges are made of rock-hard ice.

Class O - Pelagic

Age
Diameter
Location
Surface

Temperature
Composition

Atmosphere
Evolution
Life Forms

Habitability

4-10 billion years
10,000 – 15,000 km
eco zone
mostly liquid water; archipelagos
5° to 55° C
silicon, iron, magnesium, aluminum
nitrogen, oxygen, argon
none
prime conditions for vast populations of cetacean life
suitable for humanoid colonization

Any world with more than eighty percent of its surface covered with liquid water is considered Class O: Pelagic. These warm, tropical worlds are able to support vast cetacean populations, as well as abundant plant and animal life on their sparse tracts of land. Though humanoids can easily colonize pelagic worlds, it is rare for civilizations to naturally arise due to the limited space proffered by its landmasses.

Due to the warm atmosphere and vast oceans, violent tropical cyclones are common on pelagic worlds.

Class P - Glaciated

Age
Diameter
Location 
Surface
Temperature
Composition
Atmosphere
Evolution
Life Forms
Habitability

4-10 billion years
10,000 – 15,000 km
eco zone, cold zone
cold, glaciated
-19° C – 15° C
silicon, iron, magnesium, ice
nitrogen, oxygen, argon
none
hearty flora and fauna
inhabitable

On the distant edge of a star system’s ecosphere, habitable planets are still numerous, but they are a far cry from the lush garden worlds closer in. Cold and barren, more than eighty percent of a Class P: Glaciated planet is covered in solid ice, and while many possess narrow stripes of green along the equator, where hearty plant and animal life may flourish, many glaciated worlds are entirely frozen.

Despite the harsh conditions, humanoid life can thrive on a glaciated world.

Class Q - Variable

Age
Diameter
Location
Surface
Temperature
Composition
Atmosphere

Evolution
Life Forms
Habitability

2-10 billion years
4,000 – 15,000 km
hot / eco / cold zone
varies greatly
-100° to 75° C
silicon, iron, magnesium
nitrogen, oxygen, argon; can be either very tenuous or very dense
none
none
uninhabitable

Class Q: Variable planets are exceedingly rare in the universe. They typically have highly eccentric orbits or form around a star with a variable output. As a result, conditions the planet’s surface are widely varied and often quite extreme; deserts and rain forests can coexist in the span of a few kilometers, and glaciers might simultaneously loom near the equatorial regions… only to melt off in the span of a
few days. If the planet has an eccentric orbit, the entire planet might spend decades completely frozen in the far reaches of the star system, only to turn into a hothouse as it approaches its parent star.

Though pockets of the surface might occasionally become habitable, in general, the constant instability makes long term survival for any species virtually impossible.

Class R - Rogue

Age
Diameter
Location 
Surface
Temperature

Composition

Atmosphere


Evolution
Life Forms


Habitability

2-10 billion years
4,000 – 500,000 km
interstellar space
can be terrestrial or gaseous
-100°C to 20°C (terrestrial)
-220°C (gaseous)
silicate, iron (terrestrial)
hydrogen, helium (gaseous)
volcanic outgassing (terrestrial)
dense hydrogen, helium (gaseous)
turns into Class C or I soon
may support non-photosynthetic plants and some animals
limited inhabitable

A rogue planet usually forms within a star system, but at some point during its evolution, the planet is ejected from a stable orbit an expelled into interstellar space. Typically this is caused by a catastrophic asteroid impact or interaction with a wandering Class U: Ultra Giant.

Many rogue planets simply turn into frozen wastelands and die, but geologically active planets can maintain a habitable surface via volcanic outgassing and geothermal venting, especially those with satellites, as tidal interactions serve to maintain geothermal activity.

While many rogue planets are terrestrial worlds, gas giants are just as likely to be expelled from a star system, and while they are technically Class R, these wandering giants function like any other gaseous world.

Class S - Gas Supergiant

Age
Diameter
Location
Surface
Temperature
Composition
Atmosphere
Evolution
Life Forms
Habitability

2-10 billion years
500,000 – 50,000,000 km
cold zone
liquid metallic hydrogen
-220°
hydrogen and helium
hydrogen and helium
none
none
uninhabitable

Aside from their colossal size, there is little that differentiates a Class S: Super Giant world from its Class J: Jovian counterpart. Located in a star system’s cold zone, they often boast impressive ring systems and harbor dozens of moons.

Giant worlds like Class S and the other gaseous planetoids tend to act as “shields” for the terrestrial planets in the ecosphere, as their powerful gravitational fields tend to divert comets and asteroids away from the interior of a star system.

Class U - Gas Ultragiant

Age
Diameter
Location 
Surface
Temperature
Inner Core
Atmosphere
Evolution

Life Forms
Habitability

2-10 billion years
50,000,000 – 120,000,000 km
hot / cold zone
-220°C (cold zone), 850°C (hot zone)
liquid hdyrogen, deuterium
hydrogen and helium
can evolve into Class X planet or a red dwarf star
none
uninhabitable

These titanic gaseous worlds represent the upper limits of planetary masses. Structurally similar to their Class J and S counterparts, only on a far more grandiose scale, these planets have astounding diameter between 50,000,000 and 120,000,000 kilometers. Most Class U planets are content to loom in the cold zone of a star system, but if the planet is sufficiently massive (13 times the size of Jupiter), nuclear fusion ignites the deuterium within the core, and the planet becomes a red dwarf star, creating a binary star system.

The great mass of ultra giants that do not transition into stars occasionally force them to assume eccentric orbits. This causes them to spiral inward toward the heart of the star system and become a “Hot Jupiter,” a gas giant orbiting extremely close to its parent star. This destructive process disrupts the entire star system, ejecting smaller planets into interstellar space, and ultimately ends with the Class U planet’s demise as a desolate Class X: Chthonian world.

Class X - Chthonian

Age
Diameter
Location
Surface
Temperature
Inner Core
Atmosphere
Evolution

Life Forms
Habitability

3-10 billion years
1,000 – 10,000 km
hot zone
barren, rich in resources
-260° – 800° C
molten iron
none
ultimately destroyed by parent star
none
uninhabitable

When a highly eccentric orbit causes a Class U planet to spiral into the hot zone of a star system, it eventually settles into orbit perilously close to the parent star. This proximity typically results in gravitational forces stripping the atmosphere from the planet; the end result is a Class X: Chthonian planet, which is nothing more than the exposed core of the Class U world.

In stark contrast to a Class U planet, Chthonian worlds are tiny, measuring no more than 10,000 kilometers in diameter. Any atmosphere that remains is tenuous at best, and composed primarily of hydrogen and helium. As a result, the daylight side of the planet is scorched by temperatures up to 800°C, but without an atmosphere, none of this heat is retained, and nighttime temperatures may drop as low as -260°C.

Chthonian worlds are utterly barren, but rich in natural resources. Most have surfaces composed of iron and deuterium, but some are composed almost entirely of diamond. While they are wholly unsuitable for life of any kind, short-term mining operations are often established on
Class X planets to gather the valuable materials proffered by these tiny worlds before it’s too late—Chthonian worlds are extremely short-lived. Doomed by the inward spiral set into motion in the cold zone, Class X planets are ultimately absorbed by their parent star and completely obliterated.

Class Y - Demon

Age
Diameter
Location 
Surface

Temperature
Inner Core
Atmosphere
Evolution
Life Forms
Habitability

2-10 billion years
10,000 – 15,000 km
hot zone
barren; iron, deuterium, silicate rocks
200°C
molten iron, sulfur
turbulent, with toxic radiation
none
only rare biomimetic life
uninhabitable

Perhaps the most environmentally unfriendly planets in the galaxy, Class Y: Demon worlds are inhospitable to life in every way imaginable. Most often found in the hot zone of a star system, these worlds possess extremely thick, highly toxic atmospheres which are plagued with violent storms that discharge thermionic radiation; surface temperatures exceed 200°C, and winds may reach 500kph. Incredibly, a type of biomimetic life form was discovered on a Class Y planet in the Delta Quadrant in 2374.

Although Class Y planets are extremely rare in the universe, they are generally unremarkable, average-sized planets when their harsh atmosphere is taken out of the equation. The surface is predictably barren, and composed primarily of iron, deuterium, and silicate rocks, with a diameter between 10,000 and 15,000 kilometers.