Here is a world classification system I developed for my Stardust Sequence. If you see any room for improvement, let me know.
World Classification
Class A (Gas Giants) over 50 Terran Units of mass
A1) These are the ‘super’ giants. That is gas giants that are between 3 and 13 times the mass of Jupiter. Anything over 13 Jovian Units will become a Brown Dwarf star.
A2) Giant (Hydrogen & Helium) These are the gas giants that consist of primarily hydrogen, with helium coming into second place. This would be Jupiter and Saturn.
A3) Giant (Hydrogen + heavier elements) This has no equivalent in the Sol System. They are gas giants but with plenty of elements heavier than helium. These would be the gas giants that are closer towards their star, where temperatures are high enough to vaporize many elements.
Class B (Sub-Giants, Icy Giants, or even Gas Dwarves) 10-50 Terran Units of mass
B1) Sub-giant (Hydrogen) These would best be described as Gas Dwarves. They consist of almost exclusively hydrogen.
B2) Sub-giant (Helium) They are made of hydrogen, but have more than 20% helium. Uranus and Neptune may be considered this; their classification is not clear, for they have cores with plenty of water, and they have concentrations of methane in their atmospheres.
B3) Sub-giant (Water) This is one of the types of icy giants. It has a core and mantle comprised mostly of water, with some mixed rocky material.
B4) Sub-giant (Ammonia) Same as a B3, but with Ammonia in the place of water.
B5) Sub-giant (Methane) Same as B3, but with Methane.
B6) Sub-giant (Nitrogen) Same as B3, but with Nitrogen.
Class C (Barren) planets without atmosphere.
C1) Barren (Metal) A planet with a crust comprised of metals, usually high density. Mercury could be classified as such.
C2) Barren (Silicate) A planet with a silicate crust. Luna is a C2.
C3) Barren (Carbon) A carbonous world, forms between 2.0 to 6.0 AU from a star with one solar mass and one solar unit of metallicity. The lesser the metal in a star, the closer C3 worlds will form.
C4) Barren (Water) These are worlds made of solid water and other ices. Moons of Saturn, Uranus and Neptune are mostly C4.
Class D (Desert) planets without liquid water.
D1) Desert (Greenhouse) A desert world with a thick, dense atmosphere of greenhouse gases. Venus is a prime example.
D2) Desert (Hot) A desert world with a thin atmosphere, and no water in any form. It orbits close enough to the star for the average temperature to be above the melting point of water.
D3) Desert (Cold) A desert world with an average temperature below the melting point of water. It often has deposits of ices in its regolith. Think Mars.
Class E (Arid) has less than 50% of the surface under water.
E) Arid: A habitable planet with less than 50% of its surface covered by water. Often the water coverage drops to 10% or less. This is what I would classify Arrakis as.
Class F (Ocean) more than 50% of its surface covered by ocean.
F1) Ocean (Metal) The best way to describe an F1 is that it is a planet in the process of forming. During the formation of systems, continuous impacts from asteroids and even other planets leave forming worlds in a liquid state. There are no F1 planets older than a few million years; by then, the planet solidifies and turns into something else.
F2) Ocean (Silicate) It is a planet with an ocean of liquid glass. For the most part, it is in the same category as an F1, only existing during planetary formation. However, for reasons not fully understood, Dakar is an F2, perhaps due to a planetary collision.
F3) Ocean (Water) Does this really need explanation? It is the fabled earth-like planet.
F4) Ocean (Ammonia) A world with an ocean of liquid ammonia. Because ammonia has properties similar to water, F4 worlds are capable of developing life. Due to the cold temperatures ammonia exists at a liquid, F4 life will always be slower than F3 life.
F5) Ocean (Methane) A world where methane exists in a liquid form. Titan would be classified as F5, even if more than 50% of its surface is not under methane.
F6) Ocean (Nitrogen) A world so cold that its oceans are comprised of liquid nitrogen. Such worlds are rare.
Prefixes Added to other classifications to create sub-classes.
+U) Volcanic/Thermic; A world that is extremely geologically active, with a volcano always erupting somewhere on the surface. Io would be a UC2
+V) Snowball World; an ocean world that has its surface covered in ice. Only applies to F3, F4, F5 and F6. The global glaciation allows for airless worlds to be ocean worlds. Europa could be classified as a VF3.
+X) Terraformed; A world that was either a C type or D type world that has been rendered habitable by extensive planetary engineering. If Mars were terraformed, it would become a XE type world. E type worlds can also be terraformed. The Ancient Atlantians extended great resources trying to extend the hydrosphere of Apocalypse by shipping in water from space.
+Y) Heavy Element; A world enriched with elements heavier than iron, and in large concentrations.
+Z) Rouge Planet; Simple enough, a world that is not permanently bound to a star. They could be interstellar planets, or they could be worlds in binary systems that constantly switches orbits from one star to another.
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