Planetary Classification

Facts

• Planetary classification system established by mid-23rd century
• Descends from similar classification systems established by Vulcan Science Directorate, UESPA (ENT, TOS)

Classifications

Class A

• Geothermal rocky body
• Young world not yet solidified
• Insufficiently large to maintain atmosphere, heat following solidification
• Primarily hydrogen-based atmosphere
• Eventually cools to class C

Class B

• Geomorteus rocky body
• Orbits close to primary
• Surface temperature high; portions of surface potentially molten
• Little to no atmosphere due to heat and solar ablation

Class C

• Geoinactive rocky body
• Transition from Class A
• Geological processes largely halted
• Atmosphere cooled, potentially frozen

Class D

• Airless body, often an asteroid or moon

Class E

• Geoplastic rocky body
• Young world not yet solidified
• Essentially a large class A
• Transitions to class F

Class F

• Geometallic rocky body
• Transition from class E
• Surface partially solidified, geological and geothermal activity still high
• Largely uninhabitable by humanoid life
• Transitions to class G

Class G

• Geocrystalline rocky body
• Transition from class F
• Surface essentially solidified
• Temperatures cooling, potentially survivable range
• Thick atmosphere
  • Often low oxygen content due to oxygen binding in crust
• Potentially habitable with technological aid
• Transition to class H, K-P

Class H

• Rocky body
• Very arid world
  • High surface temperature
• Little to no surface water
• Atmosphere often saturated with heavy gasses or metallic vapors
• Generally uninhabitable to humanoid life

Class I

• Ice giant
• Atmospheric envelope dominated by ammonia, water, methane
  • Hydrogen levels very low (20% or lower) as compared to class J planets

Class J

• Cold gas giant
  • Divided into three subtypes by temperature, predominant cloud formation
    • Type 1: 150K or lower, dominated by ammonia clouds
    • Type 2: 150K-250K, dominated by water vapor clouds
    • Type 3: 250K-800K, cloudless
• Known as Fesoan-class by Andorians (ENT Novel: Live by the Code)

Class K

• Adaptable
• Generally thin atmopshere
• Little to no surface water
• Surface conditions suitable for life with aid of pressure domes or life support technology (TOS: "I, Mudd")
  • Often pre-terraforming state

Class L

• Marginal
• Transitioned from class G
• Barely habitable or habitable with minimal technological aid
  • Often pre-terraforming state
• Native life often limited to flora
  • Possibly in process of transitioning to class M

Class M

• Terrestrial
• Previously known as Minshara-class by Vulcan Science Directorate, Earth-type by UESPA (ENT, TOS)
• Significant surface water, below 80% coverage
• Atmosphere consisting of nitrogen, oxygen, trace elements
• Habitable by humanoid life
• Subdivided into type 1-4, indicating most to least suitable for humanoid life (TNG: "The Bonding")

Class N

• Reducing
• Thick atmosphere, often saturated with sulfur compounds
• High surface temperature due to atmospheric greenhouse effect
• Generally uninhabitable for humanoid life

Class O

• Pelagic
• Largely identical in composition and habitability to class M
• Liquid water covers 80+% of surface
• Life generally only present if sufficient landmasses to allow for mineral circulation
• Common alternative to class M planets in systems with gravitational disruption of protoplanetary disk leading to increased cometary bombardment (TNG Novel: The Buried Age)
  • Examples include distant stellar companions, multiple inward-migrating Jovians (TNG Novel: The Buried Age)

Class P

• Glaciated
• Largely identical in composition to class M
• Surface temperatures below freezing
• Generally covered in water ice
• Often transitioning from prior class due to changes in available surface heat
• Scientific study often based from subglacial bases constructed on rocky core beneath planetary ocean/slush mantle, ice crust (TNG Novel: The Buried Age)

Class Q

• Variable
• Previously known as Quaris-class by Vulcan Science Directorate
• Unpredictable surface and environmental conditions
  • Generally wide variations from region to region
• Often caused by variable star output, highly eccentric orbit

Class R

• Rogue
• Generally uninhabitable
  • Geothermal heat can sometimes maintain ecosystem on surface

Class S

• Gas dwarf
• Generally protoplanetary stage progressing towards class I

Class T

• Hot gas giant
  • Divided into two subtypes by temperature, predominant cloud formation
    • Type 4: 900K-1400K, dominated by alkali clouds
    • Type 5: 1400+K, dominated by silicate clouds

Class X

• Miscellaneous classification
• Any planetary status not falling under standard natural planetary development
• Unsubstantiated

Class Y

• Terrestrial
• Colloquially known as "demon"-class (VOY: "Demon")
• Extremely high surface temperatures
• Corrosive and/or toxic atmosphere
• Thermionic discharges from surface-level metallic hydrogen deposits common (VOY: "Demon")
• Planetary conditions can lead to vessel hazards even at standard orbital altitude (VOY: "Demon")

Notes

• Star Charts classifications flawed w.r.t. giant planets
  • Radii are physically impossible
    • Jupiter diameter ~140,000km, maximum theoretical possible planetary diameter ~400,000km, but class T planets go up to 120m km?
    • Adding more mass generally tends to increase density, not radius; proximity to sun increases size but only so far
  • No acknowledgement of ice giants (not widespread distinction yet at time of writing) despite physically distinct characteristics
    • All Star charts giant classes are predominantly hydrogen-based, ice giants (i.e. Neptune, Uranus) are water/ammonia-based with very slim hydrogen envelopes
  • All gas giants listed as being in "cold zone" beyond ecosphere, which doesn't fit later discoveries
    • Written before discovery of numerous hot Jupiters by various exoplanet surveys
• Alteration:
  • Class I ice giant
  • Class J cold gas giant
    • Type 1 through 3 using Sudarsky's schema
  • Class S gas dwarf
    • Theoretical protoplanet state leading to ice giants, potentially also gas giant that has lost atmosphere to solar wind ablation
  • Class T hot gas giant (fka hot Jupiter)
    • Type 4 through 5 using Sudarsky's schema

(Star Charts unless otherwise referenced)