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 D

• Terrestrial
• Airless body, often an asteroid or moon

Class H

• Terrestrial
• Arid world
• Atmosphere often saturated with heavy gasses or metallic vapors
• Little to no surface water
• Generally uninhabitable to humanoid life

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

Class K

• Terrestrial
• Adaptable world
• 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

• Terrestrial
• Barely habitable or habitable with minimal technological aid
  • Often pre-terraforming state

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

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

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 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)