Steve Watson

Planetary Categorization

The explosion in exoplanet discoveries has revealed that the Solar System is very far from a typical stellar system, and a need has arisen for a categorization of planetary types that includes far more than the kinds of planets found orbiting the Sun. To form these classification appeal has been made to a range of characteristics that are at least observable. The principal such characteristics are

• r = Radius (r_E)
• m = Mass (m_E)
• D = Density (x H₂O)
• T = Temperature (eq. to 1AU from Sun) – referring to the irradiation of the planet
• a = Atmospheric composition (n)
  • n = 0: No atmosphere (P < 0.001b,)
  • else: n = mean molecular mass of atmospheric gases.

Together with a few other observables these can be used to produce the following table of categorizations.

• Common            
  • Orbital focus
    • Rogue                   Not orbiting a star
    • Circumbinary     Orbiting a binary star
    • Circumtriple        Orbiting a triple star
  • Orbital character
    • Eccentric              Highly eccentric orbit
    • Double                  Two planets orbiting each other
    • Trojan                   Co-orbiting with another planet
• Solid                                           r < 1.7                    m < 6                                   D ≈ 5
  • Structure
    • Coreless               No metallic core, thus essentially all mantle
  • Composition
    • Carbon                 Iron core w/ carbon-based mantle
    • Iron                        Iron core w/ minimal mantle
    • Ocean (1)              Significant hydrosphere around core/mantle       
    • Silicate                  Iron core w/ silicone-based mantle
  • History
    • Chthonian           Close to star. Ex-fluid with envelope stripped away
  • Surface
    • Desert                   Dry desert (Arid, Dune)
    • Hycean                 Ocean w/ Hydrogen envelope
    • Ice                          Frozen volatiles (eg. water, ammonia, methane)
    • Lava                      Lava
    • Ocean (2)             Liquid (usually H₂O.) Oceans may be sub-surface
  • Habitability
    • Goldilocks            T ≈ 1
  • Size
    • Sub-Earth            r < 0.8                   m < 0.5
    • Earth(-sized)       0.8 < r < 1.7        0.5 < m < 6
    • Super-Earth                                         6 < m < 10
    • Mega-Earth                                        10 < m
• Fluid                                           1.7 < r,                   6 < m                                  D << 5  
  • Irradiation
    • Hot–                       T >> 1
    • Cold-                      T << 1
  • Size
    • Super-Earth (Gas Dwarf)               6 < m < 10
    • Mini-Neptune     1.7 < r < 3.9           6 < m < 20 (sic) 
    • Sub-Neptune       1.7 < r < 3              m < 10
    • Neptunian            3 < r < 5                10 < m < 20
    • Super-Puff           4 < r                       m < 4                                  D < 0.8
    • Super-Neptune    5 < r < 7                20 < m < 80
    • Jovian                   7 < r < 10              80 < m < 400
    • Puff planet                                          80 < m < 400                    D < 0.8
    • Super-Jupiter     10 < r                     400 < m < 4000
  • Composition
    • Gas Giant            0 < a < 4
    • Helium                  a ≈ 4
    • Ice Giant              4 < a