Steve Watson

Encounter Classifications for Terran Worlds

1. Size
   1. S              Small                           r < 0.8r_E,                  m < 0.5m_E                g[1] < 0.8
   2. M             Medium               8r_E < r < 1.7r_E       0.5m_E < m < 6m_E         0.8 < g < 2.0
   3. L              Large                                                  6m_E < m < 10m_E          0 < g < 2.2
   4. G              Giant                                                10m_E < m                      2 < g
2. Dominant Hard Surface Type
   1. V              Vulcanic              Dominant surface is due to vulcanism
   2. R              Rocky                 Heavy minerals, silicates, carbonates, varieties of regolith
   3. I               Icy                     Frozen volatiles (eg. water, ammonia, methane)
3. Persistent Surface Liquid Types
   1. N              Anhygric             None – indicating liquid cannot persist on the surface
   2. W              Hydrohygric        Water
   3. C              Anthracohygric    Methane, ethane, etc
   4. A              Allohygric            Other
4. Surface Liquid Coverage
   1. X              Xeric                  No surface liquid
   2. D              Drosic                Insignificant or transient liquidity
   3. L              Limnic                Significant but not extensive liquid coverage
   4. T              Thalassic            Extensive but not total liquid coverage
   5. O              Oceanic              Total liquid cover.
5. Effective Surface Radiation (R, mSv per annum)
   1. M              Radiominimal     R < 0.1
   2. S              Radionormal        0.1 < R < 10         1 is the ICRP recommended max
   3. C              Radiocritical        10 < R < 1000   
   4. T              Radiotoxic           1000 < R
6. Surface Temperature (T, ^oC)
   1. 1              Super-Cold          -273 < T < -100
   2. 2              Cold                    -100 < T < 4
   3. 3              Temperate          4 < T < 40            Approximate human habitable zone
   4. 4              Hot                     40 < T < 100
   5. 5              Super-Hot           100 < T
7. Surface Atmospheric Pressure (P, b)
   1. 1              Microbaric           P < 0.001b
   2. 2              Hypobaric           001b < P < 0.3b
   3. 3              Mesobaric           0.3b < P < 3b          Approximate human habitable zone
   4. 4              Hyperbaric          3b < P < 100b
   5. 5              Megabaric           100b < P
8. Surface Atmospheric Composition (by dominant[2] component)
   1. A              Airless                Microbaric worlds with an exosphere
   2. P              Primordial air      H₂, He₂
   3. C              Compound air     Common compound: CO₂, CH₄, NH₃, H₂O
   4. N              Nitrogen air        N₂ 
   5. O[3]         Oxygen air          Breathable levels of O₂
   6. L              Complex air        No dominant component
   7. X              Exotic air                             
9. Biocomplexity[4]^{, [5],[6]} (K, 10^Kbytes)
   1. 0              Abiotic                K = 0
   2. 1              Protobiotic[7]      4 < K < 5              
   3. 2              Deuterobiotic      5 < K < 6               5 = approximate minimal level for a cell
   4. 3              Triobiotic             6 < K < 7               Coli
   5. 4              Tetrobiotic           7 < K < 8               Fungus, Fruit fly
   6. 5              Pentobiotic          8 < K < 9               Mouse, Human
   7. 6              Hexobiotic           9 < K                      Pine
10. Biodensity[8] (D, Gt/m²)
    1. 0              Nonvital              D = 0
    2. 1              Rarivital              0 < D < 0.001
    3. 3              Paulivital             0.001 < D < 0.1
    4. 4              Plenivital             0.1 < D < 10             Earth = 1
    5. 5              Supervital           10 < D

[1] Rough estimates only for values of surface gravity (g, g_E) based on r and m.
[2] Dominant means > 75%
[3] This classification takes priority over any other applicable classification
[4] Kolmogorov system complexity for biological organisms. Note that such organisms are distinguishable at any level of K by their reciprocal entropy. See C. Mayer (2020) Life in The Context of Order and Complexity – PMC. They may alternatively be distinguished as just the complex systems historically evolutionarily responsive to environmental pressures.
[5] The biocomplexity of a world is marked as the complexity of the highest scoring biological organism on the world.
[6] The examples use the genome size of terrestrial organisms (rather than genes identified) to calculate their complexity. The genome is a first approximation only to a proxy for complexity.
[7] Since the scale only applies to biological organisms, 0 is assigned to all non-biological entities. It is assumed, on the basis of plausibility and the evidence of terrestrial life, that complexity below 4 is not possible for biological organisms.
[8] Total biomass / surface area of world. A rough measure of the degree to which life has occupied the world. See Bar-On, Yinon M.; Philips, Rob; Milo, Ron (2017). “The biomass distribution on Earth”. Proceedings of the National Academy of Sciences. 115 (25): 1. Note that the current definition references only the mass of carbon contained in living things. This definition may or may not be adequate in considering alien ecologies. Note also the comment in the Abstract to the referenced article:

We find that the kingdoms of life concentrate at different locations on the planet; plants (≈450 Gt C, the dominant kingdom) are primarily terrestrial, whereas animals (≈2 Gt C) are mainly marine, and bacteria (≈70 Gt C) and archaea (≈7 Gt C) are predominantly located in deep subsurface environments. We show that terrestrial biomass is about two orders of magnitude higher than marine biomass and estimate a total of ≈6 Gt C of marine biota, doubling the previous estimated quantity.