Colony and Habitat Viability Indices

The Colony Viability Index (CVI) and the Habitat Viability Index (HVI) are two scores, each from 0 to 100, that the Pax Abyssi simulation computes for every planet it generates. The CVI asks whether people could live on the surface in the open: breathe the air, drink the water, grow food and walk outside without a suit. The HVI asks a more modest question: whether people could live there permanently inside pressurised habitats, bringing their own air, because there is some reason to be there. A world can fail the first test and pass the second. Mars does exactly that. Both indices are the game's own design, built from published limits of human physiology; they are not an astronomical standard, and no telescope measures them.

Why the game needs its own scores

Astronomy already has habitability indices, but they answer different questions. The Earth Similarity Index (ESI) compares a planet's radius, density, escape velocity and surface temperature with Earth's and combines the ratios in a weighted geometric mean, giving 1 for Earth itself ^[1]. It measures resemblance, which is useful for sorting telescope targets, but a planet the size of Earth at Earth's temperature scores highly even if its air is pure carbon dioxide. The Statistical-likelihood Exo-Planetary Habitability Index (SEPHI) estimates, from a few observable quantities such as mass, radius and orbit, how likely a planet is to be habitable for life like Earth's ^[2]. Neither says whether a person could stand on the surface and breathe.

The CVI and HVI score the things a settler would care about, one dimension at a time, and show every sub-score as well as the total, so a player can see why a world scores the way it does.

How the Colony Viability Index works

The CVI adds up points from seven dimensions. Three of them are survival barriers and can cap the whole score; four are quality-of-life penalties that only subtract their own points.

Within breathability the weakest sub-score decides: perfect oxygen does not help if hydrogen sulphide is at a lethal level. Each hard dimension then sets a ceiling equal to its own percentage. A world scoring 5 of 12 on radiation, 42%, cannot score above 42 overall, however good its soil. In symbols, with $s_k$ the score of hard dimension $k$ out of its maximum $m_k$ and $C_\mathrm{raw}$ the sum of all seven,

$$\mathrm{CVI} = \min\left(C_\mathrm{raw},\ \max\left(100\,\min_k \frac{s_k}{m_k},\ 0.15\,C_\mathrm{raw}\right)\right).$$

The last term is a floor the game adds on purpose: even an unbreathable world keeps 15% of its raw score, so a Mars-like planet with good ground and accessible ice reads as "you need a suit, but there is something here" rather than a flat zero.

Image pendingFile:CVI_ceiling_diagram.svgSeven horizontal bars for the CVI dimensions of a Mars-like world; the breathability bar is empty and a line shows the capped score far below the raw sum.

Where the thresholds come from

The numbers inside each dimension are taken from human physiology and occupational safety, not invented for the game.

• Pressure. Below about 6.3 kilopascals, one sixteenth of Earth's sea-level pressure, water boils at body temperature. This is the Armstrong limit, and the CVI awards no pressure points below it.
• Oxygen. Sea-level air supplies about 21 kPa of oxygen. The index gives full marks for 19 to 23 kPa and none below 8 kPa, where people lose consciousness, or above 50 kPa, where oxygen itself becomes toxic over time.
• Toxic gases. Carbon dioxide, hydrogen sulphide, sulphur dioxide, carbon monoxide and ammonia are scored against the concentrations that the US National Institute for Occupational Safety and Health rates as immediately dangerous to life or health: 40,000 parts per million for carbon dioxide, 100 for hydrogen sulphide and sulphur dioxide, 1,200 for carbon monoxide, 300 for ammonia ^[3].
• Heat. What kills in heat is the wet-bulb temperature, the temperature a wet thermometer reads, because it sets whether sweat can cool the body. The theoretical limit is 35 °C ^[4], but in laboratory trials young, healthy volunteers could no longer hold their core temperature steady at wet-bulb temperatures of about 30 to 31 °C in warm, humid air ^[5]. The CVI uses 31 °C as its lethal line.
• Radiation. Earth's atmosphere and magnetic field keep surface doses low. On the surface of Mars the Curiosity rover measured about 0.64 millisieverts a day, some 230 millisieverts a year ^[6].
• Day length. The human body clock runs on a cycle of about 24.2 hours and can be pulled only a little way from it ^[7], so very long or very short days count against a world.
• Light for crops. Photosynthesis uses light between 400 and 700 nanometres. A cool red dwarf puts only a small fraction of its output there ^[8], so a planet in the habitable zone of one of the coolest M dwarfs receives far less of this light than Earth's surface does, and photosynthetic productivity falls with it ^[9]. The agriculture dimension reflects that.

Gravity is the least certain dimension. Months in weightlessness cost astronauts muscle and bone ^[10], but nobody has lived for years at the Moon's 0.17 g or Mars's 0.38 g, still less at twice Earth's gravity, so the index's gravity bands are informed guesses.

How the Habitat Viability Index works

The HVI assumes people live indoors and bring their own air, so breathability drops out and other questions take over.

Gravity moves to the top because it is the one thing a habitat cannot engineer around. Tiers run Excellent (80 and above), Good (60 to 79), Marginal (40 to 59), Outpost only (20 to 39) and Non-viable (below 20).

The difference between the indices shows up plainly in the game's data. Several generated Earth-like worlds with a surface gravity of about 1.66 g score CVI 85 to 90, because heavy gravity is only a soft penalty when the question is whether people can walk outside, yet their HVI stops at 47, because for a permanent settlement the same gravity is a hard ceiling.

The derived scores

Most rocky worlds have no free oxygen, so their CVI is capped at a few points and says little else. Three further scores fill the gap:

• Adjusted CVI reruns the CVI with Earth's oxygen partial pressure and ozone supplied and everything else unchanged. A world with CVI 8 and Adjusted CVI 85 is a good planet behind one barrier; one with Adjusted CVI 12 has other problems.
• Photosynthetic Oxygenation Potential (POP, 0 to 100) asks whether introduced photosynthetic microbes could thrive and slowly oxygenate the air, weighing usable light, surface water, temperature, carbon dioxide, ultraviolet and whether the planet can hold on to its gases.
• Terraforming Suitability Index (TSI, 0 to 100) asks whether deliberate engineering of the atmosphere would last, weighing gravity and magnetic field, the volatile inventory, geological activity and how far the temperature is from comfortable.

Notable examples

These are the values the game carries for the Solar System's planets, where the inputs are known best.

The highest-scoring generated worlds in the 2026-09-27 data are Earth-like planets, for example the third planet of Eta Circini (catalogue HYG 73548) at CVI 85.2 and HVI 78.1.

See also

• Habitable zone
• Mixed world, the Earth-like class that holds almost all high-CVI worlds
• Arid world, the Mars-like class that shows the CVI and HVI diverging
• Subsurface ocean world
• Red dwarf
• Star system generation
• Science in Pax Abyssi

References

1. 1Schulze-Makuch, D. et al. (2011). A Two-Tiered Approach to Assessing the Habitability of Exoplanets. Astrobiology 11, 1041-1052. doi:10.1089/ast.2010.0592
2. 2Rodríguez-Mozos, J. M. and Moya, A. (2017). Statistical-likelihood Exo-Planetary Habitability Index (SEPHI). Monthly Notices of the Royal Astronomical Society 471, 4628-4636. doi:10.1093/mnras/stx1910
3. 3National Institute for Occupational Safety and Health. Immediately Dangerous to Life or Health (IDLH) Values. Centers for Disease Control and Prevention. www.cdc.gov/niosh/idlh/
4. 4Sherwood, S. C. and Huber, M. (2010). An adaptability limit to climate change due to heat stress. Proceedings of the National Academy of Sciences 107, 9552-9555. doi:10.1073/pnas.0913352107
5. 5Vecellio, D. J. et al. (2022). Evaluating the 35 °C wet-bulb temperature adaptability threshold for young, healthy subjects (PSU HEAT Project). Journal of Applied Physiology 132, 340-345. doi:10.1152/japplphysiol.00738.2021
6. 6Hassler, D. M. et al. (2014). Mars' Surface Radiation Environment Measured with the Mars Science Laboratory's Curiosity Rover. Science 343, 1244797. doi:10.1126/science.1244797
7. 7Czeisler, C. A. et al. (1999). Stability, Precision, and Near-24-Hour Period of the Human Circadian Pacemaker. Science 284, 2177-2181. doi:10.1126/science.284.5423.2177
8. 8Covone, G. et al. (2021). Efficiency of the oxygenic photosynthesis on Earth-like planets in the habitable zone. Monthly Notices of the Royal Astronomical Society 505, 3329-3335. doi:10.1093/mnras/stab1357
9. 9Lehmer, O. R. et al. (2018). The Productivity of Oxygenic Photosynthesis around Cool, M Dwarf Stars. The Astrophysical Journal 859, 171. doi:10.3847/1538-4357/aac104
10. 10Juhl, O. J. et al. (2021). Update on the effects of microgravity on the musculoskeletal system. npj Microgravity 7, 28. doi:10.1038/s41526-021-00158-4
11. 11Heller, R. and Armstrong, J. (2014). Superhabitable Worlds. Astrobiology 14, 50-66. doi:10.1089/ast.2013.1088