--- title: Colony and Habitat Viability Indices summary: Two 0 to 100 scores the simulation gives every planet, one for whether people could live on its surface unprotected, one for whether they could live there permanently in pressurised habitats. science_status: [sim] categories: [Habitability, Life and habitability, Game mechanics, Indices and scores] aliases: [CVI, HVI, Colony Viability Index, Habitat Viability Index, Adjusted CVI, Photosynthetic Oxygenation Potential, POP, Terraforming Suitability Index, TSI, Colonisation Suitability Index] infobox: type: index name: Colony Viability Index and Habitat Viability Index abbreviations: [CVI, HVI] scale: "0 to 100 each" questions: CVI: "Could people live on the surface without suits, breathing the air, drinking the water and growing food?" HVI: "Could people live here permanently in pressurised habitats, bringing their own air?" cvi_dimensions: "breathability 20 (hard ceiling); temperature and climate 15 (hard ceiling below 5 points); radiation 12 (hard ceiling); water 15; agriculture 15; gravity 12; geophysical and day-length stability 11" hvi_dimensions: "gravity 20 (hard ceiling); radiation and shielding 15 (hard ceiling); resources and in-situ manufacturing 15; water 15; energy 12; temperature and thermal management 12; atmospheric utility 11" cvi_tiers: "Exceptional 85 to 100; High 70 to 84; Moderate 50 to 69; Low 30 to 49; Very low 10 to 29; Non-viable 0 to 9" hvi_tiers: "Excellent 80 to 100; Good 60 to 79; Marginal 40 to 59; Outpost only 20 to 39; Non-viable 0 to 19" derived_scores: "Adjusted CVI (the CVI with Earth's oxygen supplied), Photosynthetic Oxygenation Potential (POP), Terraforming Suitability Index (TSI)" literature_relatives: "Earth Similarity Index (Schulze-Makuch et al. 2011); SEPHI (Rodríguez-Mozos and Moya 2017)" science_status: "SIM: a Pax Abyssi design, built on published human-physiology limits; not an astronomical standard" sim_source: "colony assessment module (CVI and HVI calculators, 25 sub-variable scorers)" live_counts_2026_09_27: "8,751 planets in 5,160 generated systems: 13 score CVI 70 or more; 2,016 score HVI 60 or more; moons not yet scored" sim: entity: index.cvi_hvi refs: - id: schulzemakuch2011 type: article-journal author: [{family: Schulze-Makuch, given: Dirk}, {family: Méndez, given: Abel}, {family: Fairén, given: Alberto G.}, {literal: "et al."}] title: "A Two-Tiered Approach to Assessing the Habitability of Exoplanets" container-title: Astrobiology issued: 2011 volume: 11 page: "1041-1052" DOI: 10.1089/ast.2010.0592 - id: rodriguezmozos2017 type: article-journal author: [{family: Rodríguez-Mozos, given: J. M.}, {family: Moya, given: A.}] title: "Statistical-likelihood Exo-Planetary Habitability Index (SEPHI)" container-title: Monthly Notices of the Royal Astronomical Society issued: 2017 volume: 471 page: "4628-4636" DOI: 10.1093/mnras/stx1910 - id: niosh_idlh type: webpage author: [{literal: "National Institute for Occupational Safety and Health"}] title: "Immediately Dangerous to Life or Health (IDLH) Values" container-title: Centers for Disease Control and Prevention URL: https://www.cdc.gov/niosh/idlh/ accessed: 2026-09-27 - id: sherwood2010 type: article-journal author: [{family: Sherwood, given: Steven C.}, {family: Huber, given: Matthew}] title: "An adaptability limit to climate change due to heat stress" container-title: Proceedings of the National Academy of Sciences issued: 2010 volume: 107 page: "9552-9555" DOI: 10.1073/pnas.0913352107 - id: vecellio2022 type: article-journal author: [{family: Vecellio, given: Daniel J.}, {family: Wolf, given: S. Tony}, {family: Cottle, given: Rachel M.}, {family: Kenney, given: W. Larry}] title: "Evaluating the 35 °C wet-bulb temperature adaptability threshold for young, healthy subjects (PSU HEAT Project)" container-title: Journal of Applied Physiology issued: 2022 volume: 132 page: "340-345" DOI: 10.1152/japplphysiol.00738.2021 - id: hassler2014 type: article-journal author: [{family: Hassler, given: Donald M.}, {family: Zeitlin, given: Cary}, {family: Wimmer-Schweingruber, given: Robert F.}, {literal: "et al."}] title: "Mars' Surface Radiation Environment Measured with the Mars Science Laboratory's Curiosity Rover" container-title: Science issued: 2014 volume: 343 page: "1244797" DOI: 10.1126/science.1244797 - id: czeisler1999 type: article-journal author: [{family: Czeisler, given: Charles A.}, {family: Duffy, given: Jeanne F.}, {family: Shanahan, given: Theresa L.}, {literal: "et al."}] title: "Stability, Precision, and Near-24-Hour Period of the Human Circadian Pacemaker" container-title: Science issued: 1999 volume: 284 page: "2177-2181" DOI: 10.1126/science.284.5423.2177 - id: lehmer2018 type: article-journal author: [{family: Lehmer, given: Owen R.}, {family: Catling, given: David C.}, {family: Parenteau, given: Mary N.}, {family: Hoehler, given: Tori M.}] title: "The Productivity of Oxygenic Photosynthesis around Cool, M Dwarf Stars" container-title: The Astrophysical Journal issued: 2018 volume: 859 page: "171" DOI: 10.3847/1538-4357/aac104 - id: covone2021 type: article-journal author: [{family: Covone, given: Giovanni}, {family: Ienco, given: Riccardo M.}, {family: Cacciapuoti, given: Luca}, {family: Inno, given: Laura}] title: "Efficiency of the oxygenic photosynthesis on Earth-like planets in the habitable zone" container-title: Monthly Notices of the Royal Astronomical Society issued: 2021 volume: 505 page: "3329-3335" DOI: 10.1093/mnras/stab1357 - id: juhl2021 type: article-journal author: [{family: Juhl, given: Otto J.}, {family: Buettmann, given: Evan G.}, {family: Friedman, given: Michael A.}, {literal: "et al."}] title: "Update on the effects of microgravity on the musculoskeletal system" container-title: npj Microgravity issued: 2021 volume: 7 page: "28" DOI: 10.1038/s41526-021-00158-4 - id: heller2014 type: article-journal author: [{family: Heller, given: René}, {family: Armstrong, given: John}] title: "Superhabitable Worlds" container-title: Astrobiology issued: 2014 volume: 14 page: "50-66" DOI: 10.1089/ast.2013.1088 images_wanted: - file: File:CVI_HVI_planet_panel.avif subject: "The galaxy map's planet panel for a generated Earth-like world, with the ESI, CVI and HVI readouts in their tier colours" source: sim note: "shot list: capture from the galaxy map (planet panel) on a world with CVI above 70, for example HYG 73548 III in the Eta Circini system (CVI 85.2, HVI 78.1 in the 2026-09-27 data)" - file: File:CVI_ceiling_diagram.svg subject: "Diagram: seven horizontal bars (the CVI dimensions, lengths proportional to 20, 15, 12, 15, 15, 12, 11 points) for a Mars-like world, the breathability bar empty and marked as a hard ceiling, a dashed line showing the raw sum and a solid line showing the capped score" source: other note: "shot list: diagram, CC BY-SA by Pax Abyssi" --- 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 :cite[schulzemakuch2011]. 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 :cite[rodriguezmozos2017]. 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. | Dimension | Points | Type | What it looks at | |---|---|---|---| | Breathability | 20 | hard ceiling | oxygen partial pressure, total pressure, carbon dioxide, toxic gases, ozone | | Temperature and climate | 15 | hard ceiling below 5 points | mean temperature, peak wet-bulb temperature, daily and seasonal swings | | Radiation | 12 | hard ceiling | surface dose, magnetic field, ozone | | Water | 15 | soft | fresh water, rainfall, ocean cover | | Agriculture | 15 | soft | usable sunlight, growing season, soil, arable land | | Gravity | 12 | soft | surface gravity, day length against the human body clock | | Geophysical stability | 11 | soft | tectonics, carbon cycling, axial tilt, orbital eccentricity | 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. | CVI | Tier | Meaning | |---|---|---| | 85 to 100 | Exceptional | Shirtsleeve world: drink from the rivers, farm the land | | 70 to 84 | High | Liveable with minor adaptation, such as crop selection or water treatment | | 50 to 69 | Moderate | Surface living with serious infrastructure | | 30 to 49 | Low | One or two serious limitations; suits or domes for some activities | | 10 to 29 | Very low | Capped by a survival barrier; suited work only | | 0 to 9 | Non-viable | Several lethal dimensions | ::figure{src="File:CVI_ceiling_diagram.svg" size=wide alt="Seven 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." caption="Diagram: how a hard ceiling works. A Mars-like world earns points for water, ground and gravity, but its empty breathability bar caps the whole score."} ### 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 :cite[niosh_idlh]. - **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 :cite[sherwood2010], 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 :cite[vecellio2022]. 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 :cite[hassler2014]. - **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 :cite[czeisler1999], 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 :cite[covone2021], 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 :cite[lehmer2018]. The agriculture dimension reflects that. Gravity is the least certain dimension. Months in weightlessness cost astronauts muscle and bone :cite[juhl2021], 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. | Dimension | Points | Type | What it looks at | |---|---|---|---| | Gravity | 20 | hard ceiling | long-term health in the local gravity | | Radiation and shielding | 15 | hard ceiling | surface dose and how much burial or water shielding would cost | | Resources and manufacturing | 15 | soft | minerals, usable regolith, feedstock for making things locally | | Water | 15 | soft | ice, groundwater or rain for life support | | Energy | 12 | soft | sunlight, geothermal heat, nuclear fuel in the crust | | Temperature and thermal management | 12 | soft | the cost of heating or cooling a habitat | | Atmospheric utility | 11 | soft | whether the air is a resource (carbon dioxide to split for oxygen, nitrogen for buffer gas) or a hazard (acid) | 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. | World | CVI | HVI | Reading | |---|---|---|---| | Earth | 95 | 95 | The reference case | | Mars | 5 | 55 | Unbreathable air and a surface dose of about 230 mSv a year, but water ice, carbon dioxide to split for oxygen, and 0.38 g: a Marginal habitat site | | Mercury | 0 | 15 | No atmosphere, extreme temperature swings | | Venus | 0 | 3 | 90 bar of hot carbon dioxide under sulphuric acid clouds | 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. :::callout{type=science title="What these scores cannot know"} Every input to the CVI is something a physics model can estimate from a planet's size, orbit, star and atmosphere. Much of what would decide a real settlement is beyond that: whether a native biosphere is harmless, edible or hostile; what trace chemistry the soil holds; how human bodies and pregnancies fare over generations at 0.4 or 1.6 g. The indices rank worlds by the risks we can model. A real survey would need people on the ground. Ideas such as the "superhabitable" planet, a world more suitable for life than Earth :cite[heller2014], concern life in general and do not map onto either index. ::: :::callout{type=sim title="In Pax Abyssi"} Every generated planet carries a CVI and an HVI, shown with the Earth Similarity Index on the galaxy map's planet panel in tier colours. As of 2026-09-27 the game's 5,160 generated systems hold 8,751 planets: 13 score a CVI of 70 or more, and 2,016 score an HVI of 60 or more. The rarity of shirtsleeve worlds is intended. Moons are not yet scored and read 0 on both indices. The Solar System's values are set by hand from the known planets rather than computed. Neither index is an observation, and a high score means "scored as a strong candidate for settlement" in the simulation's model, nothing more. ::: ## 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]]