--- title: Dry habitable world canonical_url: https://paxabyssi.com/wiki/Dry_habitable_world markdown_url: https://paxabyssi.com/wiki/Dry_habitable_world.md type: wiki-page revision_id: 99 revision_view: stable last_updated: 2026-09-27 license: CC BY-SA 4.0 license_url: https://creativecommons.org/licenses/by-sa/4.0/ science_status: - model - sim summary: A rocky planet with liquid water on its surface but only a little of it, in lakes, brines and polar seas rather than oceans. Climate models suggest such land planets can stay habitable over a wider range of distances than an Earth-like world. categories: - Planets - Planet classes - Terrestrial planets - Habitability aliases: - TDH - T2-D - Land planet - Dry planet - Desert planet - Dune planet - Arid habitable world - Terrestrial dry habitable world infobox: type: planet_class code: TDH name: Dry habitable world image: File:Dry_habitable_world_sim.avif level: type series: Terrestrial with atmosphere (T) subtypes: - TDH-CB Cold brine - TDH-CD Clear desert - TDH-DV Dust-veiled - TDH-WL Warm living - TDH-LW life-bearing variants mass_earth: sim: 0.3 to 2.5 sim_source: Dry habitable physics engine, dry-habitable atmosphere and archetype modules, dry-habitable life module; science set TDH_00 to TDH_20 bond_albedo: sim: 0.20 to 0.55 model: higher than Earth's for bright deserts legacy_code: T2-D water_cover: sim: 0.1 to 18% radius_earth: sim: R = M^0.27, 0.7 to 1.4 last_verified: 2026-09-27 real_examples: - None confirmed - Early Mars and perhaps early Venus as past analogues typical_orbit: Habitable zone and its warm inner margin science_status: - model - sim frequency_in_sim: 180 of 8,742 generated planets (2.1%), in the committed sheets as of 2026-09-27 atmosphere_classes: - arid_young_reducing - arid_reducing - arid_biotic - arid_mature_biotic - arid_co2_thick - arid_thin_clear - arid_dusty - arid_brine habitable_zone_model: "Land planet (Abe et al. 2011, Sun-like star, low obliquity): liquid water from about 77% to 170% of Earth's sunlight, against about 90% to 135% for a water-covered planet in the same model" surface_pressure_bar: sim: 0.3 to 10, nitrogen-rich defining_criteria_sim: Rocky planet with stable surface liquid water, but covering only 0.1 to 18% of the surface literature_equivalent: Land planet (Abe et al. 2011); 'dune' planet surface_temperature_k: sim: about 205 to 350 equilibrium_temperature_k: model: a wider range than for Earth-like planets related: - https://paxabyssi.com/wiki/Ocean_world.md - https://paxabyssi.com/wiki/Mixed_world.md - https://paxabyssi.com/wiki/Arid_world.md - https://paxabyssi.com/wiki/Barren_rock_world.md - https://paxabyssi.com/wiki/Greenhouse_world.md - https://paxabyssi.com/wiki/Ice_world.md --- # Dry habitable world > Source: https://paxabyssi.com/wiki/Dry_habitable_world > > Licence: [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0/). Text by Pax Abyssi Wiki contributors; history at https://paxabyssi.com/wiki/Dry_habitable_world/history > > Revision 99, 27 September 2026 A **dry habitable world** is a rocky planet with liquid water on its surface, but only a little of it: lakes, salty brines and small polar seas rather than oceans. Planetary scientists call such planets **land planets**. They are a natural outcome of planet formation, which delivers very different amounts of water to different planets, and climate models suggest a surprising advantage: with less water, a planet can keep some of it liquid over a wider range of distances from its star than an ocean-covered Earth can. None has been identified; for now the class rests on models. ## Characteristics ### Why less water can mean a wider habitable zone On Earth, water sets both ends of the [Habitable zone](https://paxabyssi.com/wiki/Habitable_zone.md). Near the inner edge, warm oceans put so much water vapour into the air that the planet traps more heat than it can radiate, and runs away into a steam atmosphere. Near the outer edge, snow and sea ice reflect sunlight and cool the planet further, until it freezes over. A land planet sidesteps both. Abe, Abe-Ouchi, Sleep and Zahnle (2011) modelled a planet like Earth but with water only in a few places, and compared it with an "aqua planet" covered by ocean [1]. Three effects mattered: - **The tropics stay dry.** With little water to evaporate, air over the low latitudes is far from saturated, and dry air lets infrared heat escape to space. The planet can absorb more sunlight before a runaway greenhouse begins. - **Less water reaches the upper atmosphere,** so less is broken apart by ultraviolet light, and hydrogen escapes more slowly. The water lasts longer. - **Less snow and ice** form in the cold, because there is less water to make them, so the ice-albedo runaway toward a frozen planet is weaker. In their model, for a Sun-like star and low obliquity, the land planet kept liquid water from about 77 per cent of Earth's present sunlight up to about 170 per cent, while the aqua planet managed only about 90 to 135 per cent [1]. Three-dimensional climate models with different amounts and arrangements of surface water confirm the trend: the less water on the surface, the more sunlight a planet can take before its water runs away [2]. In the most extreme case, a very dry planet with low humidity and a bright surface could keep liquid water much closer to a Sun-like star than Earth does [3]. These limits sit alongside the standard habitable zone for Earth-like planets [4], and they come from models of idealised planets. (Image pending: Two horizontal bars on a scale of sunlight relative to Earth's: a short bar for an ocean-covered planet and a longer one for a land planet, extending further at both ends.) *Figure 1.* Diagram: the range of sunlight over which a land planet and an ocean-covered planet keep liquid water, in the climate model of Abe et al. (2011). ### Where the water is On a land planet the water collects where it is coldest or lowest. With a tilt like Earth's, the equator is a dry desert and the water sits near the poles, in cold seas and around ice caps, or in lakes in low basins. Much of it may be **brine**. Dissolved salts lower the freezing point: salt water with sodium chloride stays liquid down to about 252 K, and with calcium chloride to about 223 K, so a cold dry world can keep liquid pools that pure water could not. On Mars, briny films may form in the soil even today, though only briefly. ### Climate regulation Earth's long-term thermostat, the weathering of rock by rain that draws carbon dioxide out of the air, needs both rain and exposed rock [5]. A land planet has rock in abundance but little rain, so the thermostat works more weakly and more locally, near its lakes and seas. How well a dry world can regulate its carbon dioxide over billions of years is an open question. ## Formation A dry habitable world can be born dry or become dry. **Born dry.** Rocky planets in the habitable zone form from material too warm for water ice, and receive their water later from bodies scattered inward from farther out. How much arrives varies by orders of magnitude between planetary systems in formation simulations [6]. Some planets get oceans; some get only a trace. For planets around red dwarfs, one study found that most Earth-mass planets in the habitable zone should end up either as dry "dune" planets or as deep ocean worlds, with Earth-like water fractions the exception [7]. **Become dry.** A planet can also lose water. Red dwarfs are very bright in their youth, and a planet now in the habitable zone may have spent hundreds of millions of years in a runaway greenhouse, losing much of its water to space as hydrogen escaped [8]. Water can also be drawn into rock by chemical reactions with the crust, and a giant impact can blow part of it away. A planet that ends up with only a few per cent of Earth's water could be a dry habitable world. ## How we know Nothing yet distinguishes a land planet from an ocean world or a desert at interstellar distances. The case for the class comes from climate models of the kind described above, from planet-formation models that predict a wide spread of water contents, and from the Solar System's own history. **Early Mars** had lakes and rivers more than 3.5 billion years ago, as river deltas and lake-bed sediments show, but probably never an Earth-like ocean covering most of the planet, which makes it the nearest example of a land planet with surface water (see [Arid world](https://paxabyssi.com/wiki/Arid_world.md)). ::figure{src="File:Mars_Jezero_delta_PIA23386.jpg" size=wide alt="An orbital view of a crater rim with a fan-shaped sediment deposit spreading from a channel onto the crater floor." caption="Observation: the delta in Jezero crater on Mars, built where a river flowed into a lake more than 3.5 billion years ago. NASA's Perseverance rover has explored the crater since 2021."} **Venus** is another possibility: if it began with less water than Earth, Abe and colleagues suggested, it could have remained a habitable land planet for longer than an ocean-covered Venus would have, possibly until as recently as a billion years ago [1]. Several known planets orbit in or near the habitable zones of red dwarfs, among them TRAPPIST-1 d, e and f [9] and Proxima Centauri b [10]. Their water contents are unknown; they could be dry, wet or airless. Future telescopes that can take a spectrum of reflected light from such a planet might recognise a land planet by its bright, dry surface, a small water signal and little cloud. > **In Pax Abyssi** > > Dry habitable worlds are rocky planets of 0.3 to 2.5 Earth masses with liquid water covering between 0.1 and 18 per cent of their surface, nitrogen-rich air of 0.3 to 10 bar, and mean surface temperatures of roughly 205 to 350 K. A world with more water than that becomes a [mixed world](https://paxabyssi.com/wiki/Mixed_world.md); one whose water freezes or boils away becomes an [arid world](https://paxabyssi.com/wiki/Arid_world.md). The sim first picks a character for the planet and then checks it against the physics: **cold brine** worlds (TDH-CB) whose liquid water survives as salty pools, **clear desert** worlds (TDH-CD) under cloudless skies, and **dust-veiled** worlds (TDH-DV) hazed by windblown dust. Dry worlds are wired to the sim's life system, and those where life has taken hold are coded **warm living** (TDH-WL) or given a life-bearing variant (TDH-LW). Their atmospheres range from young, oxygen-free ones to thick carbon dioxide, clear thin air, dusty skies and briny cold. In the committed system sheets, 180 of 8,742 generated planets are dry habitable worlds, 14 of them with life. In the game each is drawn as a prebaked texture plate chosen by its subtype code. ## See also - [Habitable zone](https://paxabyssi.com/wiki/Habitable_zone.md) - [Mixed world](https://paxabyssi.com/wiki/Mixed_world.md) - [Ocean world](https://paxabyssi.com/wiki/Ocean_world.md) - [Arid world](https://paxabyssi.com/wiki/Arid_world.md) - [Red dwarf habitability](https://paxabyssi.com/wiki/Red_dwarf_habitability.md) - [Life in Pax Abyssi](https://paxabyssi.com/wiki/Life_in_Pax_Abyssi.md) - [Planet classification](https://paxabyssi.com/wiki/Planet_classification.md) ## References 1. Abe, Y. et al. (2011). Habitable Zone Limits for Dry Planets. Astrobiology 11, 443-460. 2. Kodama, T. et al. (2019). Inner Edge of Habitable Zones for Earth-Sized Planets With Various Surface Water Distributions. Journal of Geophysical Research: Planets 124, 2306-2324. 3. Zsom, A. et al. (2013). Toward the Minimum Inner Edge Distance of the Habitable Zone. The Astrophysical Journal 778, 109. 4. Kopparapu, R. K. et al. (2013). Habitable zones around main-sequence stars: new estimates. The Astrophysical Journal 765, 131. 5. Walker, J. C. G., Hays, P. B. and Kasting, J. F. (1981). A negative feedback mechanism for the long-term stabilization of Earth's surface temperature. Journal of Geophysical Research: Oceans 86, 9776-9782. 6. Raymond, S. N., Quinn, T. and Lunine, J. I. (2004). Making other earths: dynamical simulations of terrestrial planet formation and water delivery. Icarus 168, 1-17. 7. Tian, F. and Ida, S. (2015). Water contents of Earth-mass planets around M dwarfs. Nature Geoscience 8, 177-180. 8. Luger, R. and Barnes, R. (2015). Extreme Water Loss and Abiotic O2 Buildup on Planets Throughout the Habitable Zones of M Dwarfs. Astrobiology 15, 119-143. 9. Agol, E. et al. (2021). Refining the Transit-timing and Photometric Analysis of TRAPPIST-1: Masses, Radii, Densities, Dynamics, and Ephemerides. The Planetary Science Journal 2, 1. 10. Faria, J. P. et al. (2022). A candidate short-period sub-Earth orbiting Proxima Centauri. Astronomy & Astrophysics 658, A115. ## Infobox (planet class) | Field | Value | | --- | --- | | Code | TDH | | Name | Dry habitable world | | Image | File:Dry_habitable_world_sim.avif | | Level | type | | Series | Terrestrial with atmosphere (T) | | Subtypes | TDH-CB Cold brine, TDH-CD Clear desert, TDH-DV Dust-veiled, TDH-WL Warm living, TDH-LW life-bearing variants | | Sim source | Dry habitable physics engine, dry-habitable atmosphere and archetype modules, dry-habitable life module; science set TDH_00 to TDH_20 | | Legacy code | T2-D | | Last verified | 2026-09-27 | | Real examples | None confirmed, Early Mars and perhaps early Venus as past analogues | | Typical orbit | Habitable zone and its warm inner margin | | Science status | model, sim | | Frequency in sim | 180 of 8,742 generated planets (2.1%), in the committed sheets as of 2026-09-27 | | Atmosphere classes | arid_young_reducing, arid_reducing, arid_biotic, arid_mature_biotic, arid_co2_thick, arid_thin_clear, arid_dusty, arid_brine | | Habitable zone model | Land planet (Abe et al. 2011, Sun-like star, low obliquity): liquid water from about 77% to 170% of Earth's sunlight, against about 90% to 135% for a water-covered planet in the same model | | Defining criteria sim | Rocky planet with stable surface liquid water, but covering only 0.1 to 18% of the surface | | Literature equivalent | Land planet (Abe et al. 2011); 'dune' planet | ## Related pages - [Ocean world](https://paxabyssi.com/wiki/Ocean_world.md): A planet whose surface is almost entirely water, from Earth-like worlds with a few scattered islands to true water worlds with oceans hundreds of kilometres deep. None is confirmed, but several planets are strong candidates. - [Mixed world](https://paxabyssi.com/wiki/Mixed_world.md): A rocky planet with both continents and oceans, a temperate climate and liquid water at the surface. Earth is the only known example, and the model every search for habitable planets starts from. - [Arid world](https://paxabyssi.com/wiki/Arid_world.md): A rocky planet with a thin, cold atmosphere and no stable liquid water at the surface, only ice and the traces of ancient rivers. Mars is the Solar System's example. - [Barren rock world](https://paxabyssi.com/wiki/Barren_rock_world.md): A rocky planet with no real atmosphere, whose surface lies bare to starlight, cosmic rays and meteorites. Mercury is the Solar System's example, and JWST has found several around other stars. - [Greenhouse world](https://paxabyssi.com/wiki/Greenhouse_world.md): A rocky planet smothered by a thick carbon dioxide atmosphere whose greenhouse effect bakes the surface far above the boiling point of water. Venus is the Solar System's example, a near-twin of Earth in size. - [Ice world](https://paxabyssi.com/wiki/Ice_world.md): A cold world whose surface is made of frozen water, nitrogen, methane and carbon monoxide, like Pluto and Triton, where ices behave like rock and even the atmosphere can freeze out. Categories: [Planets](https://paxabyssi.com/wiki/Category:Planets.md), [Planet classes](https://paxabyssi.com/wiki/Category:Planet_classes.md), [Terrestrial planets](https://paxabyssi.com/wiki/Category:Terrestrial_planets.md), [Habitability](https://paxabyssi.com/wiki/Category:Habitability.md)