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Planet class · TDH · T2-D

Dry habitable world

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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. 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.

Figure 1Diagram: 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).

::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.

See also

References

  1. 1Abe, Y. et al. (2011). Habitable Zone Limits for Dry Planets. Astrobiology 11, 443-460. doi:10.1089/ast.2010.0545
  2. 2Kodama, 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. doi:10.1029/2019je006037
  3. 3Zsom, A. et al. (2013). Toward the Minimum Inner Edge Distance of the Habitable Zone. The Astrophysical Journal 778, 109. doi:10.1088/0004-637x/778/2/109
  4. 4Kopparapu, R. K. et al. (2013). Habitable zones around main-sequence stars: new estimates. The Astrophysical Journal 765, 131. doi:10.1088/0004-637x/765/2/131
  5. 5Walker, 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. doi:10.1029/jc086ic10p09776
  6. 6Raymond, 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. doi:10.1016/j.icarus.2003.11.019
  7. 7Tian, F. and Ida, S. (2015). Water contents of Earth-mass planets around M dwarfs. Nature Geoscience 8, 177-180. doi:10.1038/ngeo2372
  8. 8Luger, 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. doi:10.1089/ast.2014.1231
  9. 9Agol, 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. doi:10.3847/psj/abd022
  10. 10Faria, J. P. et al. (2022). A candidate short-period sub-Earth orbiting Proxima Centauri. Astronomy & Astrophysics 658, A115. doi:10.1051/0004-6361/202142337