Inner Edge of Habitable Zones for Earth‐Sized Planets With Various Surface Water Distributions

• Published in: Journal of geophysical research. Planets 2019-08, Vol.124 (8), p.2306-2324
• Main Authors: Kodama, T., Genda, H., O'ishi, R., Abe‐Ouchi, A., Abe, Y.
• Format: Article
• Language: English
• Subjects: Astrophysics; Atmosphere; Atmospheric circulation; Atmospheric circulation models; Atmospheric models; Circumstellar habitable zone; Earth; Earth and Planetary Astrophysics; Exoplanet; Extrasolar planets; GCM; Habitable planet; Insolation; Land planet; Landforms; Liquid surfaces; Luminosity; Mars; Mars surface; Moisture content; Oceans; Physics; Planet detection; Planetary surfaces; Planets; Radiation; Surface area; Surface water; Terrestrial planets; The runaway greenhouse effect; Tropical atmosphere; Tropical environments; Venus; Venus surface; Water content; Water distribution; Water engineering
• ISSN: 2169-9097; 2169-9100
• DOI: 10.1029/2019JE006037

When planets receive insolation above a certain critical value called the runaway threshold, liquid surface water vaporizes completely, which forms the inner edge of the habitable zone. Because land planets can emit a large amount of radiation from the dry tropics, they have a higher runaway threshold than aqua planets do. Here we systematically investigated the runaway threshold for various surface water distributions using a three‐dimensional dynamic atmosphere model. The runaway threshold for the meridionally uniform surface water distribution increases from the typical value for the aqua planet regime (~130% S0) to one for the land planet regime (~155% S0) as the dry surface area increases, where S0 is the present Earth's insolation. Although this result is similar to the previous work considering zonally uniform surface water distributions, the runaway threshold for the land planet regime is quite low compared to that of the previous work. This is because a part of the tropical atmosphere is always wet for the meridionally uniform case. We also considered the surface water distributions determined by the Earth's, Mars's, and Venus's topographies. We found that their runaway thresholds are close to that for the meridionally uniform cases, and the amount of water at the boundary between an aqua planet regime and land planet regime is around 10% of the Earth's ocean. This clearly shows that the runaway threshold is not determined uniquely by the luminosity of the central star, but it has a wide range caused by the surface water distribution of the terrestrial water planet itself. Plain Language Summary Some of the detected exoplanets are expected to be Earth‐sized rocky planets, probably with liquid water on their surface due to an adequate orbital distance from the central star. The region where liquid water is stable on the planetary surface is called the habitable zone. Previous studies showed that the habitable zone for a planet with a small amount of water (called land planets) is wider than that of a planet with a large amount of water (aqua planets). By using a three‐dimensional model for the atmospheric circulation, we investigated the inner edge of the habitable zone considering the effects of various surface water distributions. As the previous studies have done, we also recognized two climate regimes (aqua planet regime and land planet regime). The insolation that a planet receives at the inner edge of the habitable zone increases with an incr