Habitable Zone Limits for Dry Planets

Abstract

Most discussion of habitable planets has focused on Earth-like planets with globally abundant liquid water. For an "aqua planet" like Earth, the surface freezes if far from its sun, and the water vapor greenhouse effect runs away if too close. Here we show that "land planets" (desert worlds with limited surface water) have wider habitable zones than aqua planets. For planets at the inner edge of the habitable zone, a land planet has two advantages over an aqua planet: (i) the tropics can emit longwave radiation at rates above the traditional runaway limit because the air is unsaturated and (ii) the dry air creates a dry stratosphere that limits hydrogen escape. At the outer limits of the habitable zone, the land planet better resists global freezing because there is less water for clouds, snow, and ice. Here we describe a series of numerical experiments using a simple three-dimensional global climate model for Earth-sized planets. Other things (CO(2), rotation rate, surface pressure) unchanged, we found that liquid water remains stable at the poles of a low-obliquity land planet until net insolation exceeds 415 W/m(2) (170% that of modern Earth), compared to 330 W/m(2) (135%) for the aqua planet. At the outer limits, we found that a low-obliquity land planet freezes at 77%, while the aqua planet freezes at 90%. High-obliquity land and aqua planets freeze at 58% and 72%, respectively, with the poles offering the last refuge. We show that it is possible that, as the Sun brightens, an aqua planet like Earth can lose most of its hydrogen and become a land planet without first passing through a sterilizing runaway greenhouse. It is possible that Venus was a habitable land planet as recently as 1 billion years ago.

The role of sea-ice albedo in the climate of slowly rotating aquaplanets

“…The point of global glaciation of our reference simulation with the present-day rotation of Earth occurs between 90 and 93% of TSI 0 . This glaciation limit is equal to the limit found by Abe et al (2011) with the CCSR/NIES model and nearly equal to the glaciation limit in the CCSM4 model that occurs between 92 and 93 % of TSI 0 Shields et al (2013). In order to determine the outer edge of the habitable zone of the aquaplanet as a function of the rotation period, a denser CO 2 atmosphere should be considered because of the silicate-weathering feedback (Kasting et al 1993;Kopparapu et al 2014), which is expected to trap more CO 2 in the atmosphere as the temperature drops.…”

Tidal Venuses: Triggering a Climate Catastrophe via Tidal Heating

“…Various complications are worthy of note. Water on a dry planet will get trapped at the poles, which makes a moist or runaway greenhouse harder to achieve and means that the inner edge of the HZ is nearer the star (Abe et al, 2011). We do not include that limit in Fig.…”

“…Red curves assume the CTL model, blue the CPL with discrete rotation states; see Appendix E.1. Solid curves assume the Pierrehumbert (2011) runaway greenhouse model, and dotted the dry world model of Abe et al (2011); see Fig. A1 in Appendix A.…”

“…For these latter worlds, we choose Q = 100, as they do not have oceans. Thick lines are for a 10 M Earth planet and thin for 1 M Earth [Abe et al (2011) For the lowest-mass M dwarfs, the HZ is significantly reduced due to tidal heating. For masses larger than 0.25 M Sun , a tidal greenhouse in the IHZ is only possible on large-mass planets with very large eccentricities.…”

“…''Wet'' planets are terrestrial exoplanets that have a water content similar to that of Earth. ''Dry'' exoplanets have far less, *3 cm deep if condensed and spread globally, but are habitable (see Abe et al, 2011). ''Desiccated'' planets have lost all their water through a desiccating greenhouse.…”