Assessing the habitability of planets with Earth-like atmospheres with 1D and 3D climate modeling

Abstract

Context. The habitable zone (HZ) describes the range of orbital distances around a star where the existence of liquid water on the surface of an Earth-like planet is in principle possible. The applicability of one-dimensional (1D) climate models for the estimation of the HZ boundaries has been questioned by recent three-dimensional (3D) climate studies. While 3D studies can calculate the water vapor, ice albedo, and cloud feedback self-consistently and therefore allow for a deeper understanding and the identification of relevant climate processes, 1D model studies rely on fewer model assumptions and can be more easily applied to the large parameter space possible for extrasolar planets. Aims. We evaluate the applicability of 1D climate models to estimate the potential habitability of Earth-like extrasolar planets by comparing our 1D model results to those of 3D climate studies in the literature. We vary the two important planetary properties, surface albedo and relative humidity, in the 1D model. These depend on climate feedbacks that are not treated self-consistently in most 1D models.Methods. We applied a cloud-free 1D radiative-convective climate model to calculate the climate of Earth-like planets around different types of main-sequence stars with varying surface albedo and relative humidity profile. We compared the results to those of 3D model calculations available in the literature and investigated to what extent the 1D model can approximate the surface temperatures calculated by the 3D models. Results. The 1D parameter study results in a large range of climates possible for an Earth-sized planet with an Earth-like atmosphere and water reservoir at a certain stellar insolation. At some stellar insolations the full spectrum of climate states could be realized, i.e., uninhabitable conditions due to surface temperatures that are too high or too low as well as habitable surface conditions, depending only on the relative humidity and surface albedo assumed. When treating the surface albedo and the relative humidity profile as parameters in 1D model studies and using the habitability constraints found by recent 3D modeling studies, the same conclusions about the potential habitability of a planet can be drawn as from 3D model calculations.

The habitability of a stagnant-lid Earth

“…When assuming a relative humidity profile comparable to that of the Earth (Manabe & Wetherald 1967), this albedo leads to the global mean temperature of the Earth (288 K). Assumptions about the relative humidity and surface albedo can have a large impact on the planetary climate as shown, for example, by Godolt et al (2016). We chose a relative humidity similar to those in Kasting et al (1993b) and Kopparapu et al (2013).…”

“…However, recent studies have shown that liquid water on a planetary surface is also possible for global mean temperatures below 273 K with temperatures as low as ∼ 235 K since, locally, the surface temperatures are above 273 K and may still allow for liquid water (e.g. Charnay et al 2013;Wolf & Toon 2013;Kunze et al 2014;Shields et al 2014;Godolt et al 2016).…”

“…Leconte et al 2013a). The impact of different relative humidities upon 1D climate modelling results and their comparison to 3D model results has been investigated by Godolt et al (2016). The assumption of a fully saturated atmosphere overestimates the surface temperatures at higher temperatures.…”

The effect of high nitrogen pressures on the habitable zone and an appraisal of greenhouse states

“…The situation is very different for inner edge calculations, however, as differences in convective, cloud, and radiation schemes produce a wide scatter across different 1-D and 3-D models (Yang et al 2013;Kopparapu et al 2016;Kopparapu et al 2017;Bin et al 2018;Ramirez 2018a). These differences partially arise from the difficulty in simulating the water vapour feedback at higher temperatures, which is not a major concern at the HZ outer edge, where water vapour absorption is weaker (Godolt et al 2016;Ramirez and Levi 2018). Nevertheless, the trend that higher background N2 pressures push the inner edge closer to the star would likely be found in many other models as well.…”

SVEEEETIES: singular vector expansion to estimate Earth-like exoplanet temperatures from infrared emission spectra

“…von Paris et al 2015). As input data for the Earth-like planets orbiting G-and K-type stars in their habitable zone we use the modeled temperature and concentration profiles from Godolt et al (2016). These temperature profiles are also depicted in Fig.…”