Abstract
AbstractThe energy balance and climate of planets can be affected by the reflective properties of their land, ocean, and frozen surfaces. Here we investigate the effect of host star spectral energy distribution (SED) on the albedo of these surfaces using a one-dimensional energy balance model. Incorporating spectra of M-, K-, G-, and F-dwarf stars, we determined the effect of varying fractional and latitudinal distribution of land and ocean surfaces as a function of host star SED on the overall planetary albedo, climate, and ice-albedo feedback response. While noting that the spatial distribution of land masses on a given planet will have an effect on the overall planetary energy balance, we find that terrestrial planets with higher average land/ocean fractions are relatively cooler and have higher albedo regardless of star type. For Earth-like planets orbiting M-dwarf stars, the increased absorption of water ice in the near-infrared, where M-dwarf stars emit much of their energy, resulted in warmer global mean surface temperatures, ice lines at higher latitudes, and increased climate stability as the ice-albedo feedback became negative at high land fractions. Conversely, planets covered largely by ocean, and especially those orbiting bright stars, had a considerably different energy balance due to the contrast between the reflective land and the absorptive ocean surface, which in turn resulted in warmer average surface temperatures than land-covered planets and a stronger potential ice-albedo feedback. While dependent on the properties of individual planetary systems, our results place some constraints on a range of climate states of terrestrial exoplanets based on albedo and incident flux.
Highlights
The potential habitability of a given planetary environment —that is the ability of that environment to support the activity of at least one organism (Cockell et al 2016)—is strongly dependent on the amount of energy available in that system
energy balance model (EBM) simulations were carried out to investigate the effect of varying land fraction and star type on broadband planetary albedo, and mean global surface temperature
Ocean-dominated planets and land planets with oceanic equatorial belts have higher global mean surface temperatures when compared to a uniform land fraction distribution by latitude
Summary
The potential habitability of a given planetary environment —that is the ability of that environment to support the activity of at least one organism (Cockell et al 2016)—is strongly dependent on the amount of energy available in that system. Planetary surface temperature provides the strongest control on the extent and distribution of habitable conditions, and is, in most cases, controlled by three main factors: the amount of incoming stellar radiation; the albedo or reflectivity of the surface on which that radiation is incident; and any potential greenhouse effect that may be caused by the absorption and remission of outgoing radiation by atmospheric gases such as carbon dioxide (CO2) and water vapor (H2O). Unless otherwise stated, “albedo” refers to the Bond albedo of a planet, which describes the total proportion or fraction of incident stellar flux, across all wavelengths, that is reflected back to space. This is in contrast to the geometric, spherical, or V-band albedo, which is wavelength and phase angle dependent. The relationship between albedo, temperature, and global ice cover represents
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