Andrew J. Rushby (UC Irvine): "Effects of Land Coverage and Host Star Spectral Energy Distribution on the Planetary Albedo of Terrestrial Worlds"


Title: Effects of Land Coverage and Host Star Spectral Energy Distribution on the Planetary Albedo of Terrestrial Worlds

Abstract: The energy balance and climate of planets can be affected by the reflective properties of their land, ocean, and frozen surfaces. In this talk, I present results from several studies by our group at UC Irvine focussed on investigating the climates of terrestrial exoplanets orbiting M-dwarf stars. In particular, I will discuss the effect of host spectral energy distribution (SED) on the albedo of planetary surfaces, and implications for climate stability and potential habitability. Using spectra from M-, K-, G- and F-dwarf stars combined with a one-dimensional (1-D) energy balance model, we investigated the effect of varying fractional and latitudinal coverage of land and ocean surfaces as a function of host star SED on the overall planetary albedo, climate, and ice-albedo feedback response of terrestrial planets.  Our results showed that planets dominated by land are relatively cooler and brighter regardless of star type, but that planets orbiting M-dwarf host stars remain warmer relative to planets orbiting brighter stars and exhibit ice-lines at higher latitudes, as well as a weakened sensitivity of the climate system to perturbations in radiative forcing. This is due to the formation of water ice that has a lower Bond albedo in the infrared, where cool stars like M-dwarfs emit a higher proportion of their flux. This absorptive ice surface results in a lower climate sensitivity to the ice-albedo feedback, an effect especially pronounced at high land fractions, and one that considerably increases climatic stability on these planets. Conversely, planets covered largely by ocean, and especially those orbiting brighter stars, we found to have a considerably different relative energy balance due to the darker, more absorptive ocean surface, which results in warmer average surface temperatures and a stronger potential ice-albedo feedback.