The Effect of Host Star Spectral Energy Distribution and Ice-Albedo Feedback on the Climate of Extrasolar Planets

TL;DR

This study investigates how the spectral energy distribution of host stars and ice-albedo feedback influence the climate and habitability of extrasolar planets using multi-model simulations.

Contribution

It introduces a hierarchical modeling approach combining radiative transfer and climate models to assess star type effects on planetary ice cover and habitability.

Findings

Ice-covered conditions are more easily triggered around F-dwarfs with minimal instellation reduction.

Higher atmospheric CO2 levels can mask the climatic effects of ice-albedo feedback.

Planets orbiting M-dwarfs require less CO2 to maintain open water compared to those around hotter stars.

Abstract

Planetary climate can be affected by the interaction of the host star spectral energy distribution with the wavelength-dependent reflectivity of ice and snow. Here we explore this effect using a one dimensional (1-D), line-by-line, radiative-transfer model to calculate broadband planetary albedos as input to a seasonally varying, 1-D energy-balance climate model. A three-dimensional general circulation model is also used to explore the atmosphere's response to changes in incoming stellar radiation, or instellation, and surface albedo. Using this hierarchy of models we simulate planets covered by ocean, land, and water ice of varying grain size, with incident radiation from stars of different spectral types. Terrestrial planets orbiting stars with higher near-UV radiation exhibit a stronger ice-albedo feedback. We find that ice-covered conditions occur on an F-dwarf planet with only a 2% reduction in instellation relative to the present instellation on Earth, assuming fixed CO2 (present atmospheric level on Earth). A similar planet orbiting the Sun at an equivalent flux distance requires an 8% reduction in instellation, while a planet orbiting an M-dwarf star requires an additional 19% reduction in instellation to become ice-covered, equivalent to 73% of the modern solar constant. The surface ice-albedo feedback effect becomes less important at the outer edge of the habitable zone, where atmospheric CO2 can be expected to be high in order to maintain clement conditions for surface liquid water. We show that 3-10 bars of CO2 will entirely mask the climatic effect of ice and snow, leaving the outer limits of the habitable zone unaffected by the spectral dependence of water ice and snow albedo. However, less CO2 is needed to maintain open water for a planet orbiting an M-dwarf star, than would be the case for hotter main-sequence stars.