The Effect of Star-Planet Interactions on Planetary Climate
Shields, Aomawa Lalahie
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The goal of the work presented here is to explore the unique interactions between a host star, an orbiting planet, and additional planets in a stellar system, and to develop and test methods that include both radiative and gravitational effects on planetary climate and habitability. These methods can then be used to identify and assess the possible climates of potentially habitable planets in observed planetary systems. In this work I explored key star-planet interactions using a hierarchy of models, which I modifed to incorporate the spectrum of stars of different spectral types. Using a 1-D energy-balance climate model, a 1-D line-by-line, radiative-transfer model, and a 3-D general circulation model, I simulated planets covered by ocean, land, and water ice of varying grain size, with incident radiation from stars of different spectral types. I find that terrestrial planets orbiting stars with higher near-UV radiation exhibit a stronger ice-albedo feedback. Ice extent is much greater on a planet orbiting an F-dwarf star than on a planet orbiting a G-dwarf star at an equivalent flux distance, and ice-covered conditions occur on an F-dwarf planet with only a 2% reduction in instellation (incident stellar radiation) relative to the present instellation on Earth, assuming fixed CO<sub>2</sub> (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 reduction in instellation must be larger for planets orbiting cooler stars due in large part to the stronger absorption of longer-wavelength radiation by icy surfaces on these planets, in addition to stronger absorption by water vapor, CO<sub>2</sub>, and clouds in their atmospheres, providing increased downwelling longwave radiation. The surface ice-albedo feedback effect becomes less important at the outer edge of the habitable zone, where atmospheric CO<sub>2</sub> can be expected to be high. I show that ∼3-10 bars of CO<sub>2</sub> will entirely mask the climatic effect of ice and snow, leaving the traditional outer limit of the habitable zone unaffected by the spectral dependence of water ice and snow albedo. Simulations of the equilibrium climate response of a planet to increasing instellation from an F-, G-, or M-dwarf star indicate that the exit out of global ice cover is also sensitive to host star spectral energy distribution. Under fixed CO<sub>2</sub> conditions, a planet orbiting an M-dwarf star exhibits a smaller resistance to melting out of a frozen state, requiring a smaller instellation to initiate deglaciation than planets orbiting hotter, brighter stars. This is due to the combined effects of surface ice and snow absorption of the large fraction of near-IR radiation emitted by M-dwarfs, and atmospheric near-IR absorption, which weakens the Hadley circulation, reducing the climate hysteresis (the range over which multiple stable equilibia are possible) of M-dwarf planets. Given their greater climatic stability, planets orbiting cooler, lower-mass stars may be the best candidates for long-term habitability and life beyond the Solar System. As lower-mass stars are likely candidates to host multiple rocky planets, it is important to consider whether gravitational interactions among planets may have significant effects on climate and habitability over long timescales. Using an <italic>n</italic>-body integrator with inputs from a method I developed to determine the locations of all planets in a given system at the same epoch using transit timing data, a specific case is explored—that of Kepler-62f (Borucki et al. 2013), a potentially habitable planet in a five-planet system orbiting a K-dwarf star. The maximum stable initial eccentricity possible for Kepler-62f is identified as e = 0.32. Simulations using a 3-D GCM indicate that Kepler-62f would have areas of the planet with surface temperatures above the freezing point of water with 1 bar or more of CO<sub>2</sub> in its atmosphere. If it has an active carbon cycle, Kepler-62f could have ample amounts of greenhouse gases in its atmosphere to maintain atmospheric stability and habitable surface conditions while staying well below the maximum CO<sub>2</sub> greenhouse limit. In a low-CO<sub>2</sub> case (Earth-like levels), increases in planetary obliquity and orbital eccentricity coupled with an orbital configuration that places the summer solstice at or near pericenter generate regions of the planet with above-freezing surface temperatures, which may cause surface melting of an ice sheet formed during an annual cycle. If Kepler-62f is synchronously rotating and has an ocean, significant cloud cover could develop at the substellar point, increasing planetary albedo and reducing surface temperatures. The methods presented here serve as tested tools that can be used to assess the possible climates of potentially habitable planets in systems with a wide range of orbital architectures as they are discovered.
- Astronomy