Energy Budgets for Terrestrial Extrasolar Planets

Abstract

The pathways through which incoming energy is distributed between the surface and atmosphere has been analyzed for the Earth. However, the effect of the spectral energy distribution of a host star on the energy budget of an orbiting planet may be significant given the wavelength-dependent absorption properties of atmospheric CO 2 and water vapor, and surface ice and snow. We have quantified the flow of energy on aqua planets orbiting M-, G-, and F-dwarf stars, using a 3D Global Climate Model with a static ocean. The atmosphere and surface of an M-dwarf planet receiving an instellation equal to 88% of the modern solar constant at the top of the atmosphere absorb 12% more incoming stellar radiation than those of a G-dwarf planet receiving 100% of the modern solar constant, and 17% more radiation than a F-dwarf planet receiving 108% of the modern solar constant, resulting in climates similar to modern-day Earth on all three planets, assuming a 24-hr rotation period and fixed CO 2 . At 100% instellation, a synchronously-rotating M-dwarf planet exhibits smaller flux absorption in the atmosphere and on the surface of the dayside, and a dayside mean surface temperature that is 37 K colder than its rapidly-rotating counterpart. Energy budget diagrams are included to illustrate the variations in global energy budgets as a function of host star spectral class, and can contribute to habitability assessments of planets as they are discovered.

Sensitivity of the Atmospheric Water Cycle within the Habitable Zone of a Tidally Locked, Earth-like Exoplanet

“…Consistent with a sun-like star irradiance, the planet surface absorbs ≈ 48% of the incoming SW radiation. This number would decrease for a red dwarf star because the planet atmosphere would absorb a larger amount of SW radiation (Shields et al (2019)).…”

Implications of different stellar spectra for the climate of tidally locked Earth-like exoplanets

“…This is due to the wavelength dependence of the ice albedo, which decreases with wavelength above 0.5 µm, leading to a lower contrast between ice and water (Joshi & Haberle 2012). Shields et al (2019) took this further to find that a planet orbiting an M dwarf absorbs 12% more incident solar energy than its G dwarf counterpart for an Earth-like configuration with a 24-hour rotational period. Meanwhile, Yang et al (2019a) found that an increase in atmospheric absorption of stellar radiation led to an increase in relative humidity at higher altitudes globally, causing a significant decrease in OLR.…”

Using the Climate App to learn about Planetary Habitability and Climate Change

“…The width of each arrow is proportional to the importance of that energy flow as compared to the amount of starlight that reaches the planet. This sort of schematic showing energy flows in a planet's atmosphere is sometimes called a Trenberth Diagram and are commonplace in climate studies of Earth and other worlds [7][8][9].…”