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Project Progress

In this task we use computer models to study the atmospheres of extrasolar “super-Earths,” planets that orbit other stars and are 2-10 times more massive than the Earth. Significant progress was made this year on two models, one that calculates how the upper atmosphere of the super-Earth is affected by radiation coming from its parent star and another that calculates the surface temperature and change in atmospheric temperature with altitude for super-Earth atmospheres.

With our atmospheric escape model, we are investigating planetary upper atmospheres with compositions ranging from Venus-like (CO₂-dominant) to Earth-like (N₂-O₂-dominant) under different levels of extreme-UV (XUV) radiation. This effort will be helpful to better understand the habitability of M-star super-Earth planets. Generally speaking we found that planets in the HZ of M-stars could lose significant amount of their nitrogen inventory if their atmospheres are not dominated by CO₂, which would have limited the habitability of planets around low mass M-stars. (Tian, 2011).

We have also used a radiative transfer model to explore the Habitable Zone Limits around low mass stars such as M dwarfs.. Previous work by Kasting et al. (1993) obtained HZ boundaries for stars with effective temperatures between 3700 K and 7200 K— limits that do not include main-sequence M-dwarfs. In this study we use an updated 1-D radiativeconvective, cloud-free climate model to estimate the width of the HZ around these low mass stars. Significant improvements in our climate model include: (1) updated absorption coefficients for H₂O-CO₂ mixed atmospheres using new spectral databases (HITRAN 2008 & HITEMP 2010), (2) updated collision-induced absorption coefficients for CO₂ (critical for dense CO₂ atmospheres at the outer edge), and (3) a revised Rayleigh scattering coefficient for H₂O (important for water loss at the inner edge). We find that for Earth-like planets with CO₂/H₂O/N₂ atmospheres, the width of the HZ is 0.24-0.44 AU around an early M star (Teff = 3600 K) and 0.05-0.09 AU for a late M star (Teff = 2800 K) (Kopparapu et al. 2011), As our model does not include the radiative effects of clouds, the actual HZ boundaries may extend further in both directions than our conservative estimates.

Nonetheless, current ground-based surveys (e.g., the MEARTH project) and future spacebased characterization missions (e.g., JWST/TPF) may be able to use these HZ boundaries to help guide their efforts to find habitable planets around low mass stars.