Photoevaporation of Earth and Super-Earth Atmospheres in the Habitable Zones of M Dwarfs
Abstract
Kepler data show that multiple terrestrial-sized planets (i.e., Earths / super-Earths), packed in very close to the central star, are the norm in exoplanetary systems around low-mass stars. Around M dwarfs, a significant fraction of these planets reside within the Habitable Zone (HZ). This has kindled intense excitement about the possibility of finding habitable planets around these cool red stars. However, M dwarfs also remain extremely magnetically active for much longer than solar-type stars: e.g., an M3 dwarf evinces saturated levels of coronal and chromospheric activity over Gyr timescales, compared to ~100 Myr for solar-mass stars. Thus, basal levels of coronal/chromospheric X-ray/EUV emission from M dwarfs, integrated over their saturated activity lifetimes, may severely photoevaporate the atmospheres of terrestrial planets in M dwarf HZs; this would only be exacerbated by flares (which are correspondingly more intense in active M dwarfs). Here we present detailed hydrodynamic calculations of such photoevaporation for planets spanning a range of Earth/super-Earth sizes, residing in the HZ of M dwarfs of various spectral sub-types, over Gyr evolutionary timescales. Our calculations include the effects of: (1) simultaneous X-ray and EUV heating, using state-of-the-art stellar XUV SED models; (2) the change in the stellar XUV SED over evolutionary timescales; (3) realistic radiative losses (which can both dominate and vary in time); (4) thermal evolution of the planetary core; and (5) a range of initial planetary entropies (i.e.,`hot' or `cold' start) and core compositions. The analysis yields the location and extent of the HZ as a function of planetary mass, core composition, initial conditions and M sub-type. We will focus on H/He dominated (i.e., solar abundance) atmospheres; however, we will also discuss qualtitative trends for CO2 / H2O dominated atmospheres, which we are beginning to explore by coupling a detailed photochemical code with our hydrodynamic simulations. The predictions of these studies can be tested in the near future by missions such as JWST, and will also guide their search for habitable planets.
- Publication:
-
IAU General Assembly
- Pub Date:
- August 2015
- Bibcode:
- 2015IAUGA..2258190M