Coronal Mass Ejection (CME) Activity of Low Mass M Stars as An Important Factor for The Habitability of Terrestrial Exoplanets. II. CME-Induced Ion Pick Up of Earth-like Exoplanets in Close-In Habitable Zones

Abstract

Atmospheric erosion of CO2-rich Earth-size exoplanets due to coronal mass ejection (CME)-induced ion pick up within close-in habitable zones of active M-type dwarf stars is investigated. Since M stars are active at the X-ray and extreme ultraviolet radiation (XUV) wave-lengths over long periods of time, we have applied a thermal balance model at various XUV flux input values for simulating the thermospheric heating by photodissociation and ionization processes due to exothermic chemical reactions and cooling by the CO2 infrared radiation in the 15 microm band. Our study shows that intense XUV radiation of active M stars results in atmospheric expansion and extended exospheres. Using thermospheric neutral and ion densities calculated for various XUV fluxes, we applied a numerical test particle model for simulation of atmospheric ion pick up loss from an extended exosphere arising from its interaction with expected minimum and maximum CME plasma flows. Our results indicate that the Earth-like exoplanets that have no, or weak, magnetic moments may lose tens to hundreds of bars of atmospheric pressure, or even their whole atmospheres due to the CME-induced O ion pick up at orbital distances <or=0.2 astronomical units. We have found that, when exposed to intense XUV fluxes, atmospheres with CO2/N2 mixing ratios lower than 96% will show an increase in exospheric temperatures and expanded thermosphere-exosphere environments. Hence, they suffer stronger atmospheric erosion, which can result in the total loss of several hundred bars even if an exoplanet is protected by a "magnetic shield" with its boundary located at 1 Earth radius above the surface. Furthermore, our study indicates that magnetic moments of tidally locked Earth-like exoplanets are essential for protecting their expanded upper atmospheres because of intense XUV radiation against CME plasma erosion. Therefore, we suggest that larger and more massive terrestrial-type exoplanets may better protect their atmospheres against CMEs, because the larger cores of such exoplanets would generate stronger magnetic moments and their higher gravitational acceleration would constrain the expansion of their thermosphere-exosphere regions and reduce atmospheric escape.

Tidal Venuses: Triggering a Climate Catastrophe via Tidal Heating

“…Many other critical phenomena were also left out, such as atmospheric erosion by flaring, stellar activity, and magnetic dynamo generation Lammer et al, 2007Lammer et al, , 2010Tian, 2009;Segura et al, 2010;Driscoll and Olson, 2011). Tidal heating provides an interesting counterbalance to atmospheric erosion, as it may increase the outgassing rates and maintain a permanent atmosphere.…”

“…The recently discovered Y dwarf (which is not a star) WISEP J1828+2650 (Cushing et al, 2011), with an effective temperature below 300 K, is an example of such a primary. For warmer stars, HZs may still be close enough in that nonradiative processes may impact habitability, such as stellar flaring (e.g., Khodachenko et al, 2007;Lammer et al, 2007Lammer et al, , 2010Tian, 2009;Segura et al, 2010), decreased initial volatile inventory (Lissauer, 2007;Raymond et al, 2007), or tidal effects (e.g., Kasting et al, 1993;Joshi et al, 1997;Correia et al, 2008;Jackson et al, 2008a;Barnes et al, 2009a;Heller et al, 2011). As the HZ of our Sun is too distant for these phenomena to affect Earth, we can currently only explore their role theoretically; consequently, many scientists consider close-in planets less favorable candidates for habitability.…”

Spectroscopy of planetary atmospheres in our Galaxy

“…As the stellar spectrum of the M-star is redshifted compared with the solar one, Rayleigh scattering (proportional to λ −4 ) is decreased, as well as the albedo of the planet. The increased magnetic activity of M-stars may lead to a stronger stellar wind, more stellar flares and possibly more atmospheric erosion (Lammer et al 2007;Penz et al 2008). Joshi et al (1997) and Wordsworth et al (2011) have used three-dimensional simulations to estimate the climate and habitability of terrestrial planets in orbit around M stars.…”

Exoplanet atmospheres: A theoretical outlook

“…A suggestion of terrestrial planets with sulfur cycles dominating over carbon cycles is described in Kaltenegger & Sasselov (2010). Others have attempted to quantify the atmospheric escape of Earths and super Earths, with little success due to the unknown initial mass and star's activity history (e.g., Lammer et al 2007).…”