The Loss of Nitrogen-rich Atmospheres from Earth-like Exoplanets within M-star Habitable Zones

摘要

After the first discovery of massive Earth-like exoplanets around M-type dwarf stars, the search for exoplanets which resemble more an Earth analogue continues. The discoveries of super-Earth planets pose questions on habitability and the possible origin of life on such planets. Future exoplanet space projects designed to characterize the atmospheres of terrestrial exoplanets will also search for atmospheric species which are considered as bio-markers (e.g. O_3, H_2O, CH_4, etc.). By using the Earth with its atmosphere as a proxy and in agreement with the classical habitable zone concept, one should expect that Earth-like exoplanets suitable for life as we know it should have a nitrogen atmosphere and a very low CO_2 content. Whether a water bearing terrestrial planet within its habitable zone can evolve into a habitable world similar than the Earth, depends on the capability of its water-inventory and atmosphere to survive the period of high radiation of the young and/or active host star. Depending on their size and mass, lower mass stars remain at high X-ray and EUV (XUV) activity levels for hundreds of Ma's to Ga's. XUV flux values which are 10 or 20 times higher than that of the present Sun can heat the thermosphere and expand the exobase of N2-rich Earth-like exoplanets to altitudes well above their expected magnetopause distances. This results in magnetically non-protected upper atmospheres and high non-thermal escape rates. We studied this plasma induced N+ ion pick up escape and applied a numerical test-particle stellar wind plasma - exosphere interaction model. Our results indicate that Earth-analogue exoplanets with atmosphere compositions similar to that of present Earth will lose their nitrogen inventories if they are exposed over a sufficient period of time to XUV fluxes ≥ 10 times that of the present Sun. Because most M-type stars are active in XUV radiation we suggest that these planets will undergo a different atmospheric evolution than the Earth so that life as we know it may not evolve on their surfaces.