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| Paper: |
The Loss of Nitrogen-rich Atmospheres from Earth-like Exoplanets within M-star Habitable Zones |
| Volume: |
450, Molecules in the Atmospheres of Extrasolar Planets |
| Page: |
139 |
| Authors: |
Lammer, H.; Lichtenegger, H. I. M.; Khodachenko, M. L.; Kulikov, Y. N.; Griessmeier, J. |
| Abstract: |
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. O3, H2O,
CH4, 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 CO2
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. |
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