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Helium Atmospheres on Warm Neptune- and Sub-Neptune-Sized Exoplanets and Applications to GJ 436 b | | Renyu Hu
; Sara Seager
; Yuk L. Yung
; | | Date: |
9 May 2015 | | Abstract: | Warm Neptune- and sub-Neptune-sized exoplanets in orbits smaller than
Mercury’s are thought to have experienced extensive atmospheric evolution. Here
we propose that a potential outcome of this atmospheric evolution is the
formation of helium-dominated atmospheres. The hydrodynamic escape rates of
Neptune- and sub-Neptune-sized exoplanets are comparable to the
diffusion-limited escape rate of hydrogen, and therefore the escape is heavily
affected by diffusive separation between hydrogen and helium. A helium
atmosphere can thus be formed -- from a primordial hydrogen-helium atmosphere
-- via atmospheric hydrodynamic escape from the planet. The helium atmosphere
has very different abundances of major carbon and oxygen species from those of
a hydrogen atmosphere, leading to distinctive transmission and thermal emission
spectral features. In particular, the hypothesis of a helium-dominated
atmosphere can explain the thermal emission spectrum of GJ 436 b, a warm
Neptune-sized exoplanet, while also consistent with the transmission spectrum.
This model atmosphere contains trace amounts of hydrogen, carbon, and oxygen,
with the predominance of CO over CH4 as the main form of carbon. With our
atmospheric evolution model, we find that if the mass of the initial atmosphere
envelope is 1E-3 planetary mass, hydrodynamic escape can reduce the hydrogen
abundance in the atmosphere by several orders of magnitude in ~10 billion
years. Observations of exoplanet transits may thus detect signatures of helium
atmospheres and probe the evolutionary history of small exoplanets. | | Source: | arXiv, 1505.2221 | | Services: | Forum | Review | PDF | Favorites | |
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