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Helium Atmospheres on Warm Neptune- and Sub-Neptune-sized Exoplanets and Applications to GJ 436b

Hu, Renyu and Seager, Sara and Yung, Yuk L. (2015) Helium Atmospheres on Warm Neptune- and Sub-Neptune-sized Exoplanets and Applications to GJ 436b. Astrophysical Journal, 807 (1). Art. No. 8. ISSN 0004-637X.

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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 436b, a warm Neptune-sized exoplanet, while also being 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 10^−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.

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Additional Information:© 2015 The American Astronomical Society. Received 19 February 2015; accepted for publication 5 May 2015; published 24 June 2015. We appreciate comments on the manuscript made by members of the Yuk Yung research group at the California Institute of Technology. R.H. thanks Robert Johnson, Jeffrey Linsky, and Edwin Kite for helpful discussion. This work has utilized the MUSCLES M dwarf UV radiation database. Support for this work was provided by NASA through Hubble Fellowship grant #51332 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5–26555. Y.L.Y. and R.H. (in the later phase of this work) were supported in part by an NAI Virtual Planetary Laboratory grant NASA grant NNX09AB72G to the California Institute of Technology. Part of the research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.
Funding AgencyGrant Number
NASA Hubble Fellowship51332
NASANAS 5–26555
Subject Keywords: atmospheric effects; planetary systems; planets and satellites: individual (GJ 436b); radiative transfer; techniques: spectroscopic
Record Number:CaltechAUTHORS:20150825-081127411
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Official Citation:Helium Atmospheres on Warm Neptune- and Sub-Neptune-sized Exoplanets and Applications to GJ 436b Renyu Hu et al. 2015 ApJ 807 8
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:59872
Deposited By: Jason Perez
Deposited On:25 Aug 2015 19:24
Last Modified:25 Aug 2015 19:24

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