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Compositional Diversity in the Atmospheres of Hot Neptunes, with Application to GJ 436b

Moses, J. I. and Line, M. R. and Visscher, C. and Richardson, M. R. and Nettelmann, N. and Fortney, J. J. and Barman, T. S. and Stevenson, K. B. and Madhusudhan, N. (2013) Compositional Diversity in the Atmospheres of Hot Neptunes, with Application to GJ 436b. Astrophysical Journal, 777 (1). Art. No. 34. ISSN 0004-637X. PMCID PMC6398956. doi:10.1088/0004-637X/777/1/34.

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Neptune-sized extrasolar planets that orbit relatively close to their host stars—often called "hot Neptunes"—are common within the known population of exoplanets and planetary candidates. Similar to our own Uranus and Neptune, inefficient accretion of nebular gas is expected produce hot Neptunes whose masses are dominated by elements heavier than hydrogen and helium. At high atmospheric metallicities of 10-10,000 times solar, hot Neptunes will exhibit an interesting continuum of atmospheric compositions, ranging from more Neptune-like, H_2-dominated atmospheres to more Venus-like, CO_2-dominated atmospheres. We explore the predicted equilibrium and disequilibrium chemistry of generic hot Neptunes and find that the atmospheric composition varies strongly as a function of temperature and bulk atmospheric properties such as metallicity and the C/O ratio. Relatively exotic H_2O, CO, CO_2, and even O_2-dominated atmospheres are possible for hot Neptunes. We apply our models to the case of GJ 436b, where we find that a CO-rich, CH_4-poor atmosphere can be a natural consequence of a very high atmospheric metallicity. From comparisons of our results with Spitzer eclipse data for GJ 436b, we conclude that although the spectral fit from the high-metallicity forward models is not quite as good as the best fit obtained from pure retrieval methods, the atmospheric composition predicted by these forward models is more physically and chemically plausible in terms of the relative abundance of major constituents. High-metallicity atmospheres (orders of magnitude in excess of solar) should therefore be considered as a possibility for GJ 436b and other hot Neptunes.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper CentralArticle
Moses, J. I.0000-0002-8837-0035
Line, M. R.0000-0002-2338-476X
Fortney, J. J.0000-0002-9843-4354
Barman, T. S.0000-0002-7129-3002
Stevenson, K. B.0000-0002-7352-7941
Madhusudhan, N.0000-0002-4869-000X
Additional Information:© 2013 American Astronomical Society. Received 2013 June 21; accepted 2013 August 26; published 2013 October 11. We thank Nikole Lewis for providing her dayside-average GCM thermal profiles for GJ 436b and for motivating the cloud discussion, Heather Knutson for several illuminating conversations in relation to the paper and her latest GJ 436b transit data, and the anonymous reviewer for a thorough manuscript review and useful suggestions (and additional motivation for the cloud discussion). This work was supported by the NASA Planetary Atmospheres Program grant number NNX11AD64G.
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Subject Keywords:planetary systems; planets and satellites: atmospheres; planets and satellites: composition; planets and satellites: individual (GJ 436b) stars: individual (GJ 436b)
Issue or Number:1
PubMed Central ID:PMC6398956
Record Number:CaltechAUTHORS:20131121-144801542
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Official Citation:Compositional Diversity in the Atmospheres of Hot Neptunes, with Application to GJ 436b J. I. Moses et al. 2013 ApJ 777 34
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:42631
Deposited By: Ruth Sustaita
Deposited On:21 Nov 2013 23:03
Last Modified:10 Nov 2021 16:26

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