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

Authors

J. I. Moses
M. R. Line
Channon Visscher, Dordt CollegeFollow
M. R. Richardson
N. Nettleman
J. J. Fortney
T. S. Barman
K. B. Stevenson
N. Madhusudhan

Document Type

Article

Publication Date

2013

Department

Physics and Astronomy

Keywords

planetary systems, planets and satellites: atmospheres, planets and satellites: composition, planets and satellites: individual (GJ 436b), stars: individual (GJ 436b)

Abstract

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, H₂-dominated atmospheres to more Venus-like, CO₂-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 H₂O, CO, CO₂, and even O₂-dominated atmospheres are possible for hot Neptunes. We apply our models to the case of GJ 436b, where we find that a CO-rich, CH₄-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.

Comments

Copyright © The American Astronomical Society 2013.

Source Publication Title

The Astrophysical Journal

Publisher

The American Astronomical Society

Volume

777

Issue

1

First Page

34

DOI

10.1088/0004-637X/777/1/34

Recommended Citation

Moses, J. I., Line, M. R., Visscher, C., Richardson, M. R., Nettleman, N., Fortney, J. J., Barman, T. S., Stevenson, K. B., & Madhusudhan, N. (2013). Compositional Diversity in the Atmospheres of Hot Neptunes, With Application to GJ 436b. The Astrophysical Journal, 777 (1), 34. https://doi.org/10.1088/0004-637X/777/1/34