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Forward and Inverse Modeling of the Emission and Transmission Spectrum of GJ 436b: Investigating Metal Enrichment, Tidal Heating, and Clouds

Morley, Caroline V. and Knutson, Heather and Line, Michael and Fortney, Jonathan J. and Thorngren, Daniel and Marley, Mark S. and Teal, Dillon and Lupu, Roxana (2017) Forward and Inverse Modeling of the Emission and Transmission Spectrum of GJ 436b: Investigating Metal Enrichment, Tidal Heating, and Clouds. Astronomical Journal, 153 (2). Art. No. 86. ISSN 0004-6256.

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The Neptune-mass GJ 436b is one of the most studied transiting exoplanets with repeated measurements of its thermal emission and transmission spectra. We build on previous studies to answer outstanding questions about this planet, including its potentially high metallicity and tidal heating of its interior. We present new observations of GJ 436b's thermal emission at 3.6 and 4.5 μm, which reduce uncertainties in estimates of GJ 436b's flux at those wavelengths and demonstrate consistency between Spitzer observations spanning more than 7 yr. We analyze the Spitzer thermal emission photometry and Hubble WFC3 transmission spectrum. We use a dual-pronged modeling approach of both self-consistent and retrieval models. We vary the metallicity, intrinsic luminosity from tidal heating, disequilibrium chemistry, and heat redistribution. We also study clouds and photochemical hazes, but do not find strong evidence for either. The self-consistent and retrieval models combine to suggest that GJ 436b has a high atmospheric metallicity, with best fits at or above several hundred times solar metallicity, tidal heating warming its interior with best-fit intrinsic effective temperatures around 300–350 K, and disequilibrium chemistry. High metal enrichments (>600× solar) occur from the accretion of rocky, rather than icy, material. Assuming the interior temperature T int ~ 300–350 K, we find a dissipation factor Q' ~ 2 × 10^5–10^6, larger than Neptune's Q', implying a long tidal circularization timescale for the orbit. We suggest that Neptune-mass planets may be more diverse than imagined, with metal enhancements spanning several orders of magnitude, to perhaps over 1000× solar metallicity. High-fidelity observations with instruments like the James Webb Space Telescope will be critical for characterizing this diversity.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Morley, Caroline V.0000-0002-4404-0456
Knutson, Heather0000-0002-0822-3095
Line, Michael0000-0002-2338-476X
Fortney, Jonathan J.0000-0002-9843-4354
Thorngren, Daniel0000-0002-5113-8558
Marley, Mark S.0000-0002-5251-2943
Additional Information:© 2017 The American Astronomical Society. Received 2016 June 29; revised 2016 December 13; accepted 2016 December 14; published 2017 January 26. We thank the anonymous referee for their helpful suggestions. We also thank Konstantin Batygin and Greg Laughlin for helpful discussions that improved the paper. This work was performed in part under contract with the Jet Propulsion Laboratory (JPL) funded by NASA through the Sagan Fellowship Program executed by the NASA Exoplanet Science Institute. H.A.K. acknowledges support from the Sloan Foundation and from NASA through an award issued by JPL/Caltech. J.J.F. acknowledges Hubble grants HST-GO-13501.06-A and HST-GO-13665.004-A and NSF grant AST-1312545. M.S.M. acknowledges support from the NASA Origins program. M.R.L. acknowledges support provided by NASA through Hubble Fellowship grant no. 51362 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under the contract NAS 5-26555. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. Software: CHIMERA (Line et al. 2012, 2013, 2014), CEA (Gordon et al. 1984), KINETICS (Allen et al. 1981; Yung et al. 1984; Moses et al. 2005), disort (Stamnes et al. 1988; Buras et al. 2011), PYMULTINEST (Buchner et al. 2014).
Group:Infrared Processing and Analysis Center (IPAC), Astronomy Department
Funding AgencyGrant Number
Alfred P. Sloan FoundationUNSPECIFIED
NASA Hubble FellowshipHST-GO-13501.06-A
NASA Hubble FellowshipHST-GO-13665.004-A
NASA Hubble Fellowship51362
NASANAS 5-26555
Subject Keywords:planets and satellites: atmospheres – planets and satellites: composition – planets and satellites: gaseous planets
Issue or Number:2
Record Number:CaltechAUTHORS:20170127-142303179
Persistent URL:
Official Citation:Caroline V. Morley et al 2017 AJ 153 86
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:73786
Deposited By: Tony Diaz
Deposited On:27 Jan 2017 22:57
Last Modified:20 Apr 2020 08:47

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