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Investigating Trends in Atmospheric Compositions of Cool Gas Giant Planets Using Spitzer Secondary Eclipses

Wallack, Nicole L. and Knutson, Heather A. and Morley, Caroline V. and Moses, Julianne I. and Thomas, Nancy H. and Thorngren, Daniel P. and Deming, Drake and Désert, Jean-Michel and Fortney, Jonathan J. and Kammer, Joshua A. (2019) Investigating Trends in Atmospheric Compositions of Cool Gas Giant Planets Using Spitzer Secondary Eclipses. Astronomical Journal, 158 (6). Art. No. 217. ISSN 1538-3881. doi:10.3847/1538-3881/ab2a05.

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We present new 3.6 and 4.5 μm secondary eclipse measurements for five cool (T 1000 K) transiting gas giant planets: HAT-P-15b, HAT-P-17b, HAT-P-18b, HAT-P-26b, and WASP-69b. We detect eclipses in at least one bandpass for all planets except HAT-P-15b. We confirm and refine the orbital eccentricity of HAT-P-17b, which is also the only planet in our sample with a known outer companion. We compare our measured eclipse depths in these two bands, which are sensitive to the relative abundances of methane versus carbon monoxide and carbon dioxide, respectively, to predictions from 1D atmosphere models for each planet. For planets with hydrogen-dominated atmospheres and equilibrium temperatures cooler than ~1000 K, this ratio should vary as a function of both atmospheric metallicity and the carbon-to-oxygen ratio. For HAT-P-26b, our observations are in good agreement with the low atmospheric metallicity inferred from transmission spectroscopy. We find that all four of the planets with detected eclipses are best matched by models with relatively efficient circulation of energy to the nightside. We see no evidence for a solar-system-like correlation between planet mass and atmospheric metallicity, but instead identify a potential (1.9σ) correlation between the inferred CH₄/(CO + CO₂) ratio and stellar metallicity. Our ability to characterize this potential trend is limited by the relatively large uncertainties in the stellar metallicity values. Our observations provide a first look at the brightness of these planets at wavelengths accessible to the James Webb Space Telescope, which will be able to resolve individual CH₄, CO, and CO₂ bands and provide much stronger constraints on their atmospheric compositions.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Wallack, Nicole L.0000-0003-0354-0187
Knutson, Heather A.0000-0002-0822-3095
Morley, Caroline V.0000-0002-4404-0456
Moses, Julianne I.0000-0002-8837-0035
Thomas, Nancy H.0000-0003-1989-4860
Thorngren, Daniel P.0000-0002-5113-8558
Deming, Drake0000-0001-5727-4094
Désert, Jean-Michel0000-0002-0875-8401
Fortney, Jonathan J.0000-0002-9843-4354
Kammer, Joshua A.0000-0002-3441-3757
Additional Information:© 2019 The American Astronomical Society. Received 2018 November 20; revised 2019 June 10; accepted 2019 June 14; published 2019 November 6. 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 a contract with NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech. J.M.D acknowledges that the research leading to these results has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement no. 679633; Exo-Atmos). J.M. acknowledges support from NASA grant NNX16AC64G.
Group:Astronomy Department
Funding AgencyGrant Number
European Research Council (ERC)679633
Subject Keywords:Exoplanet atmospheric composition; Exoplanet atmospheres; Exoplanets; Broad band photometry; Infrared photometry
Issue or Number:6
Classification Code:Unified Astronomy Thesaurus concepts: Exoplanet atmospheric composition (2021); Exoplanet atmospheres (487); Exoplanets (498); Broad band photometry (184); Infrared photometry (792)
Record Number:CaltechAUTHORS:20190904-131931232
Persistent URL:
Official Citation:Nicole L. Wallack et al 2019 AJ 158 217
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:98412
Deposited By: Tony Diaz
Deposited On:04 Sep 2019 21:00
Last Modified:16 Nov 2021 17:38

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