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In this work we present Spitzer 3.6 and 4.5 micron secondary eclipseobservations of five new cool (<1200 K) transiting gas giant planets:HAT-P-19b, WASP-6b, WASP-10b, WASP-39b, and WASP-67b. We compare our measuredeclipse depths to the predictions of a suite of atmosphere models and toeclipse depths for planets with previously published observations in order toconstrain the temperature- and mass-dependent properties of gas giant planetatmospheres. We find that the dayside emission spectra of planets less massivethan Jupiter require models with efficient circulation of energy to the nightside and/or increased albedos, while those with masses greater than that ofJupiter are consistently best-matched by models with inefficient circulationand low albedos. At these relatively low temperatures we expect the atmosphericmethane to CO ratio to vary as a function of metallicity, and we therefore useour observations of these planets to constrain their atmospheric metallicities.We find that the most massive planets have dayside emission spectra that arebest-matched by solar metallicity atmosphere models, but we are not able toplace strong constraints on metallicities of the smaller planets in our sample.Interestingly, we find that the ratio of the 3.6 and 4.5 micron brightnesstemperatures for these cool transiting planets is independent of planettemperature, and instead exhibits a tentative correlation with planet mass. Ifthis trend can be confirmed, it would suggest that the shape of these planets'emission spectra depends primarily on their masses, consistent with thehypothesis that lower-mass planets are more likely to have metal-richatmospheres.

Spitzer Secondary Eclipse Observations of Five Cool Gas Giant Planets and Empirical Trends in Cool Planet Emission Spectra