Investigating the Relationship Between Planetary Mass and Atmospheric Metallicity Using Spitzer Secondary Eclipses for Five New Planets

Friday 6 July, 09:20

We present new 3.6 and 4.5 micron secondary eclipse measurements for five cool (T < 1000K) 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, likely due to a poorly constrained orbital eccentricity. 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. As a result of their low temperatures, we expect that the relative abundances of methane and CO in these atmospheres should reflect their overall metallicities. We compare our measured eclipse depths in the 3.6 and 4.5 micron bands, which are sensitive to the relative amount of methane and CO, respectively, to predictions from 1D atmosphere models. We confirm the low atmospheric metallicity of HAT-P-26b inferred from transmission spectroscopy at shorter wavelengths, and find that all four of the planets with detected eclipses are best-matched by models with relatively efficient planet-wide energy circulation. We find only weak evidence for a trend in metallicity as a function of planet mass, but our ability to detect this trend is limited by the relative paucity of Neptune-mass planets in our sample and the large dispersion in the inferred metallicities of HAT-P-26b versus GJ 436b. 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 these methane and CO features and provide stronger constraints on their atmospheric compositions.

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