The atmospheres of two hot, dense planets

Previous transmission spectroscopy campaigns have tended to focus on relatively low-density planets with hydrogen-rich atmospheres, as they are the most favorable targets for this technique.  We present infrared transmission spectra for two dense planets with high-metallicity stars: HD 149026b and 55 Cnc e.  The former is an unusually dense Saturn, while the latter is potentially a rocky super Earth whose atmospheric properties are much debated in the literature.  We derive transmission spectra for both planets by jointly fitting HST WFC3 (1.1-1.6 µm) and Spitzer IRAC (3.6 and 4.5 µm) transits for each planets, thus constraining transit parameters more precisely than is possible with one instrument alone.  For HD 149026b, we find an essentially flat spectrum indicative of a high-altitude cloud layer. For 55 Cnc e, we find an upward sloping spectrum across at least the second half of the WFC3 range, consistent with the results of previous work. We analyze the sensitivity of these results to changes in the HST calibration pipeline.  Finally, we use these spectra, along with previously published Spitzer phase curve data, to put constraints on the planets’ atmospheric chemical compositions and cloud properties.

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