Ensemble Atmospheric Properties of Small and Temperate Exoplanets around M/K Dwarfs

Thursday 5 July, 10:20

With the growing number of small planets discovered by the Kepler mission and ground-base surveys, efforts have been made towards the atmosphere characterization of these uncovered new worlds. While hot Jupiters exhibit diverse atmosphere compositions with a range of cloud/haze levels, atmospheric features of smaller planets (super-Earth/mini-Neptunes) have been difficult to study due to the faintness of most of their host stars. In this work, a sample of 28 planets in a constrained size (1-3 R_earth) and temperature (300-500K) range around M dwarfs with Kepler results and Spitzer observations are put together, complemented by the well-studied GJ1214b and GJ1132b. We evaluated their transit depths on the Spitzer 4.5 micron bandpass using the Pixel-Level Decorrelation method. Together with their Kepler results, we measure the transmission spectral slope, $\alpha$, for the entire sample. We find that the distribution of $\alpha is not well-described by a single Gaussian distribution. Rather, a comparison between Bayesian evidences of the likeliest 1-component and 2-component Gaussian models strongly favors the latter. The first component indicates hazy/cloudy atmospheres or bare cores with atmosphere evaporated. A smaller but significant second component represents atmospheres with potentially detectable molecular features. We conclude that atmospheres of small temperature planets are far from uniformly flat, and a subset are particularly favorable for follow-up observation from space-based platforms like HST and JWST. Aside from ensemble properties, I will also give an update on the detailed transmission spectral analysis of a mini-Neptune, HD97658b, using combined data from 4 HST visits and 9 Spitzer transits.

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