Temperate Super-Earths/Mini-Neptunes Around M/K Dwarfs Consist of Two Populations Distinguished by Kepler and Spitzer Transit Depth Variations

Studies of the atmospheres of hot Jupiters reveal a diversity of atmospheric composition and haze properties. Similar studies on individual smaller, temperate planets are rare owing to the inherent difficulty of the observations and also to the average faintness of their host stars. To investigate their ensemble atmospheric properties, we construct a sample of 28 similar planets, all of which possess equilibrium temperature within 300–500 K, have similar size (1–3 R⊕), and orbit early M dwarfs and late K dwarfs with effective temperatures within a few hundred kelvin of one another. In addition, NASA’s Kepler/K2 and Spitzer missions gathered transit observations of each planet, producing a uniform transit data set in both wavelength and coarse planetary type. With the transits measured in Kepler’s broad optical bandpass and Spitzer’s 4.5 μm wavelength bandpass, we measure the transmission spectral slope, Δz/H, for the entire sample. While this measurement is too uncertain in nearly all cases to infer the properties of any individual planet, the distribution of Δz/H among several dozen similar planets encodes a key trend. We find that the distribution of Δz/H is not well described by a single Gaussian distribution. Rather, a ratio of the Bayesian evidences between the likeliest one-component and two-component Gaussian models favors the latter by a ratio of 100:3. In the best-fit two-component Gaussian model, one Gaussian is centered around an average Δz/H = −0.1 with a standard deviation of 8.2, while a smaller but significant second population (20% ± 10% of all) is located at significantly higher Δz/H center values with higher uncertainty. Although the exact shape and locations of the Gaussian models are sensitive to our assumptions in mean molecular weight, we can conclude that (1) there exist two populations of small and temperate planets regarding their effective radius variations between the Kepler bandpass and Spitzer bandpass and (2) atmospheres of this kind of planets are far from uniformly flat, and a fraction of them may be particularly favorable for follow-up observations from space-based platforms like the Hubble Space Telescope and the James Webb Space Telescope.