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Studies of the atmospheres of hot Jupiters reveal a diversity of atmosphericcomposition and haze properties. Similar studies on individual smaller,temperate planets are rare due to the inherent difficulty of the observationsand also to the average faintness of their host stars. To investigate theirensemble atmospheric properties, we construct a sample of 28 similar planets,all possess equilibrium temperature within 300-500K, have similar size (1-3R_e), and orbit early M dwarfs and late K dwarfs with effective temperatureswithin a few hundred Kelvin of one another. In addition, NASA's Kepler/K2 andSpitzer missions gathered transit observations of each planet, producing anuniform transit data set both in wavelength and coarse planetary type. With thetransits measured in Kepler's broad optical bandpass and Spitzer's 4.5 micronwavelength bandpass, we measure the transmission spectral slope, alpha, for theentire sample. While this measurement is too uncertain in nearly all cases toinfer the properties of any individual planet, the distribution of alpha amongseveral dozen similar planets encodes a key trend. We find that thedistribution of alpha is not well-described by a single Gaussian distribution.Rather, a ratio of the Bayesian evidences between the likeliest 1-component and2-component Gaussian models favors the latter by a ratio of 100:1. One Gaussianis centered around an average alpha=-1.3, indicating hazy/cloudy atmospheres orbare cores with atmosphere evaporated. A smaller but significant secondpopulation (20+\-10% of all) is necessary to model significantly higher alphavalues, which indicate atmospheres with potentially detectable molecularfeatures. We conclude that the atmospheres of small and temperate planets arefar from uniformly flat, and that a subset are particularly favorable forfollow-up observation from space-based platforms like HST and JWST.

Temperate super-Earths/mini-Neptunes around M/K dwarfs Consist of 2 Populations Distinguished by Their Atmospheres