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 due 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 possess equilibrium temperature within 300–500 K , have similar size ( 1–3 R _ { \oplus } ) , 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 an uniform transit data set both in wavelength and coarse planetary type .
With the transits measured in Kepler ’ s broad optical bandpass and Spitzer ’ s 4.5 \mu m wavelength bandpass , we measure the transmission spectral slope , \alpha , 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 \alpha among several dozen similar planets encodes a key trend .
We find that the distribution of \alpha is not well-described by a single Gaussian distribution .
Rather , a ratio of the Bayesian evidences between the likeliest 1-component and 2-component Gaussian models favors the latter by a ratio of 100:1 .
One Gaussian is centered around an average \alpha = -1.3 , indicating hazy/cloudy atmospheres or bare cores with atmosphere evaporated .
A smaller but significant second population ( 20 \pm 10 \% of all ) is necessary to model significantly higher \alpha values , which indicate atmospheres with potentially detectable molecular features .
We conclude that the atmospheres of small and temperate planets are far from uniformly flat , and that a subset are particularly favorable for follow-up observation from space-based platforms like the Hubble Space Telescope and the James Webb Space Telescope .