  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 300-500K, have similar size (1-3
R_e), 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 micron
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+\-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 HST and JWST.

Earth and Planetary Astrophysics