  We find that Kepler exoplanet candidate (EC) host stars are preferentially
metal-rich, including the low-mass stellar hosts of small-radius ECs. The last
observation confirms a tentative hint that there is a correlation between the
metallicity of low-mass stars and the presence of low-mass and small-radius
exoplanets. In particular, we compare the J-H--g-r color-color distribution of
Kepler EC host stars with a control sample of dwarf stars selected from the
~150,000 stars observed during Q1 and Q2 of the Kepler mission but with no
detected planets. We find that at J-H = 0.30 characteristic of solar-type
stars, the average g-r color of stars that host giant ECs is 4-sigma redder
than the average color of the stars in the control sample. At the same time,
the average g-r color of solar-type stars that host small-radius ECs is
indistinguishable from the average color of the stars in the control sample. In
addition, we find that at J-H = 0.62 indicative of late K dwarfs, the average
g-r color of stars that host small-radius ECs is 4-sigma redder than the
average color of the stars in the control sample. These offsets are unlikely to
be caused by differential reddening, age differences between the two
populations, or the presence of giant stars in the control sample. Stellar
models suggest that the first color offset is due to a 0.2 dex enhancement in
[Fe/H] of the giant EC host population at M_star = 1 M_Sun, while Sloan
photometry of M 67 and NGC 6791 suggests that the second color offset is due to
a similar [Fe/H] enhancement of the small-radius EC host population at M_star =
0.7 M_Sun. These correlations are a natural consequence of the core-accretion
model of planet formation.
