  A metal-rich environment facilitates planet formation, making metal-rich
stars the most favorable targets for surveys seeking to detect new exoplanets.
Using this advantage to identify likely low-mass planet hosts, however, has
been difficult: until now, methods to determine M-dwarf metallicities required
observationally expensive data (such as parallaxes and high-resolution
spectra), and were limited to a few bright cool stars. We have obtained
moderate (R~2700) resolution K-band spectra of 17 M-dwarfs with metallicity
estimates derived from their FGK companions. Analysis of these spectra, and
inspection of theoretical synthetic spectra, reveal that an M-dwarf's
metallicity can be inferred from the strength of its Na I doublet (2.206 {\mu}m
& 2.209 {\mu}m) and Ca I triplet (2.261 {\mu}m, 2.263 {\mu}m & 2.265 {\mu}m)
absorption lines. We use these features, and a temperature-sensitive water
index, to construct an empirical metallicity indicator applicable for M-dwarfs
with near-solar metallicities (-0.5<[Fe/H]<+0.5). This indicator has an
accuracy of +/- 0.15 dex, comparable to that of existing techniques for
estimating M-dwarf metallicities, but is more observationally accessible,
requiring only a moderate resolution K-band spectrum. Applying this method to 8
known M-dwarf planet hosts, we estimate metallicities ([Fe/H]) in excess of the
mean metallicity of M-dwarfs in the solar neighborhood, consistent with the
metallicity distribution of FGK planet hosts.
