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 \sim 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 \pm 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 .