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While it is well established that giant-planet occurrence rises rapidly withhost star metallicity, it is not yet clear if small-planet occurrence aroundlate-type dwarf stars depends on host star metallicity. Using the Kepler DataRelease 25 planet candidate list and its completeness data products, we exploreplanet occurrence as a function of metallicity in the Kepler field's late-typedwarf stellar population. We find that planet occurrence increases withmetallicity for all planet radii $R_{\mathrm{p}}$ down to at least$R_{\mathrm{p}}\approx2~R_{\oplus}$ and that in the range $2~R_{\oplus}\lesssimR_{\mathrm{p}}\lesssim 5~R_{\oplus}$ planet occurrence scales linearly withmetallicity $Z$. Extrapolating our results, we predict that short-periodplanets with $R_{\mathrm{p}}\lesssim 2~R_{\oplus}$ should be rare around earlyM dwarf stars with $\mathrm{[M/H]}\lesssim-0.5$ or late M dwarf stars with$\mathrm{[M/H]}\lesssim+0.0$. This dependence of planet occurrence onmetallicity observed in the Kepler field emphasizes the need to control formetallicity in estimates of planet occurrence for late-type dwarf stars likethose targeted by Kepler's K2 extension and the Transiting Exoplanet SurveySatellite (TESS). We confirm the theoretical expectation that the small planetoccurrence--host star metallicity relation is stronger for low-mass stars thanfor solar-type stars. We establish that the expected solid mass in planetsaround late-type dwarfs in the Kepler field is comparable to the total amountof planet-making solids in their protoplanetary disks. We argue that this highefficiency of planet formation favors planetesimal accretion over pebbleaccretion as the origin of the small planets observed by Kepler aroundlate-type dwarf stars.

An Increase in Small-planet Occurrence with Metallicity for Late-type Dwarf Stars in the Kepler Field and Its Implications for Planet Formation