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We identify and study a number of new, rapidly growing instabilities of dustgrains in protoplanetary disks, which may be important for planetesimalformation. The study is based on the recognition that dust-gas mixtures aregenerically unstable to a Resonant Drag Instability (RDI), whenever the gas,absent dust, supports undamped linear modes. We show that the "streaminginstability" is an RDI associated with epicyclic oscillations; this providessimple interpretations for its mechanisms and accurate analytic expressions forits growth rates and fastest-growing wavelengths. We extend this analysis tomore general dust streaming motions and other waves, including buoyancy andmagnetohydrodynamic oscillations, finding various new instabilities. Mostimportantly, we identify the disk "settling instability," which occurs as dustsettles vertically into the midplane of a rotating disk. For small grains, thisinstability grows many orders of magnitude faster than the standard streaminginstability, with a growth rate that is independent of grain size. Growthtimescales for realistic dust-to-gas ratios are comparable to the disk orbitalperiod, and the characteristic wavelengths are more than an order of magnitudelarger than the streaming instability (allowing the instability to concentratelarger masses). This suggests that in the process of settling, dust will bandinto rings then filaments or clumps, potentially seeding dust traps,high-metallicity regions that in turn seed the streaming instability, or evenoverdensities that coagulate or directly collapse to planetesimals.

Resonant Drag Instabilities in protoplanetary disks: the streaming instability and new, faster-growing instabilities