  Interpenetrating streams of solids and gas in a Keplerian disk produce a
local, linear instability. The two components mutually interact via aerodynamic
drag, which generates radial drift and triggers unstable modes. The secular
instability does not require self-gravity, yet it generates growing particle
density perturbations that could seed planetesimal formation. Growth rates are
slower than dynamical, but faster than radial drift, timescales. Growth rates,
like streaming velocities, are maximized for marginal coupling (stopping times
comparable dynamical times). Fastest growth occurs when the solid to gas
density ratio is order unity and feedback is strongest. Curiously, growth is
strongly suppressed when the densities are too nearly equal. The relation
between background drift and wave properties is explained by analogy with
Howard's semicircle theorem. The three-dimensional, two-fluid equations
describe a sixth order (in the complex frequency) dispersion relation. A
terminal velocity approximation allows simplification to an approximate cubic
dispersion relation. To describe the simplest manifestation of this
instability, we ignore complicating (but possibly relevant) factors like
vertical stratification, dispersion of particle sizes, turbulence, and
self-gravity. We consider applications to planetesimal formation and compare
our work to other studies of particle-gas dynamics.
