  Streaming instability is a powerful mechanism which concentrates dust grains
in pro- toplanetary discs, eventually up to the stage where they collapse
gravitationally and form planetesimals. Previous studies inferred that it
should be ineffective in viscous discs, too efficient in inviscid discs, and
may not operate in local pressure maxima where solids accumulate. From a linear
analysis of stability, we show that streaming instability behaves differently
inside local pressure maxima. Under the action of the strong differential
advection imposed by the bump, a novel unstable mode develops and grows even
when gas viscosity is large. Hence, pressure bumps are found to be the only
places where streaming instability occurs in viscous discs. This offers a
promising way to conciliate models of planet formation with recent observations
of young discs.
