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A Rossby wave instability (RWI) is a phenomenon that can occur in an astrophysical disk (protoplanetary disk or accretion disk) that causes the disk change from a smooth profile to less regular. Rossby waves (e.g., in a Keplerian disk) provide positive feedback to expansion of density lumps that occur by happenstance. Some theories of planet formation depend on such an instability, considering it as producing the high-density regions that result in planets.