A new inclination instability reshapes Keplerian discs into cones: application to the outer Solar system

Abstract

Abstract Discs of bodies orbiting a much more massive central object are extremely common in astrophysics. When the orbits comprising such discs are eccentric, we show they are susceptible to a new dynamical instability. Gravitational forces between bodies in the disc drive exponential growth of their orbital inclinations and clustering in their angles of pericentre, expanding an initially thin disc into a conical shape by giving each orbit an identical ‘tilt’ with respect to the disc plane. This new instability dynamically produces the unusual distribution of orbits observed for minor planets beyond Neptune, suggesting that the instability has shaped the outer Solar system. It also implies a large disc mass (∼ 1–10 Earth masses) of scattered bodies at hundreds of au; we predict increasing numbers of detections of minor planets clustered in their angles of pericentre with high inclinations.