Mini-Oort clouds: Compact isotropic planetesimal clouds from planet-planet scattering

Starting from planetary systems with three giant planets and an outer disc of planetesimals, we use dynamical simulations to show how dynamical instabilities can transform planetesimal discs into 10²-10³ au-scale isotropic clouds. The instabilities involve a phase of planet-planet scattering that concludes with the ejection of one or more planets and the inward-scattering of the surviving gas giant(s) to remove them from direct dynamical contact with the planetesimals. 'Mini-Oort clouds' are thus formed from scattered planetesimals whose orbits are frozen by the abrupt disappearance of the perturbing giant planet. Although the planetesimal orbits are virtually isotropic, the surviving giant planets tend to have modest inclinations (typically~10°) with respect to the initial orbital plane. The collisional lifetimes of mini-Oort clouds are long (10 Myr to >10 Gyr) and there is a window of ~100 Myr or longer during which they produce spherical clouds of potentially observable dust at 70 μm. If the formation channel for hot Jupiters commonly involves planetary close encounters, we predict a correlation between this subset of extrasolar planetary systems and mini-Oort clouds.