radial drift

The phrase radial drift clearly applies to orbits shifting inward or outward in relation to the host, but is commonly used in discussions of planet formation for a particular kind of inward drift of solid objects within a protoplanetary disk. The disk's gas orbits at less than Keplerian speed because the balance of forces and inertia include gas pressure and radiation pressure. Objects sufficiently small are coupled with the gas particles and orbit with it, but somewhat larger objects are not carried along, and tend to orbit at Keplerian speed, a bit faster than surrounding gas, the relative motion resulting in a headwind. The larger object is constantly slowed and drifts inward. Another source of such drift is dynamical friction, i.e., from Lindblad torque. Such radial drift is the basis for the radial-drift barrier and is an element in streaming instability.