Evolution of the disc without any embedded planets for the two types of trap, DZ (left) and IB (right). Top: 1D surface density evolution. The colour of the lines represents the time in units of the orbital period at r = 1. Black denotes the initial profile, and the line colours pale as time proceeds. Bottom: analytical type I torque acting on a planet with ten Earth masses calculated using the disc profiles. Every horizontal line in these plots shows the specific torque at every distance from the star at a given time. The black contours represent the zero-torque locations.