Schematic representation of the relevant time scales involved in type I migration. Flow lines of gas parcels are indicated in a system of reference that rotates with the planet. The planet is indicated on the right. In the center is the star. Close to the planet, the flow lines bend and make a u-turn during a time equal τ_{u − turn}. During that time, the gas on the flow lines cool on a time scale τ_cool. One full libration around the planet (indicated by the black lines) takes a libration time, τ_lib. During this time, viscosity acts on a viscous time scale τ_visc. The corotation region lies between the two black lines. Inside and outside this region, gas parcels do not make u-turns, but have a velocity relative to the planet because of to the Keplerian sheer (gray lines).