Left panel, Top: comparing terms in Eq. (24) for 1 Earth-mass planet. Left panel, Bottom: corresponding time scales. The dotted line indicates when the “migration” time scale τ_m becomes negative, which could lead to an artificial outward migration if τ_m is used in place of τ_a in Eq. (21). Right panel: normalised torques for various planetary masses in the inner region of a disc. The torque becomes positive for an Earth-mass planet. The discontinuity around 0.1 AU is caused by the treatment of the inner disc edge (see Sect. 2.1).