Magnetic energy difference Δ E between the stellar coronal field without and with a planetary magnetosphere inside itself vs. the distance r_p of the planet from the centre of its host star. The value of Δ E is computed in the case of a closed magnetosphere (C_imf = 0) and a potential stellar field directed radially by applying Eq. (38) with n = 0. Different line styles refer to different values of the ratio of the planetary to the stellar magnetic field B_pl ∕B₀ as labelled. The radius of the planet is assumed to be 0.1 R in all the cases.