v_refl values, in kms⁻¹ calculated using the equations defined in Rodler et al. (2010, see Eqs. (5), (4) and (6)), for a range of exoplanet and stellar parameters. Each row of plots shows v_refl values for different values of stellar radii (R_⋆) and exoplanet radii (R_p). The y-axes correspond to the period of the exoplanet’s rotation on its own axis (P_rot,p). The x-axes correspond to the period of the exoplanet’s orbit around its host star and extends down to P_orb = 0.1 days). The dashed black diagonal lines show where P_rot,p = P_orb, i.e. the exoplanet is always showing the same face to its host star (as is the case for 51 Pegasi b). Each individual plot corresponds to a different period of the star’s rotation on its own axis (P_rot,⋆). The dotted black vertical lines show where P_rot,⋆ = P_orb, i.e. the star is always showing the same face to the exoplanet. The point where the dashed and dotted black lines cross mark where P_rot,p = P_rot,⋆ = P_orb, i.e. the star-exoplanet system is in a full tidal lock.