Planets that are embedded in the changing magnetic fields of their host stars can experience significant induction heating in their interiors caused by the planet's orbital motion. For induction heating to be substantial, the planetary orbit has to be inclined with respect to the stellar rotation and dipole axes. Using WX UMa, for which the rotation and magnetic axes are aligned, as an example, we show that for close-in planets on inclined orbits, induction heating can be stronger than the

more »

... heating occurring inside Jupiter's satellite Io; namely, it can generate a surface heat flux exceeding 2 W m^-2. An internal heating source of such magnitude can lead to extreme volcanic activity on the planet's surface, possibly also to internal local magma oceans, and to the formation of a plasma torus around the star aligned with the planetary orbit. A strongly volcanically active planet would eject into space mostly SO_2, which would then dissociate into oxygen and sulphur atoms. Young planets would also eject CO_2. Oxygen would therefore be the major component of the torus. If the O i column density of the torus exceeds ≈10^12 cm^-2, the torus could be revealed by detecting absorption signatures at the position of the strong far-ultraviolet O i triplet at about 1304 Å. We estimate that this condition is satisfied if the O i atoms in the torus escape the system at a velocity smaller than 1--10 km s^-1. These estimates are valid also for a tidally heated planet.