  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 tidal 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{\sc i} column density of the torus
exceeds $\approx$10$^{12}$\,cm$^{-2}$, the torus could be revealed by detecting
absorption signatures at the position of the strong far-ultraviolet O{\sc i}
triplet at about 1304\,\AA. We estimate that this condition is satisfied if the
O{\sc i} atoms in the torus escape the system at a velocity smaller than
1--10\,km\,s$^{-1}$. These estimates are valid also for a tidally heated
planet.
