  Searching for planets in the atmosphere of AGB stars is difficult, due to
confusion with the stellar wind and pulsations. The aim here is to provide a
complementary strategy for planet search in such a dense environment. The
polarization properties of SiO masers, especially their circular polarization,
are, under certain conditions, good tracers for rapid magnetospheric events. A
Jovian planet with a magnetosphere whose dipole axis is misaligned with its
rotation axis, naturally provides such conditions. Here I present several
models showing that the polarization will be periodically modulated.
Single-dish monitoring with a sufficiently dense time sampling and a carefully
calibrated polarimeter, in combination with VLBI observations, are suited to
detect and locate a periodic modulation of the circular maser polarization due
to a precessing Jovian magnetosphere. The phenomenon will be rare, because a
favourable arrangement of maser and magnetosphere is needed, otherwise the
polarization may be below the detection threshold, especially if the maser is
unsaturated. Linear polarization, though exhibiting a qualitatively similar
modulation, is likely to suffer more from confusion due to beam dilution, even
in VLBI observations.
